Class ConstantLookup
- java.lang.Object
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- org.apache.commons.configuration2.interpol.ConstantLookup
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- All Implemented Interfaces:
Lookup
public class ConstantLookup extends java.lang.Object implements Lookup
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
. Thelookup()
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 1790899 2017-04-10 21:56:46Z ggregory $
- Author:
- Commons Configuration team
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Constructor Summary
Constructors Constructor Description ConstantLookup()
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description static void
clear()
Clears the shared cache with the so far resolved constants.protected java.lang.Class<?>
fetchClass(java.lang.String className)
Loads the class with the specified name.java.lang.Object
lookup(java.lang.String var)
Tries to resolve the specified variable.protected java.lang.Object
resolveField(java.lang.String className, java.lang.String fieldName)
Determines the value of the specified constant member field of a class.
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Method Detail
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lookup
public java.lang.Object lookup(java.lang.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 theresolveField()
method and pass in the name of the class and the field.
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clear
public static void clear()
Clears the shared cache with the so far resolved constants.
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resolveField
protected java.lang.Object resolveField(java.lang.String className, java.lang.String fieldName) throws java.lang.Exception
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:
java.lang.Exception
- if an error occurs
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fetchClass
protected java.lang.Class<?> fetchClass(java.lang.String className) throws java.lang.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 thegetClass()
method of Commons Lang'sClassUtils
.- Parameters:
className
- the name of the class to be loaded- Returns:
- the corresponding class object
- Throws:
java.lang.ClassNotFoundException
- if the class cannot be loaded
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