Class ConstantLookup

  • 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. 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
    • Constructor Summary

      Constructors 
      Constructor Description
      ConstantLookup()  
    • 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.
      • 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 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 the resolveField() method and pass in the name of the class and the field.
        Specified by:
        lookup in interface Lookup
        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 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 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:
        java.lang.Exception - if an error occurs
      • 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 the getClass() method of Commons Lang's ClassUtils.
        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