Package java.lang

Class ClassLoader

java.lang.Object
java.lang.ClassLoader
Direct Known Subclasses:
SecureClassLoader
public abstract class ClassLoader
extends Object
Loads classes and resources from a repository. One or more class loaders are installed at runtime. These are consulted whenever the runtime system needs a specific class that is not yet available in-memory. Typically, class loaders are grouped into a tree where child class loaders delegate all requests to parent class loaders. Only if the parent class loader cannot satisfy the request, the child class loader itself tries to handle it.

ClassLoader is an abstract class that implements the common infrastructure required by all class loaders. Android provides several concrete implementations of the class, with dalvik.system.PathClassLoader being the one typically used. Other applications may implement subclasses of ClassLoader to provide special ways for loading classes.

See Also:
Class

  • Constructor Details

    • ClassLoader

      protected ClassLoader()
      Constructs a new instance of this class with the system class loader as its parent.

    • ClassLoader

      protected ClassLoader​(ClassLoader parentLoader)
      Constructs a new instance of this class with the specified class loader as its parent.
      Parameters:
      parentLoader - The ClassLoader to use as the new class loader's parent.

  • Method Details

    • getSystemClassLoader

      public static ClassLoader getSystemClassLoader()
      Returns the system class loader. This is the parent for new ClassLoader instances and is typically the class loader used to start the application.

    • getSystemResource

      public static URL getSystemResource​(String resName)
      Finds the URL of the resource with the specified name. The system class loader's resource lookup algorithm is used to find the resource.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      the URL object for the requested resource or null if the resource can not be found.
      See Also:
      Class.getResource(java.lang.String)

    • getSystemResources

      public static Enumeration<URL> getSystemResources​(String resName) throws IOException
      Returns an enumeration of URLs for the resource with the specified name. The system class loader's resource lookup algorithm is used to find the resource.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      an enumeration of URL objects containing the requested resources.
      Throws:
      IOException - if an I/O error occurs.

    • getSystemResourceAsStream

      public static InputStream getSystemResourceAsStream​(String resName)
      Returns a stream for the resource with the specified name. The system class loader's resource lookup algorithm is used to find the resource. Basically, the contents of the java.class.path are searched in order, looking for a path which matches the specified resource.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      a stream for the resource or null.
      See Also:
      Class.getResourceAsStream(java.lang.String)

    • defineClass

      @Deprecated protected final Class<?> defineClass​(byte[] classRep, int offset, int length) throws ClassFormatError
      Deprecated.
      Use defineClass(String, byte[], int, int) instead.
      Constructs a new class from an array of bytes containing a class definition in class file format.
      Parameters:
      classRep - the memory image of a class file.
      offset - the offset into classRep.
      length - the length of the class file.
      Returns:
      the Class object created from the specified subset of data in classRep.
      Throws:
      ClassFormatError - if classRep does not contain a valid class.
      IndexOutOfBoundsException - if offset < 0, length < 0 or if offset + length is greater than the length of classRep.

    • defineClass

      protected final Class<?> defineClass​(String className, byte[] classRep, int offset, int length) throws ClassFormatError
      Constructs a new class from an array of bytes containing a class definition in class file format.
      Parameters:
      className - the expected name of the new class, may be null if not known.
      classRep - the memory image of a class file.
      offset - the offset into classRep.
      length - the length of the class file.
      Returns:
      the Class object created from the specified subset of data in classRep.
      Throws:
      ClassFormatError - if classRep does not contain a valid class.
      IndexOutOfBoundsException - if offset < 0, length < 0 or if offset + length is greater than the length of classRep.

