Eclipse Implementation of JAXB Users Guide
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# Eclipse Implementation of JAXB Users Guide

Abstract

This document explains various interesting/complex/tricky aspects of Eclipse Implementation of JAXB, based on questions posted on the old JAXB users forum and answers provided there. This is an ongoing work-in-progress. Any feedback appreciated.

## 1. Compiling XML Schema

### 1.1. Dealing with errors

#### 1.1.1. Schema errors

Because XML Schema is so complicated, and because there are a lot of tools out there do not implement the spec correctly, it is often the case that a schema you are trying to compile has some real errors in it. When this is the case, you'll see XJC reporting somewhat cryptic errors such as `rcase-RecurseLax.2: There is not a complete functional mapping between the particles.`

The Eclipse Implementation of JAXB uses the schema correctness checker from the underlying JAXP implementation, which is the JAXP RI in a typical setup. The JAXP RI is one of the most conformant schema validators, and therefore most likely correct. So the first course of action usually is to fix problems in the schema.

However, in some situations, you might not have an authority to make changes to the schema. If that is the case and you really need to compile the schema, you can bypass the correctness check by using the `-nv` option in XJC. When you do this, keep in mind that you are possibly feeding "garbage" in, so you may see XJC choke with some random exception.

#### 1.1.2. Property 'fooBarZot' is already defined

One of the typical errors you'll see when compiling a complex schema is:

Example 1. Multiple property definitions error

    parsing a schema...
    [ERROR] Property "MiOrMoOrMn" is already defined.
      line 132 of
    file:/C:/kohsuke/Sun/JAXB/jaxb-unit/schemas/individual/MathML2/presentation/scripts.xsd

    [ERROR] The following location is relevant to the above error
      line 138 of
    file:/C:/kohsuke/Sun/JAXB/jaxb-unit/schemas/individual/MathML2/presentation/scripts.xsd

This is an actual example of the offending part of a schema, taken from MathML. If you go to line 132 of `scripts.xsd`, you'll see that it has a somewhat complicated content model definition:

Example 2. Multiple property definitions in MathML

    <xs:group name="mmultiscripts.content">
        <xs:sequence>
            <xs:group ref="Presentation-expr.class"/>
            <xs:sequence minOccurs="0" maxOccurs="unbounded">      <!-- line 132 -->
                <xs:group ref="Presentation-expr-or-none.class"/>
                <xs:group ref="Presentation-expr-or-none.class"/>
            </xs:sequence>
            <xs:sequence minOccurs="0">
                <xs:element ref="mprescripts"/>
                <xs:sequence maxOccurs="unbounded">                 <!-- line 138 -->
                    <xs:group ref="Presentation-expr-or-none.class"/>
                    <xs:group ref="Presentation-expr-or-none.class"/>
                </xs:sequence>
            </xs:sequence>
        </xs:sequence>
    </xs:group>

This is a standard technique in designing a schema. When you want to say "in this element, `B` can occur arbitrary times, but `C` can occur only up to once", you write this as `B*,(C,B*)?`. This, however, confuses Eclipse Implementation of JAXB, because it tries to bind the first `B` to its own property, then `C` to its own property, then the second `B` to its own property, and so we end up having a collision again.

In this particular case, `B` isn't a single element but it's a choice of large number of elements abstracted away in `<xs:group>`s, so they are hard to see. But if you see the same content model referring to the same element/group twice in a different place, you can suspect this.

In this case, you'd probably want the whole thing to map to a single list so that you can retain the order those elements show up in the document. You can do this by putting the same `<jaxb:property>` customization on the whole "`mmultiscripts.content`" model group, like this (or you can do it externally with XPath):

Example 3. How to fix the problem?

    <xs:groupname="mmultiscripts.content">
    <xs:annotation>
        <xs:appinfo>
            <jaxb:propertyname="content"/>
        </xs:appinfo>
    </xs:annotation>
    <xs:sequence>
    <xs:groupref="Presentation-expr.class"/>

Another way to fix this problem is to use the simpler and better binding mode in XJC, which is a Eclipse Implementation of JAXB vendor extension.

#### 1.1.3. Two declarations cause a collision in the ObjectFactory class

When schemas contain similar looking element/type names, they can result in "Two declarations cause a collision in the ObjectFactory class" errors. To be more precise, for each of all types and many elements (exactly what elements get a factory and what doesn't is bit tricky to explain), XJC produces one method on the `ObjectFactory` class in the same package. The `ObjectFactory` class is created for each package that XJC generates some files into. The name of the method is derived from XML element/type names, and the error is reported if two elements/types try to generate the same method name.

There are two approaches to fix this problem. If the collision is coming from two different schemas with different target namespaces, then you can easily avoid the collision by compiling them into different Java packages. To do this, use `<schemabindings>` customization on two schemas and specify the package name.

Another way to fix this problem is to use `<factoryMethod>` customization on two conflicting elements/types to specify different factory method names. This can be used in all cases, but if you have a large number of conflicts, you'll have to specify this customization one by one.

Notice that `<class>` customization doesn't affect the `ObjectFactory` method name by itself.

#### 1.1.4. Customization errors

##### 1.1.4.1. XPath evaluation of ... results in empty target node

External Jakarta XML Binding customizations are specified by using XPath (or using SCD.) This works by writing an XPath expression that matches a particular element in the schema document. For example, given the following schema and binding file:

Example 4. Schema and external binding file

`test.xsd`.

    <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
        <xs:complexTypename="foo"/>
    </xs:schema>

`test.xjb`.

    <bindings version="3.0" xmlns="https://jakarta.ee/xml/ns/jaxb" xmlns:xs="http://www.w3.org/2001/XMLSchema">
        <bindings schemaLocation="test.xsd">
            <bindings node="//xs:complexType[@name='foo']">
                <classname="Bar"/>
            </bindings>
        </bindings>
    </bindings>

will be interpreted as if the class customization is attached to the complex type '`foo`'.

For this to work, the XPath expression needs to match one and only one element in the schema document. When the XPath expression is incorrect and it didn't match anything, you get this "XPath evaluation of ... results in empty target node" problem.

Common causes of this problem include typos, incorrect namespace URI declarations, and misunderstanding of XPath.

### 1.2. Fixing broken references in schema

Sometimes a schema may refer to another schema document without indicating where the schema file can be found, like this:

Example 5. Schema reference without location

    <xs:import namespace="http://www.w3.org/1999/xlink" />

In other cases, a schema may refer to another schema on the network, which often slows down your compilation process and makes it unreliable. Yet in some other cases, a schema may reference another schema in relative path, and that may not match your directory structure.

XJC bundles a catalog resolver so that you can work around these situations without changing the schema documents. The main idea behind the catalog is "redirection" --- when XJC is about to fetch resources, it will consult the catalog resolver to see if it can find the resource elsewhere (which is usually your local resources.)

#### 1.2.1. Catalog format

The catalog resolver supports many different formats, but the easiest one is a line based `*.cat` format. Other than comments and empty lines, the file mainly consists of two kinds of declarations, `SYSTEM`, and `PUBLIC`.

Example 6. `sample-catalog.cat`

    --
      sample catalog file.

      double hyphens are used to begin and end a comment section.
    --

    SYSTEM "http://www.w3.org/2001/xml.xsd" "xml.xsd"

    PUBLIC "-//W3C//DTD XMLSCHEMA 200102//EN" "s4s/XMLSchema.dtd"

#### 1.2.2. Resolve by system ID

The SYSTEM entry has the format of "SYSTEM REFERENCE ACTUAL-LOCATION", which defines a simple redirection. Every time XJC loads any resource (be it schemas, DTDs, any entities referenced within), it will first resolve relative paths to absolute paths, then looks for a matching REFERENCE line. If it is found, the specified actual location is read instead. Otherwise XJC will attempt to resolve the absolutepath.

ACTUAL-LOCATION above accepts relative paths, and those are resolved against the catalog file itself (so in the above example, `xml.xsd` is assumed to be in the same directory with `sample-catalog.cat`.

What you need to be careful is the fact that the REFERENCE portion must be absolute, and when XJC finds a reference in schema, it will first convert that to the absolute path before checking the catalog. So what this means is that if your schema is written like this:

Example 7. Schema reference by relative path

    <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="xlink.xsd" />

Then your catalog entry would have to look like this:

Example 8. xlink.cat

    -- this doesn't work because xlink.xsd will be turned into absolute path --
    SYSTEM "xlink.xsd" "http://www.w3.org/2001/xlink.xsd"

    -- this will work, assuming that the above schema is in /path/to/my/test.xsd --
    SYSTEM "/path/to/my/xlink.xsd" "http://www.w3.org/2001/xlink.xsd"

#### 1.2.3. Resolve by public ID / namespace URI

Another kind of entry has the format of "PUBLIC PUBLICID ACTUAL-LOCATION" or "PUBLIC NAMESPACEURI ACTUAL-LOCATION".

The "`PUBLICID`" version is used to resolve DTDs and entities in DTDs. But this type of entry is also used to resolve `<xs:import>` statements. XJC will match the value of the namespace attribute and see if there's any matching entry. So given a schema like this:

Example 9. Schema import

    <xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="xlink.xsd" />
    <xs:import namespace="http://www.w3.org/1998/Math/MathML" />

The following catalog entries will match them.

Example 10. by-publicid.cat

    PUBLIC "http://www.w3.org/1999/xlink" "http://www.w3.org/2001/xlink.xsd"
    PUBLIC "http://www.w3.org/1998/Math/MathML" "/path/to/my/mathml.xsd"

As you can see, XJC will check the PUBLIC entries regardless of whether `<xs:import>` has the schemaLocation attribute or not. As with the case with the SYSTEM entry, the ACTUAL-LOCATION part can be relative to the location of the catalog file.

#### 1.2.4. Specifying the catalog file

Once you write a catalog file, you'd need to specify that when you invoke XJC.

CLI

  To do this from the CLI, use the `-catalog` option. See xjc -help for more details.

Ant

  Use the catalog attribute on the `<xjc>` task. See XJC ant task documentation for more details.

