org.apache.commons.configuration.interpol
Class ConstantLookup

java.lang.Object
  org.apache.commons.lang.text.StrLookup
      org.apache.commons.configuration.interpol.ConstantLookup

public class ConstantLookup

extends StrLookup

A specialized lookup implementation that allows access to constant fields of classes.

Sometimes it is necessary in a configuration file to refer to a constant defined in a class. This can be done with this lookup implementation. Variable names passed in must be of the form mypackage.MyClass.FIELD. The lookup() method will split the passed in string at the last dot, separating the fully qualified class name and the name of the constant (i.e. static final) member field. Then the class is loaded and the field's value is obtained using reflection.

Once retrieved values are cached for fast access. This class is thread-safe. It can be used as a standard (i.e. global) lookup object and serve multiple clients concurrently.

Since:

1.4

Version:

$Id: ConstantLookup.java 588329 2007-10-25 20:01:31Z oheger $

Author:

Commons Configuration team

Constructor Summary

ConstantLookup()

Method Summary

static void	clear() Clears the shared cache with the so far resolved constants.
protected Class	fetchClass(String className) Loads the class with the specified name.
String	lookup(String var) Tries to resolve the specified variable.
protected Object	resolveField(String className, String fieldName) Determines the value of the specified constant member field of a class.

Methods inherited from class org.apache.commons.lang.text.StrLookup

mapLookup, noneLookup, systemPropertiesLookup

Methods inherited from class java.lang.Object

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

Constructor Detail

ConstantLookup

public ConstantLookup()

Method Detail

lookup

public String lookup(String var)

Tries to resolve the specified variable. The passed in variable name is interpreted as the name of a static final member field of a class. If the value has already been obtained, it can be retrieved from an internal cache. Otherwise this method will invoke the resolveField() method and pass in the name of the class and the field.

Parameters:

var - the name of the variable to be resolved

Returns:

the value of this variable or null if it cannot be resolved

clear

public static void clear()

Clears the shared cache with the so far resolved constants.

resolveField

protected Object resolveField(String className,
                              String fieldName)
                       throws Exception

Determines the value of the specified constant member field of a class. This implementation will call fetchClass() to obtain the java.lang.Class object for the target class. Then it will use reflection to obtain the field's value. For this to work the field must be accessable.

Parameters:

className - the name of the class

fieldName - the name of the member field of that class to read

Returns:

the field's value

Throws:

Exception - if an error occurs

fetchClass

protected Class fetchClass(String className)
                    throws ClassNotFoundException

Loads the class with the specified name. If an application has special needs regarding the class loaders to be used, it can hook in here. This implementation delegates to the getClass() method of Commons Lang's ClassUtils.

Parameters:

className - the name of the class to be loaded

Returns:

the corresponding class object

Throws:

ClassNotFoundException - if the class cannot be loaded