Class ConstantLookup
java.lang.Object
org.apache.commons.lang.text.StrLookup
org.apache.commons.configuration.interpol.ConstantLookup
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 1210218 2011-12-04 20:57:48Z oheger $
- Author:
- Commons Configuration team
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Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionstatic void
clear()
Clears the shared cache with the so far resolved constants.protected Class<?>
fetchClass
(String className) Loads the class with the specified name.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
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Constructor Details
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ConstantLookup
public ConstantLookup()
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Method Details
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lookup
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 theresolveField()
method and pass in the name of the class and the field.- Specified by:
lookup
in classorg.apache.commons.lang.text.StrLookup
- Parameters:
var
- the name of the variable to be resolved- Returns:
- the value of this variable or null if it cannot be resolved
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clear
Clears the shared cache with the so far resolved constants. -
resolveField
Determines the value of the specified constant member field of a class. This implementation will callfetchClass()
to obtain thejava.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 classfieldName
- the name of the member field of that class to read- Returns:
- the field's value
- Throws:
Exception
- if an error occurs
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fetchClass
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 thegetClass()
method of Commons Lang'sClassUtils
.- Parameters:
className
- the name of the class to be loaded- Returns:
- the corresponding class object
- Throws:
ClassNotFoundException
- if the class cannot be loaded
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