org.anc.constants

Class Constants

java.lang.Object

org.anc.constants.Constants

public abstract class Constants
extends Object

The base class used for declaring project wide constant values.

Each sub-class defines public final fields that will be initialized with values from a properties file. The default values to use if the properties file does not exist are specified with @Default annotations on each field.

 package org.anc.example;
 public class MyConstants extends Constants
 {
    @Default("Hello world.")
    public final String HELLO_WORLD = null;
    @Default("8")
    public final Integer NTHREADS = null;
    
    public MyConstants()
    {
       super.init();
    }
 }
 

Each public field in the subclass must be initialized to null and the subclass constructor(s) must call super.init(). The Constants subclass can contain fields of type String, Boolean, Integer, Float, and Double. However, the value specified by the @Default annotation is always a String.

An instance of the subclass can then be created (usually as a static final field of the application object) to access the defined constants.

 public class Application
 {
    public static final MyConstants CONST = new MyConstants();
    
    public void run()
    {
       System.out.println(CONST.HELLO_WORLD);
    }
 }
 

The Constants class will look for the properties file conf/machineName/class.name.properties where:

• machineName is the value of the environment variable HOSTNAME (Unix based OSes) or COMPUTERNAME (Windows)
• class.name is the fully qualified Java class name.

The Constants class will first look for the properties file on the file system and then on the class path. If a properties file can not be found the fields will be initialized with the values from the @Default annotations.

A sub-class can also specify the properties file to use with the init(String) method. The String parameter passed to the init method should be the name of a Java system property or an OS environmental variable. The Constants class will first try System.getProperty and then System.getenv to obtain a file name. If neither property has been set the above method is used to locate the properties file. For example,

 // In MyConstants.java
 package org.anc.example;
 public class MyConstants extends Constants
    {
       @Default("Hello world")
       public final String HELLO_WORLD = null;
       
       public MyConstants()
       {
          super.init("org.anc.hello");
       }
       
       public static void main(String[] args)
       {
          MyConstants constants = new MyConstants();
          System.out.println(constants.HELLO_WORLD);
       }
    }
 
 # In /home/anc/hello.properties
 HELLO_WORLD=Bonjour le monde.
 
 # From the command line:
 > java -cp MyConstants.jar -Dorg.anc.hello=/home/anc/hello.properties org.anc.example.MyConstants
 > Bonjour le monde
 

A properties file containing the default values can be generated by creating an instance of the class and calling the save() method. This is convenient when you want to create properties files for use with other machines.

 public static void main(String[] args)
 {
    MyConstants constants = new MyConstants();
    constants.save();
 }
 

Author:

Keith Suderman

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	Constants.Default Annotation used to provide a default value for constants.

Constructor Summary

Constructors

Constructor and Description
Constants()

Method Summary

Modifier and Type	Method and Description
protected String	getName()
protected Properties	getProperties(String propName)
protected void	init()
protected void	init(String propertyName)
protected static boolean	isPublicFinalBoolean(Field field)
protected static boolean	isPublicFinalDouble(Field field)
protected static boolean	isPublicFinalFloat(Field field)
protected static boolean	isPublicFinalInteger(Field field)
protected static boolean	isPublicFinalString(Field field)
void	save()
void	save(File file)
void	save(String path)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Constants

public Constants()

Method Detail

save

public void save()
          throws IOException

Throws:

IOException

save

public void save(String path)
          throws IOException

Throws:

IOException

save

public void save(File file)
          throws IOException

Throws:

IOException

getProperties

protected Properties getProperties(String propName)
                            throws FileNotFoundException,
                                   IOException

Throws:

FileNotFoundException

IOException

getName

protected String getName()

init

protected void init()

init

protected void init(String propertyName)

isPublicFinalString

protected static boolean isPublicFinalString(Field field)

isPublicFinalInteger

protected static boolean isPublicFinalInteger(Field field)

isPublicFinalDouble

protected static boolean isPublicFinalDouble(Field field)

isPublicFinalFloat

protected static boolean isPublicFinalFloat(Field field)

isPublicFinalBoolean

protected static boolean isPublicFinalBoolean(Field field)