https://github.com/cglib/cglib/cglib: cglib is a powerful, high performance and quality Code Generation Library, It is used to extend JAVA classes and implements interfaces at runtime. 
/*
 * Copyright 2003,2004 The Apache Software Foundation
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package net.sf.cglib.proxy;

import java.lang.reflect.Method;


Map methods of subclasses generated by Enhancer to a particular callback. The type of the callbacks chosen for each method affects the bytecode generated for that method in the subclass, and cannot change for the life of the class. 
Note: CallbackFilter implementations are supposed to be lightweight as cglib might keep CallbackFilter objects alive to enable caching of generated classes. Prefer using static classes for implementation of CallbackFilter.


/**
 * Map methods of subclasses generated by {@link Enhancer} to a particular
 * callback. The type of the callbacks chosen for each method affects
 * the bytecode generated for that method in the subclass, and cannot
 * change for the life of the class.
 * <p>Note: {@link CallbackFilter} implementations are supposed to be
 * lightweight as cglib might keep {@link CallbackFilter} objects
 * alive to enable caching of generated classes. Prefer using {@code static}
 * classes for implementation of {@link CallbackFilter}.</p>
 */
public interface CallbackFilter {
    
Map a method to a callback.

Params:
  • method – the intercepted method
Returns:the index into the array of callbacks (as specified by Enhancer.setCallbacks) to use for the method,
/**
     * Map a method to a callback.
     * @param method the intercepted method
     * @return the index into the array of callbacks (as specified by {@link Enhancer#setCallbacks}) to use for the method, 
     */
    int accept(Method method);

    
The CallbackFilter in use affects which cached class
the Enhancer will use, so this is a reminder that
you should correctly implement equals and
hashCode for custom CallbackFilter
implementations in order to improve performance.

/**
     * The <code>CallbackFilter</code> in use affects which cached class
     * the <code>Enhancer</code> will use, so this is a reminder that
     * you should correctly implement <code>equals</code> and
     * <code>hashCode</code> for custom <code>CallbackFilter</code>
     * implementations in order to improve performance.
    */
    boolean equals(Object o);
}