libjava/classpath/javax/management/openmbean/CompositeDataInvocationHandler.java

/* CompositeDataInvocationHandler.java - Pseudo-accessors for CompositeData.
   Copyright (C) 2007 Free Software Foundation

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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02110-1301 USA.

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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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package javax.management.openmbean;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

/**
 * <p>
 * Provides an {@link java.lang.reflect.InvocationHandler} which
 * implements a series of accessor methods (those beginning with
 * {@code "get"} or {@code "is"}) using the content of a
 * {@link CompositeData} object.  An instance of {@link CompositeData}
 * consists of a series of key-value mappings.  This handler assumes
 * these keys to be the names of attributes, and thus provides
 * accessor methods by returning the associated value.
 * </p>
 * <p>
 * As an example, consider the following interface:
 * </p>
 * <pre>
 * public interface Person
 * {
 *   public String getName();
 *   public Date getBirthday();
 * }
 * </pre>
 * <p>
 * This specifies two accessor methods for retrieving the attributes,
 * {@code name} and {@code birthday}.  An implementation of this interface
 * can be provided by creating an instance of this class, using a
 * {@link CompositeData} object with appropriate key-value mappings
 * (e.g. "name" => "Fred", "birthday" => 30/06/1974), and then passing
 * that to {@link java.lang.reflect.Proxy#newProxyInstance} along with
 * the interface itself.  The invocation handler implements the methods
 * by calling {@link CompositeData#get(String)} with the appropriate key.
 * </p>
 * <p>
 * The attribute name is derived by taking the remainder of the method
 * name following {@code "get"}.  If the first letter of this substring
 * is uppercase, then two attempts are made to retrieve the attribute
 * from the {@link CompositeData} instance: one using the original substring,
 * and one with the first letter changed to its lower-case equivalent.
 * If the first letter is lowercase, only the exact substring is used.
 * </p>
 * <p>
 * An {@link Object#equals(Object)} implementation is provided.  This returns
 * true if the argument is a proxy with a {@link CompositeDataInvocationHandler}
 * using an equivalent {@link CompositeData} instance.  {@link Object#hashCode()}
 * is also defined so as to match this implementation and give equivalent instances
 * the same hashcode.
 * </p>
 *
 * @author Andrew John Hughes (gnu_andrew@member.fsf.org)
 * @since 1.6
 */
public class CompositeDataInvocationHandler
  implements InvocationHandler
{

  /**
   * The {@link CompositeData} providing the key-value mappings.
   */
  private CompositeData data;

  /**
   * Constructs a new {@link CompositeDataInvocationHandler}
   * with the specified {@link CompositeData} instance.
   *
   * @param data the {@link CompositeData} instance to use.
   * @throws IllegalArgumentException if {@code data} is {@code null}.
   */
  public CompositeDataInvocationHandler(CompositeData data)
  {
    if (data == null)
      throw new IllegalArgumentException("The CompositeData instance " +
                                         "must be non-null.");
    this.data = data;
  }

  /**
   * Returns the {@link CompositeData} instance which provides
   * the key-value mappings for this instance.  This is never
   * {@code null}.
   *
   * @return the {@link CompositeData} instance.
   */
  public CompositeData getCompositeData()
  {
    return data;
  }

  /**
   * Called by the proxy class whenever a method is called.  The
   * handler only deals with accessor methods (beginning with
   * {@code "get"} or {@code "is"}), {@code equals}, and
   * {@code "hashCode"}.  Accessor methods are implemented by
   * returning the appropriate value from the {@link CompositeData}
   * instance, while {@code equals} and {@code hashCode} allow
   * two proxies with a {@link CompositeDataInvocationHandler} using
   * the same {@link CompositeData} instance to be classified
   * as equivalent.
   *
   * @param proxy the proxy on which the method was called.
   * @param method the method which was called.
   * @param args the arguments supplied to the method.
   * @return the return value from the method.
   * @throws Throwable if an exception is thrown in the process.
   */
  public Object invoke(Object proxy, Method method, Object[] args)
    throws Throwable
  {
    String mName = method.getName();
    if (mName.equals("equals"))
      {
        if (args[0] instanceof Proxy)
          {
            InvocationHandler h = Proxy.getInvocationHandler(args[0]);
            if (h instanceof CompositeDataInvocationHandler)
              return data.equals(((CompositeDataInvocationHandler)
                                  h).getCompositeData());
          }
        return false;
      }
    if (mName.equals("hashCode"))
      {
        return data.hashCode();
      }
    String attrib = null;
    if (mName.startsWith("get"))
      attrib = mName.substring(3);
    else if (mName.startsWith("is"))
      attrib = mName.substring(2);
    if (attrib == null)
      throw new NoSuchMethodException(mName + " is not an accessor.");
    if (!data.containsKey(attrib))
      {
        if (Character.isLowerCase(attrib.charAt(0)))
          throw new NoSuchMethodException("The attribute " +
                                          attrib + " is not available " +
                                          "in the given CompositeData " +
                                          "object");
        attrib = Character.toLowerCase(attrib.charAt(0))
          + attrib.substring(1);
        if (!data.containsKey(attrib))
          throw new NoSuchMethodException("The attribute " +
                                          attrib + " is not available " +
                                          "in the given CompositeData " +
                                          "object");
      }
    return data.get(attrib);
  }

}