Maven

  For the Maven plugin, use the `<catalog>` element in the configuration:

      <plugin>
          <groupId>org.jvnet.jaxb2.maven2</groupId>
          <artifactId>maven-jaxb2-plugin</artifactId>
          <configuration>
              <!-- relative to the POM file -->
              <catalog>mycatalog.cat</catalog>
          </copnfiguration>
      </plugin>

#### 1.2.5. Debugging catalog file

If you are trying to write a catalog file and banging your head against a wall because it's not working, you should enable the verbose option of the catalog resolver. How you do this depends on what interface you use:

CLI

  Specify export XJC_OPTS="-Dxml.catalog.verbosity=999" then run XJC.

Ant/Maven

  Add `-Dxml.catalog.verbosity=999` as a command line option to Ant/Maven.

If you are otherwise invoking XJC programmatically, you can set the above system property before invoking XJC.

### 1.3. Mapping of `<xs:any />`

XJC binds `<xs:any />` in the following ways:

#### 1.3.1. `processContents="skip"`

`<xs:any />` with `processContents=skip` means any well-formed XML elements can be placed. Therefore, XJC binds this to DOM `Element` interface.

Example 11. Any/Skip schema

    <xs:element name="person">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="name" type="xs:string" />
          <xs:any processContents="skip" maxOccurs="unbounded" minOccurs="0" />
        </xs:sequence>
      </xs:complexType>
    </xs:element>

Example 12. Any/Skip binding

    import org.w3c.dom.Element;

    @XmlRootElement
    class Person {
      public String getName();
      public void setName(String);

      @XmlAnyElement
      public List<Element> getAny();
    }

#### 1.3.2. `processContents="strict"`

`<xs:any />` with `processContents=strict` (or `<xs:any />` without any processContents attribute, since it defaults to "strict") means any XML elements placed here must have corresponding schema definitions. This mode is not what people typically expect as "`wildcard`", but this is the default. The following shows this binding. (`lax=true` is unintuitive, but it's not an error in this document):

Example 13. Any/Strict schema

    <xs:element name="person">
      <xs:complexType>
        <xs:sequence>
          <xs:element name="name" type="xs:string" />
          <xs:any maxOccurs="unbounded" minOccurs="0" />
        </xs:sequence>
      </xs:complexType>
    </xs:element>

Example 14. Any/Strict binding

    @XmlRootElement
    class Person {
      public String getName();
      public void setName(String);

      @XmlAnyElement(lax=true)
      public List<Object> getAny();
    }

Jakarta XML Binding binds any such element to an `Object`, and during unmarshalling, all elements encountered are unmarshalled into corresponding Jakarta XML Binding objects (including `JAXBElement`s if necessary) and placed in this field. If it encounters elements that cannot be unmarshalled, DOM elements are produced instead.

At runtime, you can place either DOM elements or some Jakarta XML Binding objects that map to elements. A typical mistake is to put a `String` that contains XML fragment, but this won't work; you'd have to first read that into a DOM.

#### 1.3.3. `processContents="lax"`

`<xs:any />` with `processContents=lax` means any XML elements can be placed here, but if their element names match those defined in the schema, they have to be valid. XJC actually handles this exactly like processContents='strict', since the strict binding allows unknown elements anyway.

### 1.4. Mapping of `<xs:element />` to JAXBElement

Sometimes XJC binds an element declaration to `JAXBElement`. Sometimes XJC binds an element declaration to a Java class. What makes this difference?

### 1.5. How modularization of schema interacts with XJC

Over time schema authors have developed several techniques to modularize large schemas. Some of those techniques have some noteworthy interactions with XJC.

#### 1.5.1. Chameleon schema

Chameleon schema" (read more, in particular this) is a technique used to define multiple almost-identical sets of definitions into multiple namespaces from a single schema document.

For example, with this technique, you can write just one "foo" complex type and define it into namespace X and Y. In this case, one tends to hope that XJC will only give you one `Foo` class for this, but unfortunately because it's actually defined in two namespaces, Jakarta XML Binding needs two Java classes to distinguish `X:foo` and `Y:foo`, so you'll get multiple copies.

If you find this to be problematic, there are a few ways to work around the problem.

1. If you are in control of the schema, see if you can rewrite the schema to avoid using this technique. In some cases, the schema doesn't actually exploit the additional power of this technique, so this translation can be done without affecting XML instance documents. In some other cases, the chameleon schema can be argued as a bad schema design, as it duplicates definitions in many places.

2. If you are not in control of the schema, see if you can rewrite the schema nevertheless. This will only work if your transformation doesn't affect XML instance documents.

3. Perhaps there can be a plugin that eases the pain of this, such as by defining common interfaces among copies.

### 1.6. Adding behaviors

Adding behaviors to the generated code is one area that still needs improvement. Your feedback is appreciated.

Suppose if Eclipse Implementation of JAXB generated the following classes.

Example 15. Simple Eclipse Implementation of JAXB Generated Code

    package org.acme.foo;

    @XmlRootElement
    class Person {
      private String name;

      public String getName() { return name; }
      public void setName(String) { this.name=name; }
    }

    @XmlRegistry
    class ObjectFactory {
      Person createPerson() { ... }
    }

To add a behavior, first write a class that extends from `Person`. You also need to extend ObjectFactory to return this new class. Notice that neither classes have any Jakarta XML Binding annotation, and I put them in a separate package. This is because we'd like `PersonEx` class to be used in place of `Person`, and we don't want `PersonEx` to be bound to its own XML type.

Example 16. Extended Person class

    package org.acme.foo.impl;

    class PersonEx extends Person {
      @Override
      public void setName(String name) {
        if(name.length()<3) throw new IllegalArgumentException();
        super.setName(name);
      }
    }

    @XmlRegistry
    class ObjectFactoryEx extends ObjectFactory {
      @Override
      Person createPerson() {
        return new PersonEx();
      }
    }

At runtime, you can create `JAXBContext` normally, like this.

Example 17. Creating JAXBContext

    JAXBContext context = JAXBContext.newInstance(ObjectFactory.class);
    // or JAXBContext.newInstance("org.acme.foo");

`PersonEx` can be marshalled out just like `Person`:

Example 18. Marshalling

    Person p = new PersonEx();
    context.createMarshaller().marshal(p,System.out);
    // this will produce <person />

To unmarshal XML documents into `PersonEx`, you'll need to configure the unmarshaller to use your `ObjectFactoryEx` as the factory, like this:

Example 19. Unmarshalling

    Unmarshaller u = context.createUnmarshaller();
    u.setProperty("org.glassfish.jaxb.core.ObjectFactory",new ObjectFactoryEx());
    PersonEx p = (PersonEx)u.unmarshal(new StringReader("<person />"));

If you have multiple packages and thus multiple `ObjectFactory`s, you can pass in an array of them (`new Object[]{new OFEx1(),new OFEx2(),...}`.)

#### 1.6.1. Inserting your class in the middle

If you have a type hierarchy and would like to insert your class in the middle, you can use the combination of `XmlTransient` and `@implClass` of `<class>` customization. See the following example:

Example 20. Hierarchy of types and `<jaxb:class implClass>`

    <xs:schema ...>
      <xs:complexType name="vehicle">
        <xs:annotation><xs:appinfo>
          <jaxb:class implClass="MyVehicle" />
        </xs:appinfo></xs:annotation>
      </xs:complexType>

      <xs:complexType name="car">
        <xs:complexContent>
          <xs:extension base="vehicle" />
        </xs:complexContent>
      </xs:complexType>

      <xs:complexType name="bicycle">
        <xs:complexContent>
          <xs:extension base="vehicle" />
        </xs:complexContent>
      </xs:complexType>
    </xs:schema>

Example 21. This creates a class hierarchy like the following (among the generated Java code):

                Vehicle
                   ^
                   |
                MyVehicle
                   ^
              _____|______
             |            |
            Car          Bicycle

You'll then manually write `MyVehicle` class that extends from `Vehicle`. Annotate this class with `XmlTransient` to achieve the desired effect.

### 1.7. Avoid strong databinding

Under some limited circumstances, a weaker databinding is preferable for various reasons. Jakarta XML Binding does offer a few ways for you to achieve this.

#### 1.7.1. Avoid mapping to enum

The following customization will stop binding a simple type to a type-safe enum. This can be convenient when number of constants is too large to be an useful enum (by default, the Jakarta XML Binding spec won't generate enum with more than 256 constants, but even 100 might be too large for you.)

Example 22. Avoid mapping one simple type

    <xs:simpleType name="foo">
      <xs:annotation><xs:appinfo>
        <jaxb:typesafeEnumClass map="false" />
      </xs:appinfo></xs:annotation>
      <xs:restriction base="xs:string">
        <xs:enumeration value="x" />
        <xs:enumeration value="y" />
        <xs:enumeration value="z" />
      </xs:restriction>
    </xs:simpleType>

To disable such type-safe enum binding altogether for the entire schema, use a global binding setting like this (this is actually telling XJC not to generate enums if a simple type has more than 0 constants --- the net effect is no enum generation):

Example 23. Avoid generating enums at all

    <xs:schema ...>
      <xs:annotation><xs:appinfo>
        <jaxb:globalBindings typesafeEnumMaxMembers="0" />
      </xs:appinfo></xs:annotation>
      ...
    </xs:schema>

#### 1.7.2. Mapping to DOM

The `<jaxb:dom>`customization allows you to map a certain part of the schema into a DOM tree. This customization can be attached to the following schema components:

- Wildcards (`<xs:any>`)

- Type definitions (`<xs:complexType>` and `<xs:simpleType>`)

- Model groups (`<xs:choice>`,`<xs:all>`,`<xs:sequence>`)

- Model group declarations (`<xs:group>`)

- Particles

- Element declarations (`<xs:element>`)

In the following example, a wildcard is mapped to a DOM node. Each element that matches to the wildcard will be turned into a DOM tree.

Example 24. Dom Customization example

    <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
                   xmlns:jaxb="https://jakarta.ee/xml/ns/jaxb"
                   jaxb:version="3.0">

            <xs:element>
               <xs:complexType>
                  <xs:sequence>
                     <xs:any maxOccurs="unbounded" processContents="skip">
                        <xs:annotation><xs:appinfo>
                          <jaxb:dom/>
                        </xs:appinfo></xs:annotation>
                     </xs:any>
                  </xs:sequence>
               </xs:complexType>
            </xs:element>
        .
        .
        .
        </xs:schema>

This extension can be used to access wildcard content or can be used to process a part of a document by using other technologies that require "raw" XML. By default, Jakarta XML Binding generates a `getContent()` method for accessing wildcard content, but it only supports "lax" handling which means that unknown content is discarded. You may find more information in 7.12 chapter of Jakarta XML Binding 2 specification.

