Class ConstantStringLookup

  • All Implemented Interfaces:
    StringLookup

    class ConstantStringLookup
    extends AbstractStringLookup

    Looks up the value of a fully-qualified static final value.

    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 must be in the format apackage.AClass.AFIELD. The lookup(String) 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.

    Using a StringLookup from the StringLookupFactory:

     StringLookupFactory.INSTANCE.constantStringLookup().lookup("java.awt.event.KeyEvent.VK_ESCAPE");
     

    Using a StringSubstitutor:

     StringSubstitutor.createInterpolator().replace("... ${const:java.awt.event.KeyEvent.VK_ESCAPE} ..."));
     

    The above examples convert java.awt.event.KeyEvent.VK_ESCAPE to "27".

    This class was adapted from Apache Commons Configuration.

    Since:
    1.5
    • Field Summary

      Fields 
      Modifier and Type Field Description
      private static java.util.concurrent.ConcurrentHashMap<java.lang.String,​java.lang.String> CONSTANT_CACHE
      An internally used cache for already retrieved values.
      private static char FIELD_SEPARATOR
      Constant for the field separator.
      (package private) static ConstantStringLookup INSTANCE
      Defines the singleton for this class.
    • Method Summary

      All Methods Static Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      (package private) 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.String lookup​(java.lang.String key)
      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
    • Field Detail

      • CONSTANT_CACHE

        private static final java.util.concurrent.ConcurrentHashMap<java.lang.String,​java.lang.String> CONSTANT_CACHE
        An internally used cache for already retrieved values.
      • FIELD_SEPARATOR

        private static final char FIELD_SEPARATOR
        Constant for the field separator.
        See Also:
        Constant Field Values
    • Constructor Detail

      • ConstantStringLookup

        ConstantStringLookup()
    • Method Detail

      • clear

        static void clear()
        Clears the shared cache with the so far resolved constants.
      • 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
      • lookup

        public java.lang.String lookup​(java.lang.String key)
        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:
        key - the name of the variable to be resolved
        Returns:
        The value of this variable or null if it cannot be resolved
      • 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