http://www.javassist.org/: Javassist (JAVA programming ASSISTant) makes Java bytecode manipulation simple. It is a class library for editing bytecodes in Java. (Shigeru Chiba, www.javassist.org) 
/*
 * Javassist, a Java-bytecode translator toolkit.
 * Copyright (C) 1999- Shigeru Chiba. All Rights Reserved.
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License.  Alternatively, the contents of this file may be used under
 * the terms of the GNU Lesser General Public License Version 2.1 or later,
 * or the Apache License Version 2.0.
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 */

package javassist.tools;

import java.util.HashMap;
import java.util.Map;
import java.util.UUID;

import javassist.CannotCompileException;
import javassist.CtBehavior;


Creates bytecode that when executed calls back to the instance's result method.

Example of how to create and insert a callback:


ctMethod.insertAfter(new Callback("Thread.currentThread()") {
    public void result(Object[] objects) {
        Thread thread = (Thread) objects[0];
        // do something with thread...
    }
}.sourceCode());



Contains utility methods for inserts callbacks in CtBehaviour, example:


insertAfter(ctBehaviour, new Callback("Thread.currentThread(), dummyString") {
    public void result(Object[] objects) {
        Thread thread = (Thread) objects[0];
        // do something with thread...
    }
});



Author:Marten Hedborg, Shigeru Chiba
/**
 * Creates bytecode that when executed calls back to the instance's result method.
 *
 * <p>Example of how to create and insert a callback:</p>
 * <pre>
 * ctMethod.insertAfter(new Callback("Thread.currentThread()") {
 *     public void result(Object[] objects) {
 *         Thread thread = (Thread) objects[0];
 *         // do something with thread...
 *     }
 * }.sourceCode());
 * </pre>
 * <p>Contains utility methods for inserts callbacks in <code>CtBehaviour</code>, example:</p>
 * <pre>
 * insertAfter(ctBehaviour, new Callback("Thread.currentThread(), dummyString") {
 *     public void result(Object[] objects) {
 *         Thread thread = (Thread) objects[0];
 *         // do something with thread...
 *     }
 * });
 * </pre>
 *
 * @author Marten Hedborg
 * @author Shigeru Chiba
 */
public abstract class Callback {

    public static Map<String,Callback> callbacks = new HashMap<String,Callback>();

    private final String sourceCode;

    
Constructs a new Callback object.

Params:
  • src –       The source code representing the inserted callback bytecode.
                 Can be one or many single statements each returning one object.
                 If many single statements are used they must be comma separated.
/**
     * Constructs a new <code>Callback</code> object.
     *
     * @param src       The source code representing the inserted callback bytecode.
     *                  Can be one or many single statements each returning one object.
     *                  If many single statements are used they must be comma separated.
     */
    public Callback(String src){
        String uuid = UUID.randomUUID().toString();
        callbacks.put(uuid, this);
        sourceCode = "((javassist.tools.Callback) javassist.tools.Callback.callbacks.get(\""+uuid+"\")).result(new Object[]{"+src+"});";
    }

    
Gets called when bytecode is executed

Params:
  • objects –   Objects that the bytecode in callback returns
/**
     * Gets called when bytecode is executed
     *
     * @param objects   Objects that the bytecode in callback returns
     */
    public abstract void result(Object[] objects);

    @Override
    public String toString(){
        return sourceCode();
    }

    public String sourceCode(){
        return sourceCode;
    }

    
Utility method to insert callback at the beginning of the body.

Params:
  • callback –  The callback
See Also:
/**
     * Utility method to insert callback at the beginning of the body.
     *
     * @param callback  The callback
     *
     * @see CtBehavior#insertBefore(String)
     */
    public static void insertBefore(CtBehavior behavior, Callback callback)
            throws CannotCompileException
    {
        behavior.insertBefore(callback.toString());
    }

    
Utility method to inserts callback at the end of the body.
The callback is inserted just before every return instruction.
It is not executed when an exception is thrown.

Params:
  • behavior –  The behaviour to insert callback in
  • callback –  The callback
See Also:
/**
     * Utility method to inserts callback at the end of the body.
     * The callback is inserted just before every return instruction.
     * It is not executed when an exception is thrown.
     *
     * @param behavior  The behaviour to insert callback in
     * @param callback  The callback
     *
     * @see CtBehavior#insertAfter(String, boolean)
     */
    public static void insertAfter(CtBehavior behavior,Callback callback)
            throws CannotCompileException
    {
        behavior.insertAfter(callback.toString(), false);
    }

    
Utility method to inserts callback at the end of the body.
The callback is inserted just before every return instruction.
It is not executed when an exception is thrown.

Params:
  • behavior –  The behaviour to insert callback in
  • callback –  The callback representing the inserted.
  • asFinally – True if the inserted is executed
                 Not only when the control normally returns
                 but also when an exception is thrown.
                 If this parameter is true, the inserted code cannot
                 access local variables.
See Also:
/**
     * Utility method to inserts callback at the end of the body.
     * The callback is inserted just before every return instruction.
     * It is not executed when an exception is thrown.
     *
     * @param behavior  The behaviour to insert callback in
     * @param callback  The callback representing the inserted.
     * @param asFinally True if the inserted is executed
     *                  Not only when the control normally returns
     *                  but also when an exception is thrown.
     *                  If this parameter is true, the inserted code cannot
     *                  access local variables.
     *
     * @see CtBehavior#insertAfter(String, boolean)
     */
    public static void insertAfter(CtBehavior behavior, Callback callback, boolean asFinally)
            throws CannotCompileException
    {
        behavior.insertAfter(callback.toString(), asFinally);
    }

    
Utility method to inserts callback at the specified line in the body.

Params:
  • behavior –  The behaviour to insert callback in
  • callback –  The callback representing.
  • lineNum –   The line number.  The callback is inserted at the
                 beginning of the code at the line specified by this
                 line number.
See Also:
Returns:     The line number at which the callback has been inserted.
/**
     * Utility method to inserts callback at the specified line in the body.
     *
     * @param behavior  The behaviour to insert callback in
     * @param callback  The callback representing.
     * @param lineNum   The line number.  The callback is inserted at the
     *                  beginning of the code at the line specified by this
     *                  line number.
     *
     * @return      The line number at which the callback has been inserted.
     *
     * @see CtBehavior#insertAt(int, String)
     */
    public static int insertAt(CtBehavior behavior, Callback callback, int lineNum)
            throws CannotCompileException
    {
        return behavior.insertAt(lineNum, callback.toString());
    }
}