### 1.8. Working with generated code in memory

#### 1.8.1. Cloning

The generated beans (and in particular the `JAXBElement` class) do not support the clone operation. There was a suggestion by another user that beanlib has been used successfully to clone Jakarta XML Binding objects.

## 2. Customization of Schema Compilation

### 2.1. Customizing Java packages

The Jakarta XML Binding specification provides a `<jaxb:schemaBindings>` customization so that you can control which namespace goes to which package. See the example below:

Example 25. package customization

        <jaxb:schemaBindings>
          <jaxb:package name="org.acme.foo"/>
        </jaxb:schemaBindings>

You can do this as an internal customization (in which case you put this in `<xs:annotation>``<xs:appinfo>` under place it right under the `<xs:schema>` element), or do this as an external customization, like this:

Example 26. External package customization

    <bindings xmlns="https://jakarta.ee/xml/ns/jaxb" version="3.0">
      <bindings schemaLocation="../path/to/my.xsd">
        <schemaBindings>
          <package name="org.acme.foo"/>
        </schemaBindings>
      </bindings>
    </bindings>

Note that this customization is per namespace. That is, even if your schema is split into multiple schema documents, you cannot put them into different packages if they are all in the same namespace.

#### 2.1.1. Tip: get rid of the org.w3._2001.xmlschema package

Under some rare circumstances, XJC will generate some Java classes into a package called `org.w3._2001.xmlschema`. This happens when XJC decides that it needs some Java artifacts for the XML Schema built-in namespace of `http://www.w3.org/2001/XMLSchema`.

Since this package name is most often problematic, you can rename this by simply saving the following text in an .xsd file and submitting it to XJC along with the other schemas you have:

Example 27. Schemalet to get rid of org.w3._2001.xmlschema

    <schema xmlns="http://www.w3.org/2001/XMLSchema"
      targetNamespace="http://www.w3.org/2001/XMLSchema"
      xmlns:jaxb="https://jakarta.ee/xml/ns/jaxb"
      jaxb:version="3.0">
      <annotation><appinfo>
        <jaxb:schemaBindings>
          <jaxb:package name="org.acme.foo"/>
        </jaxb:schemaBindings>
      </appinfo></annotation>
    </schema>

This is bit tricky, but the idea is that since you can define a schema for one namespace in multiple schema documents, this makes XJC think that this schema is a part of the built-in "XML Schema for XML Schema".

### 2.2. Using SCD for customizations

When using an external customization file, the Jakarta XML Binding spec requires that you use XPath as a means to specify what your customization is attached to. For example, if you want to change the class name generated from a complex type, you'd write something like:

Example 28. External customization example

    <bindings xmlns="https://jakarta.ee/xml/ns/jaxb" version="3.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <bindings schemaLocation="../path/to/my.xsd" node="/xs:schema/xs:complexType[@name='foo']">
        <class name="FooType"/>
      </bindings>
    </bindings>

While the above process works, the problem with this is that the XPath+ `schemaLocation` combo tends to be verbose and error prone. It's verbose, because often a trivial target schema component like this "global complex type foo" takes up a lot of characters. The xs namespace declaration also takes up some space, although in this case we managed to avoid declaring the "tns" namespace (that represents the namespace that the schema defines.)

It's also error prone, because it relies on the way schema documents are laid out, because the schemaLocation attribute needs to point to the right schema document file. When a schema is split into multiple files for modularity (happens especially often with large schemas), then you'd have to find which schema file it is. Even though you can use relative paths, this hard-coding of path information makes it hard to pass around the binding file to other people.

JAXB RI 2.1 and onward offers a better way to do this as a vendor extension.

The key technology to solve this problem is a "`schema component designator`" (SCD.) This is a path language just like XPath, but whereas XPath is designed to refer to XML infoset items like elements and attributes, SCD is designed to refer to schema components like element declarations or complex types.

With SCD, the above binding can be written more concisely as follows:

Example 29. External customization by SCD

    <bindings xmlns="https://jakarta.ee/xml/ns/jaxb" version="3.0" xmlns:tns="http://my.namespace/">
      <bindings scd="/type::tns:foo">
        <class name="FooType"/>
      </bindings>
    </bindings>

`/type::tns:foo` can be written more concisely as `/~tns:foo`, too. If you are interested in more about the syntax of SCDs, read the example part of the spec, and maybe EBNF. If you know XPath, I think you'll find this fairly easy to learn.

Another benefit of an SCD is that tools will have easier time generating SCDs than XPath, as XPaths are often vulnerable to small changes in the schema document, while SCDs are much more robust. The downside of using SCD is as of JAXB 2.1, this feature is a vendor extension and not defined in the spec.

### 2.3. Using different datatypes

Eclipse Implementation of JAXB has a built-in table that determines what Java classes are used to represent what XML Schema built-in types, but this can be customized.

One of the common use cases for customization is to replace the `XMLGregorianCalendar` with the friendlier `Calendar` or `Date`. `XMLGregorianCalendar` is designed to be 100% compatible with XML Schema's date/time system, such as providing infinite precision in sub-seconds and years, but often the ease of use of those familiar Java classes win over the precise compatibility.

One very easy way to do this is to simply use your IDE (or even "`sed`") to replace all the references to `XMLGregorianCalendar` by `Calendar`. This is of course not a very attractive option if your build process runs XJC as a part of it.

Alternatively, the following customization file can be used to do this. When using external customization file, the Jakarta XML Binding spec requires you to use XPath as a means to specify what your customization is attached to. For example, if you want to change the class name generated from a complex type, you'd use the following customization:

Example 30. Customization to use Calendar for x`s:date`

    <bindings xmlns="https://jakarta.ee/xml/ns/jaxb" version="3.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <globalBindings>
        <javaType name="java.util.Calendar" xmlType="xs:date"
          parseMethod="jakarta.xml.bind.DatatypeConverter.parseDate"
          printMethod="jakarta.xml.bind.DatatypeConverter.printDate"
        />
      </globalBindings>
    </bindings>

Save this in a file and specify this to Eclipse Implementation of JAXB with the "-b" option.

To use the `Date` class, you'll need to do a bit more work. First, put the following class into your source tree:

Example 31. Adapter for Date

    package org.acme.foo;
    public class DateAdapter {
      public static Date parseDate(String s) {
        return DatatypeConverter.parseDate(s).getTime();
      }
      public static String printDate(Date dt) {
        Calendar cal = new GregorianCalendar();
        cal.setTime(dt);
        return DatatypeConverter.printDate(cal);
      }
    }

... then your binding file will be the following:

Example 32. Customization to use Date for x`s:date`

    <bindings xmlns="https://jakarta.ee/xml/ns/jaxb" version="3.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
      <globalBindings>
        <javaType name="java.util.Date" xmlType="xs:date"
          parseMethod="org.acme.foo.DateAadpter.parseDate"
          printMethod="org.acme.foo.DateAdapter.printDate"
        />
      </globalBindings>
    </bindings>

## 3. Annotating Your Classes

### 3.1. Mapping your favorite class

#### 3.1.1. ResultSet

Jakarta XML Binding (or any other databinding engine, for that matter) is for binding strongly-typed POJO-like objects to XML, such as `AddressBook` class, `PurchaseOrder` class, and so on, where you have fields and methods that shape a class.

There are other kinds of classes that are more close to reflection. Those classes don't have methods like `getAddress`, and instead you'd do `get("Address")`. JDBC ResultSet is one of those classes. It's one class that represents million different data structures, be it a customer table or a product table. Generally speaking, these classes does not allow Jakarta XML Binding to statically determine what the XML representation should look like. Instead, you almost always need to look at an instance to determine the shape of XML.

These classes are not really suitable for binding in Jakarta XML Binding. If this is the only object that you'd want to write out, then you'd be better off using XMLStreamWriter or some such XML infoset writing API. There are a few online articles that cover this topic. Also, many modern database offers a native ability to export a query into XML, which is supposed to work a lot faster than you'd do in Java (and saves your time of writing code.)

If you are using ResultSet as a part of your object tree that you want to marshal to Jakarta XML Binding, then you can use `XmlJavaTypeAdapter`.

#### 3.1.2. HashMap

Jakarta XML Binding spec defines a special handling for `Map` when it's used as a propety of a bean. For example, the following bean would produce XMLs like the following:

Example 33. Bean with Map

    @XmlRootElement
    class Foo {
      public HashMap<String,Integer> map;
    }

Example 34. XML representation

    <foo>
      <map>
        <entry>
          <key>a</key>
          <value>1</value>
        </entry>
        <entry>
          <key>b</key>
          <value>2</value>
        </entry>
      </map>
    </foo>

Unfortunately, as of 2.1, this processing is only defined for bean properties and not when you marshal `HashMap` as a top-level object (such as a value in `JAXBElement`.) In such case, `HashMap` will be treated as a Java bean, and when you look at `HashMap` as a bean it defines no getter/setter property pair, so the following code would produce the following XML:

Example 35. Bean with Map

    m = new HashMap();
    m.put("abc",1);
    marshaller.marshal(new JAXBElement(new QName("root"),HashMap.class,m),System.out);

Example 36. XML representation

    <root />

This issue has been recorded as #223 and the fix needs to happen in later versions of the Jakarta XML Binding spec.

In the mean time, such top-level objects have to be first adapted to a bean that Jakarta XML Binding can process. This has added benefit of being able to control XML representation better. The following code illustrates how to do this:

Example 37. Adapting HashMap

    public class MyHashMapType {
        public List<MyHashMapEntryType> entry = new ArrayList<MyHashMapEntryType>();
        public MyHashMapType(Map<String,Integer> map) {
            for( Map.Entry<String,Integer> e : map.entrySet() )
                entry.add(new MyHashMapEntryType(e));
        }
        public MyHashMapType() {}
    }

    public class MyHashMapEntryType {
        @XmlAttribute // @XmlElement and @XmlValue are also fine
        public String key;

        @XmlAttribute // @XmlElement and @XmlValue are also fine
        public int value;

        public MyHashMapEntryType() {}
        public MyHashMapEntryType(Map.Entry<String,Integer> e) {
           key = e.getKey();
           value = e.getValue();
        }
    }

    marshaller.marshal(new JAXBElement(new QName("root"),MyHashMapType.class,new MyHashMapType(m)),System.out);

If you have a lot of difference kinds of `Map`, you can instead use `Object` as the key and the value type. In that way, you'll be able to use maps with different type parameters, at the expense of seeing `xsi:type` attribute on the instance document.