    • defineClass

      protected final Class<?> defineClass​(String className, byte[] classRep, int offset, int length, ProtectionDomain protectionDomain) throws ClassFormatError
      Constructs a new class from an array of bytes containing a class definition in class file format and assigns the specified protection domain to the new class. If the provided protection domain is null then a default protection domain is assigned to the class.
      Parameters:
      className - the expected name of the new class, may be null if not known.
      classRep - the memory image of a class file.
      offset - the offset into classRep.
      length - the length of the class file.
      protectionDomain - the protection domain to assign to the loaded class, may be null.
      Returns:
      the Class object created from the specified subset of data in classRep.
      Throws:
      ClassFormatError - if classRep does not contain a valid class.
      IndexOutOfBoundsException - if offset < 0, length < 0 or if offset + length is greater than the length of classRep.
      NoClassDefFoundError - if className is not equal to the name of the class contained in classRep.

    • defineClass

      protected final Class<?> defineClass​(String name, ByteBuffer b, ProtectionDomain protectionDomain) throws ClassFormatError
      Defines a new class with the specified name, byte code from the byte buffer and the optional protection domain. If the provided protection domain is null then a default protection domain is assigned to the class.
      Parameters:
      name - the expected name of the new class, may be null if not known.
      b - the byte buffer containing the byte code of the new class.
      protectionDomain - the protection domain to assign to the loaded class, may be null.
      Returns:
      the Class object created from the data in b.
      Throws:
      ClassFormatError - if b does not contain a valid class.
      NoClassDefFoundError - if className is not equal to the name of the class contained in b.

    • findClass

      protected Class<?> findClass​(String className) throws ClassNotFoundException
      Overridden by subclasses, throws a ClassNotFoundException by default. This method is called by loadClass after the parent ClassLoader has failed to find a loaded class of the same name.
      Parameters:
      className - the name of the class to look for.
      Returns:
      the Class object that is found.
      Throws:
      ClassNotFoundException - if the class cannot be found.

    • findLoadedClass

      protected final Class<?> findLoadedClass​(String className)
      Returns the class with the specified name if it has already been loaded by the VM or null if it has not yet been loaded.
      Parameters:
      className - the name of the class to look for.
      Returns:
      the Class object or null if the requested class has not been loaded.

    • findSystemClass

      protected final Class<?> findSystemClass​(String className) throws ClassNotFoundException
      Finds the class with the specified name, loading it using the system class loader if necessary.
      Parameters:
      className - the name of the class to look for.
      Returns:
      the Class object with the requested className.
      Throws:
      ClassNotFoundException - if the class can not be found.

    • getParent

      public final ClassLoader getParent()
      Returns this class loader's parent.
      Returns:
      this class loader's parent or null.

    • getResource

      public URL getResource​(String resName)
      Returns the URL of the resource with the specified name. This implementation first tries to use the parent class loader to find the resource; if this fails then findResource(String) is called to find the requested resource.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      the URL object for the requested resource or null if the resource can not be found
      See Also:
      Class.getResource(java.lang.String)

    • getResources

      public Enumeration<URL> getResources​(String resName) throws IOException
      Returns an enumeration of URLs for the resource with the specified name. This implementation first uses this class loader's parent to find the resource, then it calls findResources(String) to get additional URLs. The returned enumeration contains the URL objects of both find operations.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      an enumeration of URL objects for the requested resource.
      Throws:
      IOException - if an I/O error occurs.

    • getResourceAsStream

      public InputStream getResourceAsStream​(String resName)
      Returns a stream for the resource with the specified name. See getResource(String) for a description of the lookup algorithm used to find the resource.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      a stream for the resource or null if the resource can not be found
      See Also:
      Class.getResourceAsStream(java.lang.String)

    • loadClass

      public Class<?> loadClass​(String className) throws ClassNotFoundException
      Loads the class with the specified name. Invoking this method is equivalent to calling loadClass(className, false).

      Note: In the Android reference implementation, the second parameter of loadClass(String, boolean) is ignored anyway.

      Parameters:
      className - the name of the class to look for.
      Returns:
      the Class object.
      Throws:
      ClassNotFoundException - if the class can not be found.

    • loadClass

      protected Class<?> loadClass​(String className, boolean resolve) throws ClassNotFoundException
      Loads the class with the specified name, optionally linking it after loading. The following steps are performed:
      1. Call findLoadedClass(String) to determine if the requested class has already been loaded.
      2. If the class has not yet been loaded: Invoke this method on the parent class loader.
      3. If the class has still not been loaded: Call findClass(String) to find the class.