### 3.2. Mapping interfaces

Because of the difference between the XML type system induced by W3C XML Schema and the Java type system, Jakarta XML Binding cannot bind interfaces out of the box, but there are a few things you can do.

#### 3.2.1. Use `@XmlRootElement`

When your interface is implemented by a large number of sub-classes, consider using `XmlRootElement` annotation like this:

Example 38. XmlRootElement for open-ended interfaces

    @XmlRootElement
    class Zoo {
      @XmlAnyElement
      public List<Animal> animals;
    }

    interface Animal {
      void sleep();
      void eat();
      ...
    }

    @XmlRootElement
    class Dog implements Animal { ... }

    @XmlRootElement
    class Lion implements Animal { ... }

This will produce XML documents like this:

Example 39. XML for XmlRootElement

    <zoo>
        <lion> ... </lion>
        <dog> ... </dog>
    </zoo>

The key characteristics of this approach is:

1. Implementations are open-ended; anyone can implement those interfaces, even by different people from different modules, provided they are all given to the `JAXBContext.newInstance` method. There's no need to list all the implementation classes in anywhere.

2. Each implementation of the interface needs to have an unique element name.

3. Every reference to interface needs to have the `XmlElementRef` annotation. The `type=Object.class` portion tells Jakarta XML Binding that the greatest common base type of all implementations would be `java.lang.Object`.

`@XmlElementWrapper` is often useful with this, as it allows you need to group them. Such as:

Example 40. XmlRootElement for open-ended interfaces

    @XmlRootElement
    class Zoo {
      @XmlElementWrapper
      @XmlAnyElement
      public List<Animal> onExhibit;
      @XmlElementWrapper
      @XmlAnyElement
      public List<Animal> resting;
    }

Example 41. Effect of XmlElementWrapper

    <zoo>
        <onExhibit>
            <lion> ... </lion>
            <dog> ... </dog>
        </onExhibit>
        <resting>
            <lion> ... </lion>
            <dog> ... </dog>
        </resting>
    </zoo>

#### 3.2.2. Use `@XmlJavaTypeAdapter`

When you use interfaces just to hide your implementation classes from exposure, and when there's 1-to-1 (or close to 1-on-1) relationship between a class and an interface, `XmlJavaTypeAdapter` can be used like below.

Example 42. XmlJavaTypeAdapter for interfaces

    @XmlJavaTypeAdapter(FooImpl.Adapter.class)
    interface IFoo {
      ...
    }
    class FooImpl implements IFoo {
      @XmlAttribute
      private String name;
      @XmlElement
      private int x;

      ...

      static class Adapter extends XmlAdapter<FooImpl,IFoo> {
        IFoo unmarshal(FooImpl v) { return v; }
        FooImpl marshal(IFoo v) { return (FooImpl)v; }
      }
    }

    class Somewhere {
      public IFoo lhs;
      public IFoo rhs;
    }

Example 43. XML of XmlJavaTypeAdapter

    <somewhere>
      <lhs name="...">
        <x>5</x>
      </lhs>
      <rhs name="...">
        <x>5</x>
      </rhs>
    </somewhere>

The key characteristics of this approach is:

1. Interface and implementation will be tightly coupled through an adapter, although changing an adapter code will allow you to support multiple implementations.

2. There's no need of any annotation in where interfaces are used.

A variation of this technique is when you have a few implementations for interface, not just one.

Example 44. XmlJavaTypeAdapter for interfaces with multiple implementations

    @XmlJavaTypeAdapter(AbstractFooImpl.Adapter.class)
    interface IFoo {
      ...
    }
    abstract class AbstractFooImpl implements IFoo {
      ...

      static class Adapter extends XmlAdapter<AbstractFooImpl,IFoo> {
        IFoo unmarshal(AbstractFooImpl v) { return v; }
        AbstractFooImpl marshal(IFoo v) { return (AbstractFooImpl)v; }
      }
    }

    class SomeFooImpl extends AbstractFooImpl {
      @XmlAttribute String name;
      ...
    }

    class AnotherFooImpl extends AbstractFooImpl {
      @XmlAttribute int id;
      ...
    }

    class Somewhere {
      public IFoo lhs;
      public IFoo rhs;
    }

Example 45. XML of XmlJavaTypeAdapter with multiple implementations

    <somewhere>
      <lhs xsi:type="someFooImpl" name="...">
      </lhs>
      <rhs xsi:type="anotherFooImpl" id="3" />
    </somewhere>

Note that `SomeFooImpl` and `AnotherFooImpl` must be submitted to `JAXBContext.newInstance` one way or the other.

To take this example a bit further, you can use `Object` instead of `AbstractFooImpl`. The following example illustarates this:

Example 46. XmlJavaTypeAdapter for interfaces with multiple implementations

    @XmlJavaTypeAdapter(AnyTypeAdapter.class)
    interface IFoo {
      ...
    }
    public class AnyTypeAdapter extends XmlAdapter<Object,Object> {
      Object unmarshal(Object v) { return v; }
      Object marshal(Object v) { return v; }
    }

    class SomeFooImpl implements IFoo {
      @XmlAttribute String name;
      ...
    }

    class Somewhere {
      public IFoo lhs;
      public IFoo rhs;
    }

Example 47. Corresponding schema

    <xs:complexType name="somewhere">
      <xs:sequence>
        <xs:element name="lhs" type="xs:anyType" minOccurs="0"/>
        <xs:element name="rhs" type="xs:anyType" minOccurs="0"/>
      </xs:sequence>
    </xs:complexType>

As you can see, the schema will generated to accept `xs:anyType` which is more relaxed than what the Java code actually demands. The instance will be the same as the above example. Starting from JAXB RI 2.1, we bundle the `AnyTypeAdapter` class in the runtime that defines this adapter. So you won't have to write this adapter in your code.

#### 3.2.3. Use `@XmlElement`

If the use of interface is very little and there's 1-to-1 (or close to) relationship between interfaces and implementations, then you might find `XmlElement` to be the least amount of work.

Example 48. XmlElement for interfaces

    interface IFoo {
      ...
    }
    class FooImpl implements IFoo {
      ...
    }

    class Somewhere {
      @XmlElement(type=FooImpl.class)
      public IFoo lhs;
    }

Example 49. XML of XmlElement

    <somewhere>
      <lhs> ... </lhs>
    </somewhere>

This effectively tells Jakarta XML Binding runtime that "even though the field is `IFoo`, it's really just `FooImpl`.

In this approach, a reference to an interface has to have knowledge of the actual implementation class. So while this requires the least amount of typing, it probably wouldn't work very well if this crosses module boundaries.

Like the `XmlJavaTypeAdapter` approach, this can be used even when there are multiple implementations, provided that they share the common ancestor.

The extreme of this case is to specify `@XmlElement(type=Object.class)`.

#### 3.2.4. Hand-write schema

Occasionally the above approaches cause the generated schema to become somewhat ugly, even though it does make the Jakarta XML Binding runtime work correctly. In such case you can choose not to use the generated schema and instead manually modify/author schemas tht better match your needs.

#### 3.2.5. Do schema-to-java

With sufficient knowlege, one can also use `<jaxb:class ref="..."/>` annotation so that you can cause XJC to use the classes you already wrote. See this thread for an example. TODO: more details and perhaps an example.

#### 3.2.6. DOESN'T WORK: Have Jakarta XML Binding generate interaces and swap different implementations

Some users attempted to use the "`generateValueClass`" customization and see if they can completely replace the generated implementations with other implementations. Unfortunately, this does not work.

Even with the interface/implementation mode, Jakarta XML Binding runtime still requires that the implementation classes have all the Jakarta XML Binding annotations. So just implementing interfaces is not sufficient. (This mode is mainly added to simplify the migration from JAXB 1.0 to Jakarta XML Binding, and that's a part of the reason why things are done this way.)

### 3.3. Evolving annotated classes

Here is the basic problem of evolution. You got your CoolApp v1, which contains class Foo that has some Jakarta XML Binding annotations. Now you are working towawrd CoolApp v2, and you want to make some changes to Foo. But you want to do so in such a way that v1 and v2 can still talk to each other.

The evolution compatibility has two different aspects. One is the schema compatibility, which is about the relationship between the v1 schema and the v2 schema. The other is about runtime compatibility, which is about reading/writing documents between two versions.

#### 3.3.1. Runtime compatibility

There are two directions in the runtime compatibility. One is whether v1 can still read what v2 write (forward compatible), and the other is whether v2 can read what v1 wrote (backward compatible).

#### 3.3.2. "`Semi-compatible`"

Jakarta XML Binding can read XML documents that don't exactly match what's expected. This is the default behavior of the Jakarta XML Binding unmarshaller, yet you can change it to a more draconian behavior (TODO: pointer to the unmarshalling section.)

When we are talking about evolving classes, it's often convenient to leave it in the default behavior, as that would allow Jakarta XML Binding to nicely ignore elements/attributes newly added in v2. So we call it backward semi-compatible if v2 can read what v1 wrote in this default unmarshalling mode, and similarly forward semi-compatible if v1 can read what v2 wrote in this default unmarshalling mode.

Technically, these are weaker than true backward/forward compatibility (since you can't do a draconian error detection), yet in practice it works just fine.

#### 3.3.3. Adding/removing/changing non-annotated things

You can add, remove, or change any non-annotated fields, methods, inner/nested types, constructors, interfaces. Those changes are both backward and forward compatible, as they don't cause any change to the XML representation.

Adding super class is backward compatible and forward semi-compatible. Similarly, removing super class is forward compatible and backward semi-compatible.

#### 3.3.4. Adding/removing/changing properties

Adding new annotated fields or methods is backward compatible and forward semi-compatible. Similarly, removing them is forward compatible and backward semi-compatible.

Changing a property is bit more tricky.

1. If you change the property name from X to Y, that would be the equivalent of deleting X and adding Y, so it would be backward and forward semi-compatible. What Jakarta XML Binding really cares is properties' XML names and not Java names, so by using the `name` parameter of `XmlElement`, `XmlAttribute` et al, you can change Java property names without affecting XML, or change XML without affecting Java properties. These are backward and forward semi-compatible. See below:

2. Example 50. Changing Java without affecting XML

       // BEFORE
       public class Foo {
           public String abc;
       }
       // AFTER: Java name changed, but XML remains the same
       public class Foo {
           @XmlElement(name="abc")
           public String def;
       }

   Example 51. Changing XML without affecting Java

       // BEFORE
       public class Foo {
           public String abc;
       }
       // AFTER: no Java change, but XML will look different
       public class Foo {
           @XmlElement(name="def")
           public String abc;
       }

3. If you change a property type, generally speaking it will be not compatible at all. For example, you can't change from `java.util.Calendar` to `int` and expect it to work. To make it a somewhat compatible change, the old type and the new type has to be related. For example, `String` can represent all `int` values, so changing `int` to `String` would be a backward compatible and forward semi-compatible change. `XmlJavaTypeAdapter` allows you to make changes to Java without affecting XML (or vice versa.)

#### 3.3.5. Changing class names

`XmlType` and `XmlRootElement` allows you to change a class name without affecting XML.

Example 52. Changing class name without affecting XML (1)

    // BEFORE
    @XmlRootElement
    public class Foo { ... }

    // AFTER: no XML change
    @XmlRootElement(name="foo")
    @XmlType(name="foo")
    public class Bar { ... }

Example 53. Changing class name without affecting XML (2)

    // BEFORE
    public class Foo { ... }

    // AFTER: no XML change
    @XmlType(name="foo")
    public class Bar { ... }

#### 3.3.6. Schema Compatibility

TODO.

### 3.4. XML layout and in-memory data layout

Your program sometimes needs to have a different in-memory data structure from its XML representation. Jakarta XML Binding has a few different ways to achieve this.

#### 3.4.1. XmlJavaTypeAdapter

`XmlJavaTypeAdapter` allows you to de-couple the in-memory representation and the XML representation by introducing an intermediate representation. The basic model is as follows:

    In-memory objects  <===>  Intermediate objects   <===>
    XML
                      adapter                         XMLBinding

Your adapter code will be responsible for converting in-memory objects to/from intermediate objects. Intermediate objects are then bound to XML by following the standard Jakarta XML Binding rules. See `XmlAdapter` for a general description of how adapters works.

Adapters extend from the `XmlAdapter` class and provide two methods "unmarshal" and "marshal" that converts values in both directions, and then the `XmlJavaTypeAdapter` annotation is used to tell Jakarta XML Binding where and what adapters kick in.

(TODO: more info about XmlJavaTypeAdapter needed)

1. adapting a class

2. adapting a property

3. adapting an external class

4. adapting a collection and its effect

5. adapting and using interfaces

#### 3.4.2. Using XmlJavaTypeAdapter for element/attribute values

One of the common use cases of `XmlJavaTypeAdapter` is to map a "value object" to a string in XML. The following example illustrates how to do this, by using `java.awt.Color` as an example.

Example 54. Mapping Color to #RRGGBB

    @XmlRootElement
    class Box {
      @XmlJavaTypeAdapter(ColorAdapter.class)
      @XmlElement
      Color fill;
    }

    class ColorAdapter extends XmlAdapter<String,Color> {
      public Color unmarshal(String s) {
        return Color.decode(s);
      }
      public String marshal(Color c) {
        return "#"+Integer.toHexString(c.getRGB());
      }
    }

This maps to the following XML representation:

Example 55. Box instance

    <box>
      <fill>#112233</fill>
    </box>

Since `XmlJavaTypeAdapter` is on a field, this adapter only kicks in for this particular field. If you have many `Color` fields and would like them all to use the same adapter, you can move the annotation to a package:

Example 56. package-info.java

    @XmlJavaTypeAdapter(type=Color.class,value=ColorAdapter.class)
    package foo;

Example 57. Box.java

    @XmlRootElement
    class Box {
      @XmlElement Color fill;
      @XmlElement Color border;
    }

This causes all the fields in the classes in the `foo` package to use the same specified adapter.

Also see the `DatatypeConverter` class that defines a series of basic conversion routines that you may find useful.

#### 3.4.3. Pair property

Another useful technique is to define two properties, one for Jakarta XML Binding and the other for your application. See the following example:

Example 58. Pair property sample

    @XmlRootElement
    class Person {
      private int age;

      // This public property is for users
      @XmlTransient
      public int getAge() {
        return age;
      }
      public void setAge(int age) {
        this.age = age;
      }

      // This property is for Jakarta XML Binding
      @XmlAttribute(name="age")
      private String getAge_() {
        if(age==-1)  return "dead";
        else         return String.valueOf(age);
      }
      private void setAge_(String v) throws NumberFormatException {
        if(v.equals("dead"))   this.age=-1;
        else                   this.age=Integer.parseInt(age);
    }

The main "`age`" property is public, but marked as `XmlTransient`, so it's exposed in your program, but Jakarta XML Binding will not map this to XML. There's another private "`age_`" property. Since this is marked with `XmlAttribute`, this is what Jakarta XML Binding is going to use to map to the attribute. The getter and setter methods on this property will handle the conversion between the in-memory representation and the XML representation.

### 3.5. Mapping cyclic references to XML

Object models designed in Java often have cycles, which prevent straight-forward conversion to XML by Jakarta XML Binding. In fact, when you try to marshal an object tree that contains a cycle, the Jakarta XML Binding marshaller reports an error, pointing out the objects that formed the cycle. This is because Jakarta XML Binding by itself cannot figure out how to cut cycles into a tree.

Thus it is your responsibility to annotate classes and use other means to "tell" Jakarta XML Binding how to handle a cycle. This chapter talks about various techniques to do this.

#### 3.5.1. Parent pointers

One of the very common forms of cycle is a parent pointer. The following example illustrates a typical parent pointer, and how this can be turned into "natural" XML:

Example 59. Classes with parent pointer

    @XmlRootElement
    class Department {
      @XmlAttribute
      String name;
      @XmlElement(name="employee")
      List<Employee> employees;
    }

    class Employee {
      @XmlTransient
      Department department;  // parent pointer
      @XmlAttribute
      String name;

      public void afterUnmarshal(Unmarshaller u, Object parent) {
        this.department = (Department)parent;
      }
    }

This will produce the following XML:

Example 60. XML view of department

    <department name="accounting">
      <employee name="Joe Chin" />
      <employee name="Adam Smith" />
      ...
    </department>

And reading this document back into Java objects will produce the expected tree with all the proper parent pointers set up correctly.

The first technique here is the use of `XmlTransient` on the parent pointer. This tells Jakarta XML Binding that you don't need this parent pointer to be represented explicitly in XML, because the fact that `employee` is always contained inside `department` implies the parent/child relationship. This causes the marshaller to produce the expected XML. However, when you unmarshal it, since this field is not bound, the `Employee.department` field will be left null.

That's where the second technique comes in, which is the use of the `afterUnmarshal` callback. This method is invoked by the Jakarta XML Binding implementation on each instance when the unmarshalling of a `Employee` object completes. Furthermore, the second parameter to the method is the parent object, which in this case is a `Department` object. So in this example, this sets up the parent pointer correctly.

This callback can be also used to perform other post-unmarshalling set up work.

#### 3.5.2. Many-to-many relationship

TBD

#### 3.5.3. `@XmlID` and `@XmlIDREF`

When a reference to another object is annotated with `XmlIDREF`, its corresponding XML it will be referenced by `xs:IDREF`, instead of containment. See below for an example:

Example of `@XmlID` and `@XmlIDREF`

    @XmlRootElement
    class Root {
      List<Foo> foos;
      List<Bar> bars;
    }
    class Foo {
      // you don't have to make it an attribute, but that's more common
      @XmlAttribute @XmlIDREF Bar bar;
    }
    class Bar {
      // you don't have to make it an attribute, but that's more common
      @XmlAttribute @XmlID String id;
    }

Example 61. Schema for above

    <xs:complexType name="foo">
      <xs:sequence/>
      <xs:attribute name="bar" type="xs:IDREF"/>
      </xs:sequence>
    </xs:complexType>
    <xs:complexType name="bar">
      <xs:sequence/>
      <xs:attribute name="id" type="xs:ID"/>
    </xs:complexType>

Example 62. A sample instance

    <root>
      <foo bar="x"/>
      <foo bar="y"/>
      <bar id="x"/>
      <bar id="y"/>
    </root>

There are a few things to consider when you do this. First, the object to be referenced must have an ID that is unique within the whole document. You'd also need to ensure that the referenced objects are contained somewhere else (like in the `Root` class in this case), or else `Bar` objects will never be marshalled. This technique can be used to remove the cyclic references, but it's only possible when your object model has an easy cut point.

#### 3.5.4. Use the CycleRecoverable interface

Starting 2.1 EA2, the Eclipse Implementation of JAXB exposes `CycleRecoverable` interface. Applications can choose to implement this interface in some of its objects. When a cyclic reference is detected during marshalling, and if the object that formed a cycle implements this interface, then the method on this interface is called to allow an application to nominate its replacement to be written to XML. In this way, the application can recover from a cycle gracefully.

This technique allows you to cope with a situation where you cannot easily determine upfront as to where a cycle might happen. On the other hand, this feature is a Eclipse Implementation of JAXB feature. Another downside of this is that unless you nominate your replacement carefully, you can make the marshalling output invalid with respect to the schema, and thus you might hit another problem when you try to read it back later.

## 4. Unmarshalling

### 4.1. `@XmlRootElement` and unmarshalling

Classes with `XmlRootElement` can be unmarshalled from XML elements simply by invoking the unmarshal method that takes one parameter. This is the simplest mode of unmarshalling.

Unmarshalling with `@XmlRootElement`

    @XmlRootElement
    class Foo {
      @XmlAttribute
      String name;
      @XmlElement
      String content;
    }

    Unmarshaller u = ...;
    Foo foo = (Foo)u.unmarshal(new File("foo.xml"));

Example 63. foo.xml

    <foo name="something">
      <content>abc</content>
    </foo>

However, sometimes you may need to unmarshal an instance of a type that does not have an `XmlRootElement`. For example, you might dynamically find out at the runtime that a certain element has a certain type. For example, the following document illustrates an XML instance where the content of `<someOtherTagName>` element is represented by the `Foo` class.