      Note: In the Android reference implementation, the resolve parameter is ignored; classes are never linked.

      Parameters:
      className - the name of the class to look for.
      resolve - Indicates if the class should be resolved after loading. This parameter is ignored on the Android reference implementation; classes are not resolved.
      Returns:
      the Class object.
      Throws:
      ClassNotFoundException - if the class can not be found.

    • resolveClass

      protected final void resolveClass​(Class<?> clazz)
      Forces a class to be linked (initialized). If the class has already been linked this operation has no effect.

      Note: In the Android reference implementation, this method has no effect.

      Parameters:
      clazz - the class to link.

    • findResource

      protected URL findResource​(String resName)
      Finds the URL of the resource with the specified name. This implementation just returns null; it should be overridden in subclasses.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      the URL object for the requested resource.

    • findResources

      protected Enumeration<URL> findResources​(String resName) throws IOException
      Finds an enumeration of URLs for the resource with the specified name. This implementation just returns an empty Enumeration; it should be overridden in subclasses.
      Parameters:
      resName - the name of the resource to find.
      Returns:
      an enumeration of URL objects for the requested resource.
      Throws:
      IOException - if an I/O error occurs.

    • findLibrary

      protected String findLibrary​(String libName)
      Returns the absolute path of the native library with the specified name, or null. If this method returns null then the virtual machine searches the directories specified by the system property "java.library.path".

      This implementation always returns null.

      Parameters:
      libName - the name of the library to find.
      Returns:
      the absolute path of the library.

    • getPackage

      protected Package getPackage​(String name)
      Returns the package with the specified name. Package information is searched in this class loader.
      Parameters:
      name - the name of the package to find.
      Returns:
      the package with the requested name; null if the package can not be found.

    • getPackages

      protected Package[] getPackages()
      Returns all the packages known to this class loader.
      Returns:
      an array with all packages known to this class loader.

    • definePackage

      protected Package definePackage​(String name, String specTitle, String specVersion, String specVendor, String implTitle, String implVersion, String implVendor, URL sealBase) throws IllegalArgumentException
      Defines and returns a new Package using the specified information. If sealBase is null, the package is left unsealed. Otherwise, the package is sealed using this URL.
      Parameters:
      name - the name of the package.
      specTitle - the title of the specification.
      specVersion - the version of the specification.
      specVendor - the vendor of the specification.
      implTitle - the implementation title.
      implVersion - the implementation version.
      implVendor - the specification vendor.
      sealBase - the URL used to seal this package or null to leave the package unsealed.
      Returns:
      the Package object that has been created.
      Throws:
      IllegalArgumentException - if a package with the specified name already exists.

    • setSigners

      protected final void setSigners​(Class<?> c, Object[] signers)
      Sets the signers of the specified class. This implementation does nothing.
      Parameters:
      c - the Class object for which to set the signers.
      signers - the signers for c.

    • setClassAssertionStatus

      public void setClassAssertionStatus​(String cname, boolean enable)
      Sets the assertion status of the class with the specified name.

      Note: This method does nothing in the Android reference implementation.

      Parameters:
      cname - the name of the class for which to set the assertion status.
      enable - the new assertion status.

    • setPackageAssertionStatus

      public void setPackageAssertionStatus​(String pname, boolean enable)
      Sets the assertion status of the package with the specified name.

      Note: This method does nothing in the Android reference implementation.

      Parameters:
      pname - the name of the package for which to set the assertion status.
      enable - the new assertion status.

    • setDefaultAssertionStatus

      public void setDefaultAssertionStatus​(boolean enable)
      Sets the default assertion status for this class loader.

      Note: This method does nothing in the Android reference implementation.

      Parameters:
      enable - the new assertion status.

    • clearAssertionStatus

      public void clearAssertionStatus()
      Sets the default assertion status for this class loader to false and removes any package default and class assertion status settings.

      Note: This method does nothing in the Android reference implementation.