Example 64. foo2.xml

    <someOtherTagName name="something">
      <content>abc</content>
    </someOtherTagName>

To unmarshal this into a `Foo` class, use the version of the `unmarshal` method that takes the 'expectedType' argument, as follows:

Example 65. Unmarshalling into a known type

    Unmarshaller u = ...;
    JAXBElement<Foo> root = u.unmarshal(new StreamSource(new File("foo.xml")),Foo.class);
    Foo foo = root.getValue();

To reduce the number of the `unmarshal` methods, this two-argument version is not defined for every single-argument version. So as in this example, you might need to perform additional wrapping of the input parameter.

This instructs Jakarta XML Binding that the caller is expecting to unmarshal `Foo` instance. Jakarta XML Binding returns a `JAXBElement` of `Foo`, and this `JAXBElement` captures the tag name of the root element.

### 4.2. Unmarshalling is not working! Help!

There are a few common causes for this problem. These causes often exhibit similar symptoms:

1. Instance documents are invalid

2. `JAXBContext` is not created correctly.

#### 4.2.1. Make sure your instance document is valid

First, use an independent schema validator to check if your document is really valid with respect to the schema you compiled. When the root element of a document is invalid, then the unmarshaller will issue "unexpected element" errors. When a portion of a document is invalid, Eclipse Implementation of JAXB skips that portion, so the end result is that the unmarshalling returns normally, yet you notice that a part of the content tree is missing. This is often the desirable behavior, but it sometimes ends up masking a problem.

Also, try to install `ValidationEventHandler` on the unmarshaller. When a portion of a document is skipped, the unmarshaller notifies a `ValidationEventHandler`, so it allows you to see what's going on.

Example 66. Installing ValidationEventHandler

    Unmarshaller u = ...;
    // this implementation is a part of the API and convenient for trouble-shooting,
    // as it prints out errors to System.out
    u.setEventHandler(new jakarta.xml.bind.helpers.DefaultValidationEventHandler());

    u.unmarshal(new File("foo.xml"));

Also consider installing a `Schema` object to the unmarshaller, so that the unmarshaller performs a schema validation while unmarshalling. Earlier I suggested that you try an independent schema validator, but for various reasons (not all tools are reliable, you might have made an error and used a different schema/instance), using validating unmarshalling is a better way to guarantee the validity of your instance document being unmarshalled. Please follow the JAXP tutorial for more about how to construct a `Schema` object from your schema.

If you are unmarshalling from XML parser APIs (such as DOM, SAX, StAX), then also make sure that the parser/DOM is configured with the namespace enabled.

#### 4.2.2. Check if your JAXBContext is correct

(TODO: This also applies to the marshaller. Think about moving it.)

The other possibility is that `JAXBContext` is not set up correctly. `JAXBContext` "knows" a set of classes, and if it doesn't know a class that it's supposed to know, then the unmarshaller may fail to perform as you expected.

To verify that you created `JAXBContext` correctly, call `JAXBContext.toString()`. It will output the list of classes it knows. If a class is not in this list, the unmarshaller will never return an instance of that class. Make you see all the classes you expect to be returned from the unmarshaller in the list. When dealing with a large schema that spans across a large number of classes and packages, this is one possible cause of a problem.

If you noticed that a class is missing, explicitly specify that to `JAXBContext.newInstance`. If you are binding classes that are generated from XJC, then the easiest way to include all the classes is to specify the generated `ObjectFactory` class(es).

### 4.3. Element default values and unmarshalling

Because of the "strange" way that element default values in XML Schema work, people often get confused about their behavior. This section describes how this works.

When a class has an element property with the default value, and if the document you are reading is missing the element, then the unmarshaller does not fill the field with the default value. Instead, the unmarshaller fills in the field when the element is present but the content is missing. See below:

Example 67. XML instance 1

    <foo />

Example 68. XML instance 2

    <foo>
      <a/>  <!-- or <a></a> -->
    </foo>

Example 69. XML instance 3

    <foo>
      <a>abc</a>
    </foo>

Example 70. Element defaults and XML

    @XmlRootElement
    class Foo {
      @XmlElement(defaultValue="value") public String a=null;
    }

    Foo foo = unmarshaller.unmarshal("instance1.xml");
    System.out.println(foo.a);   // null

    Foo foo = unmarshaller.unmarshal("instance2.xml");
    System.out.println(foo.a);   // "value". The default kicked in.

    Foo foo = unmarshaller.unmarshal("instance3.xml");
    System.out.println(foo.a);   // "abc". Read from the instance.

This is consistent with the XML Schema spec, where it essentially states that the element defaults do not kick in when the element is absent, so unfortunately we can't change this behavior.

Depending on your expectation, using a field initializer may achieve what you are looking for. See below:

Example 71. Possible changes by using field initializer

    @XmlRootElement
    class Foo {
      @XmlElement public String a="value";
    }

    Foo foo = unmarshaller.unmarshal("instance1.xml");
    System.out.println(foo.a);   // "value", because Jakarta XML Binding didn't overwrite the value

    Foo foo = unmarshaller.unmarshal("instance2.xml");
    System.out.println(foo.a);   // "", because <a> element had 0-length string in it

    Foo foo = unmarshaller.unmarshal("instance3.xml");
    System.out.println(foo.a);   // "abc". Read from the instance.

Alternatively, attribute default values work in a way that agrees with the typical expectation, so consider using that. Also, see Element default values and marshalling.

### 4.4. Dealing with large documents

Jakarta XML Binding API is designed to make it easy to read the whole XML document into a single tree of Jakarta XML Binding objects. This is the typical use case, but in some situations this is not desirable. Perhaps:

1. A document is huge and therefore the whole may not fit the memory.

2. A document is a live stream of XML (such as XMPP) and therefore you can't wait for the EOF.

3. You only need to databind the portion of a document and would like to process the rest in other XML APIs.

This section discusses several advanced techniques to deal with these situations.

#### 4.4.1. Processing a document by chunk

When a document is large, it's usually because there's repetitive parts in it. Perhaps it's a purchase order with a large list of line items, or perhaps it's an XML log file with large number of log entries.

This kind of XML is suitable for chunk-processing; the main idea is to use the StAX API, run a loop, and unmarshal individual chunks separately. Your program acts on a single chunk, and then throws it away. In this way, you'll be only keeping at most one chunk in memory, which allows you to process large documents.

See the streaming-unmarshalling example and the partial-unmarshalling example in the Eclipse Implementation of JAXB distribution for more about how to do this. The streaming-unmarshalling example has an advantage that it can handle chunks at arbitrary nest level, yet it requires you to deal with the push model --- Jakarta XML Binding unmarshaller will "`push`" new chunk to you and you'll need to process them right there.

In contrast, the partial-unmarshalling example works in a pull model (which usually makes the processing easier), but this approach has some limitations in databinding portions other than the repeated part.

#### 4.4.2. Processing a live stream of XML

The techniques discussed above can be used to handle this case as well, since they let you unmarshal chunks one by one. See the xml-channel example in the Eclipse Implementation of JAXB distribution for more about how to do this.

#### 4.4.3. Creating virtual infosets

For further advanced cases, one could always run a streaming infoset conversion outside Jakarta XML Binding API and basically curve just the portion of the infoset you want to data-bind, and feed it as a complete infoset into Jakarta XML Binding API. Jakarta XML Binding API accepts XML infoset in many different forms (DOM, SAX, StAX), so there's a fair amount of flexibility in choosing the right trade off between the development effort in doing this and the runtime performance.

For more about this, refer to the respective XML infoset API.

## 5. Marshalling

### 5.1. Changing prefixes

By default, a Jakarta XML Binding marshaller uses random namespace prefixes (such as `ns1`, `ns2`, ...) when it needs to declare new namespace URIs. While this is perfectly valid XML wrt the schema, for human readability, you might want to change them to something that makes more sense.

The Eclipse Implementation of JAXB defines NamespacePrefixMapper to allow you to do this. See the `namespace-prefix` sample in the distribution for more details.

### 5.2. Element default values and marshalling

Because of a "strange" way element default values in XML Schema work, people often get confused about its behavior. This section describes how this works.

When a class has an element property with the default value, and if a value is null, then the marshaller will not produce the corresponding element in XML:

Example 72. Element defaults and XML

    @XmlRootElement
    class Foo {
      @XmlElement(defaultValue="value") public String a=null;
    }

    marshaller.marshal(new Foo(),System.out);

Example 73. Marshalling output from above

    <foo />

This is consistent with the XML Schema spec, where it essentially states that the element defaults do not kick in when the element is absent. Attribute default values do not have this problem, so if you can change the schema, changing it to an attribute is usually a better idea. Alternatively, depending on your expectation, setting the field to a default value in Java may achieve what you are looking for.

Example 74. Possible changes

    @XmlRootElement
    class Foo {
      @XmlElement public String a="value";
    }
    @XmlRootElement
    class Bar {
      @XmlAttribute public String a;
    }

    marshaller.marshal(new Foo(),System.out);
    marshaller.marshal(new Bar(),System.out);

Example 75. Marshalling output from above

    <foo>
        <a>value</a>
    </foo>

    <bar/>

Also, see Element default values and unmarshalling.

### 5.3. Different ways of marshalling

#### 5.3.1. Different output media

The most basic notion of the marshalling is to take a Jakarta XML Binding-bound object that has `@XmlRootElement`, and write it out as a whole XML document. So perhaps you have a class like this:

Example 76. Jakarta XML Binding POJO

    class Point {
      @XmlElement
      public int x;
      @XmlElement
      public int y;
      Point(...) { ... }
    }

Then you can do:

Example 77. Plain marshalling

    marshaller.marshal( new Point(1,3), System.out );
    marshaller.marshal( new Point(1,3), new File("out.xml") );

.. and so on. There're seven `Marshaller.marshal` methods that takes different output media as the second parameter. If you are writing to a file, a socket, or memory, then you should use the version that takes `OutputStream`. Unless you change the target encoding to something else (default is UTF-8), there's a special marshaller codepath for `OutputStream`, which makes it run really fast. You also don't have to use `BufferedOutputStream`, since the Eclipse Implementation of JAXB does the adequate buffering.

You can also write to `Writer`, but in this case you'll be responsible for encoding characters, so in general you need to be careful. If you want to marshal XML into an encoding other than UTF-8, it's best to use the `JAXB_ENCODING` property and then write to `OutputStream`, as it escapes characters to things like `&#x1824;` correctly.

The next medium we support is W3C DOM. This is bit unintuitive, but you'll do it like this:

Example 78. Marshal to DOM

    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    dbf.setNamespaceAware(true);
    Document doc = dbf.newDocumentBuilder().newDocument();

    marshaller.marshal( new Point(1,3), doc );

And after the method invocation you get a complete DOM tree that represents the marshalled document.

The other versions of the marshal methods are there to write XML documents in terms of other XML APIs, such as SAX and StAX. The version that takes `ContentHandler` is useful when you need a custom formatting needs (like you want each attribute to be in new line, etc), but otherwise they are not very interesting if you are writing a whole document.

#### 5.3.2. Marshalling into a subtree

Another common use of Jakarta XML Binding is where you are writing a bigger document, and you use Jakarta XML Binding to generate part(s) of it. The Eclipse Implementation of XML Web Services is the prime example. It produces a SOAP message, and Jakarta XML Binding is only used to produce the body. When you are doing this, you first set `JAXB_FRAGMENT` property on the marshaller. This changes the behaviors of the marshaller so that it works better in this situation.

If you are writing to an `OutputStream` or `Writer` and generally sending it to someone else, you can do something like this:

Example 79. Marshalling into a subtree

    System.out.println("<envelope>");
    marshaller.marshal( object, System.out );
    System.out.println("</envelope>");

Like I mentioned, this is probably the fastest, even though `println` isn't very pretty. `JAXB_FRAGMENT` prevents the marshaller from producing an XML declaration, so the above works just fine. The downside of this approach is that if the ancestor elements declare the namespaces, Jakarta XML Binding won't be able to take advantage of them.

You can also marshal an object as a subtree of an existing DOM tree. To do this, you pass the `Element` object as the second parameter, and the marshaller will marshal an object as a child of this node.

StAX is also very convenient for doing this sort of things. You can create `XMLStreamWriter`, write some stuff, and then pass that to the marshaller. `JAXB_FRAGMENT` prevents the marshaller from producing `startDocument` and `endDocument` token. When doing this sub-tree marshaling to DOM and StAX, Jakarta XML Binding can take advantage of available in-scope namespace bindings.

Finally, you can marshal an object as a subtree into `ContentHandler`, but it requires a fair amount of SAX programming experience, and it goes beyond the scope of this entry.

#### 5.3.3. Marshalling a non-element

Another common use case is where you have an object that doesn't have `@XmlRootElement` on it. Jakarta XML Binding allows you to marshal it like this:

Example 80. Marshalling a non-element

    marshaller.marshal( new JAXBElement(
      new QName("","rootTag"),Point.class,new Point(...)));

This puts the `<rootTag>` element as the root element, followed by the contents of the object, then `</rootTag>`. You can actually use it with a class that has `@XmlRootElement`, and that simply renames the root element name.

At the first glance the second `Point.class` parameter may look redundant, but it's actually necessary to determine if the marshaller will produce (infamous) `@xsi`:type. In this example, both the class and the instance are `Point`, so you won't see `@xsi`:type. But if they are different, you'll see it.

This can be also used to marshal a simple object, like `String` or an integer.

Marshalling a non-element with `@xsi`:type

    marshaller.marshal( new JAXBElement(
      new QName("","rootTag"),String.class,"foo bar"));

But unfortunately it cannot be used to marshal objects like `List` or `Map`, as they aren't handled as the first-class citizen in the Jakarta XML Binding world.

#### 5.3.4. Connecting to other XML APIs

Because of the `Source` and `Result` support, Jakarta XML Binding objects can be easily marshalled into other XML APIs that are not mentioned here. For example, dom4j has `DocumentResult` that extends `Result`, so you can do:

Example 81. Marshalling to dom4j

    DocumentResult dr = new DocumentResult();
    marshaller.marshal( object, dr );
    o = dr.getDocument();

Similar mechanism is available for JDOM and XOM. This conversion is much more efficient than first marshalling to `ByteArrayOutputStream` and then read it back into these DOMs. The same mechanism can be used to marshal to FastInfoset or send the marshaled document to an XSLT engine (`TransformerHandler`.)

The other interesting connector is `JAXBSource`, which wraps a marshaller and allows a Jakarta XML Binding object to be used as a "source" of XML. Many XML APIs take `Source` as an input, and now Jakarta XML Binding object can be passed to them directly.

For example, you can marshal a Jakarta XML Binding object and unmarshal it into another JAXBContext like this:

Example 82. Loading into a different JAXBContext

    JAXBContext context1 = ... ;
    JAXBContext context2 = ... ;

    context1.createUnmarshaller().unmarshal( new JAXBSource(context2,object) );

This amounts to looking at the same XML by using different schema, and again this is much more efficient than going through `ByteArrayOutputStream`.

### 5.4. Interaction between marshalling and DOM

Sometimes you may notice that Jakarta XML Binding is producing XML with seemingly unnecessary namespace declarations. In this section, we'll discuss the possible causes and how to resolve this.

#### 5.4.1. Caused by DOM mapping

The #1 cause of extra namespace declarations is due to the DOM mapping. This mainly happens because of a schema construct that forces XJC to generate a property with DOM. This includes the use of wildcard `<xs:any/>` (see more about this Mapping of `<xs:any />`), as well as `xs:anyType` (which can also happen by omission, such as `<xs:element name="foo"/>`, which is interpreted as `<xs:element name="foo" type="xs:anyType" />`.

During unmarshalling, when a subtree of the input XML is converted into XML, Jakarta XML Binding copies all the in-scope namespace bindings active at that time to the root of the DOM element. So for example, given the following Java class and XML, the DOM tree that the `child` field will get will look like the following:

Example 83. Bean with wildcard

    @XmlRootElement
    class Foo {
      @XmlAnyElement
      public Element child;
    }

Example 84. Instance with subtree matching wildcard

    <foo xmlns:a="a" xmlns:b="b" xmlns:c="c">
      <subtree xmlns:c="cc">
        <data>a:xyz</data>
      </subtree>
    </foo>

Example 85. DOM tree to be stored in Foo.child

    <subtree xmlns:a="a" xmlns:b="b" xmlns:c="cc">
        <data>a:xyz</data>
      </subtree>

Note that the two namespace declarations are copied over, but `c` is not because it's overridden. Also not that Jakarta XML Binding is not touching the whitespace in document. This copying of namespace declarations is necessary to preserve the infoset in the input document. For example, if the `<data>` is a QName, its meaning would change if Jakarta XML Binding unmarshaller doesn't copy it.

Now, imagine what happens when you marshal this back to XML. Despite the fact that in this example neither `b` nor `c` prefixes are in use, Jakarta XML Binding cannot delete them, because it doesn't know if those attributes are significant to the application or not. Therefore, this could end up producing XML with "extra namespace declarations" like:

Example 86. DOM tree to be stored in Foo.child

    <foo>
      <subtree xmlns:a="a" xmlns:b="b" xmlns:c="cc">
        <data>a:xyz</data>
      </subtree>
    </foo>

Resolving this problem is not possible in the general case, but sometimes one of the following strategy works:

1. Sometimes schema author incorrectly assumes that `<xs:element name="foo"/>` means `<xs:element name="foo" type="xs:string"/>`, because attribute declarations work somewhat like this. In such a case, adding explicit `type` attribute avoids the use of DOM, so things will work as expected.

2. The wildcard processing mode " `strict`" would force a typed binding, and thereby eliminate any DOM mapping.

3. You might be able to manulally go into the DOM tree and remove unnecessary namespace declarations, if your application knows what are necessary and what are not.

## 6. Schema Generation

### 6.1. Invoking schemagen programatically

Schemagen tools by default come in as CLI, ant task, and Maven plugin. These interfaces allow you to invoke schemagen functionality from your program.

#### 6.1.1. At runtime

If the classes you'd like to generate schema from are already available as `java.lang.Class` objects (meaning they are already loaded and resolved in the current JVM), then the easiest way to generate a schema is to use the Jakarta XML Binding API:

Example 87. Generate schema at runtime

    File baseDir = new File(".");

    class MySchemaOutputResolver extends SchemaOutputResolver {
        public Result createOutput( String namespaceUri, String suggestedFileName ) throws IOException {
            return new StreamResult(new File(baseDir,suggestedFileName));
        }
    }

    JAXBContext context = JAXBContext.newInstance(Foo.class, Bar.class, ...);
    context.generateSchema(new MySchemaOutputResolver());

#### 6.1.2. CLI interface

The CLI interface (`public static int com.sun.tools.jxc.SchemaGenerator.run(String[])`) is the easiest API to access. You can pass in all the schemagen command-line arguments as a string array, and get the exit code as an int value. Messages are sent to `System.err` and `System.out`.

#### 6.1.3. Ant interface

Ant task can be invoked very easily from a non-Ant program. The schemagen ant task is defined in the SchemaGenTask class,

#### 6.1.4. Native Java API

The above two interfaces are built on top of externally committed contracts, so they'll evolve only in a compatibile way. The downside is that the amount of control you can exercise over them would be limited.

So yet another approach to invoke schemagen is to use Eclipse Implementation of JAXB's internal interfaces. But be warned that those interfaces are subject to change in the future versions, despite our best effort to preserve them. This is the API that the Eclipse Implementation of XML Web Services uses to generate schema inside WSDL when they generate WSDL, so does some other web services toolkits that work with the Eclipse Implementation of JAXB.

Most of those interfaces are defined and well-documented in the com.sun.tools.xjc.api package. You can see how the schemagen tools are eventually calling into this API at the implementaion of SchemaGenerator class.

### 6.2. Generating Schema that you want

This section discusses how you can change the generated XML schema. For changes that also affect the infoset (such as changing elements to attributes, namespaces, etc.), refer to a different section "XML layout and in-memory data layout".

#### 6.2.1. Adding facets to datatypes

As of Eclipse Implementation of JAXB 4.0.5, currently no support for this, although there has been several discussions in the users alias.

The Eclipse Implementation of JAXB project is currently lacking resources to attack this problem, and therefore looking for volunteers to work on this project. The basic idea would be to define enough annotations to cover the basic constraint facets (such as length, enumerations, pattern, etc.) The schema generator would have to be then extended to honor those annotations and generate schemas accordingly.

Some users pointed out relevance of this to Jakarta Bean Validation. If you are interested in picking up this task, let us know!

## 7. Deployment

### 7.1. Using Eclipse Implementation of JAXB with Maven

#### 7.1.1. Maven coordinates for Eclipse Implementation of JAXB artifacts

- jakarta.xml.bind:jakarta.xml.bind-api: API classes for Jakarta XML Binding. Required to compile against Jakarta XML Binding.

- org.glassfish.jaxb:jaxb-core: Contains sources required by XJC, JXC and Runtime modules.

- org.glassfish.jaxb:jaxb-runtime: Contains the main runtime used for serialization and deserialization java objects to/from xml.

- org.glassfish.jaxb:jaxb-xjc: Tool to generate Jakarta XML Binding java sources from XML representation.

- org.glassfish.jaxb:jaxb-jxc: Tool to generate XML schema from Jakarta XML Binding java sources.

#### 7.1.2. JAXB RI bundles

- com.sun.xml.bind:jaxb-core: Contains sources required by XJC, JXC and Runtime modules with dependencies.

- com.sun.xml.bind:jaxb-impl: Eclipse Implementation of JAXB runtime jar.

- com.sun.xml.bind:jaxb-xjc: Class generation tool jar.

- com.sun.xml.bind:jaxb-jxc: Schema generation tool jar.

In contrast to `org.glassfish.jaxb` artifacts, these jars have all dependency classes included inside.

#### 7.1.3. Binary distribution

- com.sun.xml.bind:jaxb-ri: Zip distribution containing tooling scripts and all dependency jars in one archive.

#### 7.1.4. Jakarta XML Binding API and Runtime

Minimum requirement to compile is `jakarta.xml.bind-api.jar`. If a client application is running on an environment where Jakarta XML Binding runtime is provided, `jakarta.xml.bind-api.jar` is all that is needed.

Example 88. API only

                    <!-- API -->
                    <dependency>
                        <groupId>jakarta.xml.bind</groupId>
                        <artifactId>jakarta.xml.bind-api</artifactId>
                        <version>4.0.2</version>
                    </dependency>

If client application needs to include the runtime, e.g. running standalone on Java SE `jaxb-impl` should be also included.

Example 89. API + Runtime

                    <!-- API -->
                    <dependency>
                        <groupId>jakarta.xml.bind</groupId>
                        <artifactId>jakarta.xml.bind-api</artifactId>
                        <version>4.0.2</version>
                    </dependency>

                    <!-- Runtime -->
                    <dependency>
                        <groupId>com.sun.xml.bind</groupId>
                        <artifactId>jaxb-impl</artifactId>
                        <version>4.0.5</version>
                    </dependency>

### 7.2. Using Eclipse Implementation of JAXB on JPMS

Java SE 11 features JSR 376 Java Platform Module System. Starting from version 2.3.2 Eclipse Implementation of JAXB supports JPMS and can be loaded and used from module path. There are only a few things to be aware of.

#### 7.2.1. Eclipse Implementation of JAXB classes openness

Eclipse Implementation of JAXB does reflectively access private members of the class, so client application if loaded from module path needs to "open" packages containing jaxb classes to Jakarta XML Binding. There are alternative Jakarta XML Binding implementations, having different module names, Jakarta XML Binding requires pojo classes to be open only to API module.

Example 90. JPMS module descriptor opening Jakarta XML Binding pojo classes to Jakarta XML Binding API

    //JPMS module descriptor
    module com.example.jaxbclasses {

        //Jakarta XML Binding module name
        requires jakarta.xml.bind;

        //open pojo package to make accessing private members possible for Jakarta XML Binding.
        opens com.example.jaxbclasses.pojos to jakarta.xml.bind;
    }

Jakarta XML Binding API will delegate openness to implementation module after resolving it with service discovery mechanism.

Example 91. Eclipse Implementation of JAXB on JPMS Command line examples

    #Both client and Eclipse Implementation of JAXB on module path:
    $ java -m com.example.jaxbclasses/com.example.jaxb.Main --module-path jaxbclient.jar:jakarta.xml.bind-api.jar:jakarta.activation-api.jar:jaxb-core.jar:jaxb-impl.jar

    #Both client and Eclipse Implementation of JAXB on classpath:
    $ java com.example.jaxb.Main -cp jaxbclient.jar:jakarta.xml.bind-api.jar:jakarta.activation-api.jar:jaxb-core.jar:jaxb-impl.jar

    #Client on classpath, Eclipse Implementation of JAXB on module path:
    $ java com.example.jaxb.Main -cp jaxbclient.jar --module-path jakarta.xml.bind-api.jar:jakarta.activation-api.jar:jaxb-core.jar:jaxb-impl.jar --add-modules jakarta.xml.bind

Jakarta XML Binding API will delegate openness to implementation module after resolving it with service discovery mechanism.

## 8. Other Miscellaneous Topics

### 8.1. Performance and thread-safety

The JAXBContext class is thread safe, but the Marshaller, Unmarshaller, and Validator classes are not thread safe.

For example, suppose you have a multi-thread server application that processes incoming XML documents by Jakarta XML Binding. In this case, for the best performance you should have just one instance of JAXBContext in your whole application like this:

Example 92. Singleton JAXBContext

    class MyServlet extends HttpServlet {
        static final JAXBContext context = initContext();

        private static JAXBContext initContext() {
            return JAXBContext.newInstance(Foo.class,Bar.class);
        }
    }

And each time you need to unmarshal/marshal/validate a document. Just create a new Unmarshaller/Marshaller/Validator from this context, like this:

Example 93. Thread local Unmarshaller

        public void doGet( HttpServletRequest req, HttpServletResponse ) {
            Unmarshaller u = context.createUnmarshaller();
            u.unmarshal(...);
        }

This is the simplest safe way to use the Eclipse Implementation of JAXB from multi-threaded applications.

If you really care about the performance, and/or your application is going to read a lot of small documents, then creating Unmarshaller could be relatively an expensive operation. In that case, consider pooling Unmarshaller objects. Different threads may reuse one Unmarshaller instance, as long as you don't use one instance from two threads at the same time.

### 8.2. Compiling DTD

The Eclipse Implementation of JAXB is shipped with an "experimental" DTD support, which let's you compile XML DTDs. It is marked "experimental" not because the feature is unstable nor unreliable, but rather because it's not a part of the JAXB specification and therefore the level of commitment to compatibility is lower.

Example 94. To compile a DTD, run the XJC binding compiler as follows:

    $ xjc.sh -dtd test.dtd

All the other command-line options of the XJC binding compiler can be applied. Similarly, the XJC ant task supports DTD. The generated code will be no different from what is generated from W3C XML Schema. You'll use the same JAXB API to access the generated code, and it is portable in the sense that it will run on any JAXB 2.0 implementation.

DTD long predates XML namespace, although people since then developed various techniques to use XML namespaces in conjunction with DTD. Because of this, XJC is currently unable to reverse-engineer the use of XML namespace from DTD. If you compile DTDs that use those techniques, you'd either manuallly modify the generated code, or you can try a tool like Trang that can convert DTD into XML Schema in ways that better preserves XML namespaces.

#### 8.2.1. Customizations

The customization syntax for DTD is roughly based on the ver.0.21 working draft of the JAXB specification, which is available at xml.coverpages.org. The deviations from this document are:

- The `whitespace` attribute of the `conversion` element takes " `preserve`", " `replace`", and " `collapse`" instead of " `preserve`", " `normalize`", and " `collapse`" as specified in the document.

- The `interface` customization just generates marker interfaces with no method.

#### 8.2.2. Compiling DTD from Maven

Example 95. The following POM snippest describes how to invoke XJC to compile DTD from a Maven project:

    <plugin>
      <groupId>org.jvnet.jaxb2.maven2</groupId>
      <artifactId>maven-jaxb2-plugin</artifactId>
      <executions>
        <execution>
          <goals>
            <goal>generate</goal>
          </goals>
          <configuration>
            <!--  if you want to put DTD somewhere else
            <schemaDirectory>src/main/jaxb</schemaDirectory>
            -->
            <extension>true</extension>
            <schemaLanguage>DTD</schemaLanguage>
            <schemaIncludes>
              <schemaInclude>*.dtd</schemaInclude>
            </schemaIncludes>
            <bindingIncludes>
              <bindingInclude>*.jaxb</bindingInclude>
            </bindingIncludes>
            <args>
              <arg>-Xinject-listener-code</arg>
            </args>
          </configuration>
        </execution>
      </executions>
      <dependencies>
        <dependency>
          <groupId>org.jvnet.jaxb2-commons</groupId>
          <artifactId>property-listener-injector</artifactId>
          <version>1.0</version>
        </dependency>
      </dependencies>
    </plugin>

Example 96. The dependencies section inside the plugin element can be used to specify additional XJC plugins. If you'd like to use more recent version of the Eclipse Implementation of JAXB, you can specify a dependency to XJC here to do so, like this:

    <dependency>
      <groupId>com.sun.xml.bind</groupId>
      <artifactId>jaxb-xjc</artifactId>
      <version>4.0.5</version>
    </dependency>

### 8.3. Designing a client/server protocol in XML

Occasionally, people try to define a custom protocol that allows multiple XML requests/responses to be sent over a single transport channel. This section discusses the non-trivial interaction between XML and sockets, and how you can design a protocol correctly.

XML1.0 requires a conforming parser to read the entire data till end of the stream (because a parser needs to handle documents like `<root/><!-- post root comment -->`). As a result, a naive attempt to keep one `OutputStream` open and marshal objects multiple times fails.

Example 97. One easy way to work around this limitation is to design your protocol so that the data on the wire will look like the following:

    <conversation>
      <!-- message 1 -->
      <message>
        ...
      </message>

      <!-- message 2 -->
      <message>
        ...
      </message>

      ...
    </conversation>

The `<conversation>` start tag is sent immediately after the socket is opened. This works as a container to send multiple "messages", and this is also an excellent opportunity to do the hand-shaking (e.g., `protocol-version='1.0'` attribute.) Once the `<conversation>` tag is written, multiple messages can be marshalled as a tree into the channel, possibly with a large time lag in between. You can use the Jakarta XML Binding marshaller to produce such message. When the sender wants to disconnect the channel, it can do so by sending the `</conversation>` end tag, followed by the socket disconnection.

Of course, you can choose any tag names freely, and each message can have different tag names.

The receiver would use the StAX API and use `XMLStreamReader` to read this stream. You'd have to use this to process the first `<conversation>` start tag. After that, every time you call a Jakarta XML Binding unmarshaller, you'll get the next message.

For the concrete code, see the `xml-channel` example in the Eclipse Implementation of JAXB distribution.

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