6.2.2、CGLib 动态代理

CGLib 全称为 Code Generation Library，是一个强大的高性能，高质量的代码生成类库，可以在运行期扩展 Java 类与实现 Java 接口，CGLib 封装了 asm，可以再运行期动态生成新的 class。和 JDK 动态代理相比较：JDK 创建代理有一个限制，就是只能为接口创建代理实例，而对于没有通过接口定义业务方法的类，则可以通过 CGLib 创建动态代理。

CGLib 代理示例

定义一个代理对象：

public class HelloService {

    public HelloService() {
        System.out.println("HelloService构造");
    }

    /**
     * 该方法不能被子类覆盖,Cglib是无法代理final修饰的方法的
     */
    final public String sayOthers(String name) {
        System.out.println("HelloService:sayOthers>>" + name);
        return null;
    }

    public void sayHello() {
        System.out.println("HelloService:sayHello");
    }
}

自定义MethodInterceptor：

import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

import java.lang.reflect.Method;

/**
 * 自定义MethodInterceptor
 */
public class MyMethodInterceptor implements MethodInterceptor{

    /**
     * sub：cglib生成的代理对象
     * method：被代理对象方法
     * objects：方法入参
     * methodProxy: 代理方法
     */
    @Override
    public Object intercept(Object sub, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
        System.out.println("======插入前置通知======");
        Object object = methodProxy.invokeSuper(sub, objects);
        System.out.println("======插入后者通知======");
        return object;
    }
}

生成CGLIB代理对象调用目标方法：

import net.sf.cglib.core.DebuggingClassWriter;
import net.sf.cglib.proxy.Enhancer;

public class CglibTest {

    public static void main(String[] args) {
        // 代理类class文件存入本地磁盘方便我们反编译查看源码
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "D:\\code");
        // 通过CGLIB动态代理获取代理对象的过程
        Enhancer enhancer = new Enhancer();
        // 设置enhancer对象的父类
        enhancer.setSuperclass(HelloService.class);
        // 设置enhancer的回调对象
        enhancer.setCallback(new MyMethodInterceptor());
        // 创建代理对象
        HelloService proxy= (HelloService)enhancer.create();
        // 通过代理对象调用目标方法
        proxy.sayHello();
    }

}

运行结果:

CGLib 代理原理

实现CGLIB动态代理必须实现 MethodInterceptor (方法拦截器)接口，源码如下:

/*
 * Copyright 2002,2003 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;

/**
 * General-purpose {@link Enhancer} callback which provides for "around advice".
 * @author Juozas Baliuka <a href="mailto:baliuka@mwm.lt">baliuka@mwm.lt</a>
 * @version $Id: MethodInterceptor.java,v 1.8 2004/06/24 21:15:20 herbyderby Exp $
 */
public interface MethodInterceptor
extends Callback
{
    /**
     * All generated proxied methods call this method instead of the original method.
     * The original method may either be invoked by normal reflection using the Method object,
     * or by using the MethodProxy (faster).
     * @param obj "this", the enhanced object
     * @param method intercepted Method
     * @param args argument array; primitive types are wrapped
     * @param proxy used to invoke super (non-intercepted method); may be called
     * as many times as needed
     * @throws Throwable any exception may be thrown; if so, super method will not be invoked
     * @return any value compatible with the signature of the proxied method. Method returning void will ignore this value.
     * @see MethodProxy
     */    
    public Object intercept(Object obj, java.lang.reflect.Method method, Object[] args,
                               MethodProxy proxy) throws Throwable;

}

这个接口只有一个intercept()方法，这个方法有4个参数：

1）obj表示增强的对象，即实现这个接口类的一个对象；

2）method表示要被拦截的方法；

3）args表示要被拦截方法的参数；

4）proxy表示要触发父类的方法对象；

在上面的Client代码中，通过 Enhancer.create( )方法创建代理对象，create()方法的源码：

    /**
     * Generate a new class if necessary and uses the specified
     * callbacks (if any) to create a new object instance.
     * Uses the no-arg constructor of the superclass.
     * @return a new instance
     */
    public Object create() {
        classOnly = false;
        argumentTypes = null;
        return createHelper();
    }

该方法含义就是如果有必要就创建一个新类，并且用指定的回调对象创建一个新的对象实例，

使用的父类的参数的构造方法来实例化父类的部分。核心内容在createHelper()中，源码如下:

    private Object createHelper() {
        validate();
        if (superclass != null) {
            setNamePrefix(superclass.getName());
        } else if (interfaces != null) {
            setNamePrefix(interfaces[ReflectUtils.findPackageProtected(interfaces)].getName());
        }
        return super.create(KEY_FACTORY.newInstance((superclass != null) ? superclass.getName() : null,
                                                    ReflectUtils.getNames(interfaces),
                                                    filter,
                                                    callbackTypes,
                                                    useFactory,
                                                    interceptDuringConstruction,
                                                    serialVersionUID));
    }

preValidate()方法校验callbackTypes、filter是否为空，以及为空时的处理。

通过newInstance()方法创建EnhancerKey对象，作为Enhancer父类AbstractClassGenerator.create()方法创建代理对象的参数。

   protected Object create(Object key) {
        try {
            Class gen = null;

            synchronized (source) {
                ClassLoader loader = getClassLoader();
                Map cache2 = null;
                cache2 = (Map)source.cache.get(loader);
                if (cache2 == null) {
                    cache2 = new HashMap();
                    cache2.put(NAME_KEY, new HashSet());
                    source.cache.put(loader, cache2);
                } else if (useCache) {
                    Reference ref = (Reference)cache2.get(key);
                    gen = (Class) (( ref == null ) ? null : ref.get()); 
                }
                if (gen == null) {
                    Object save = CURRENT.get();
                    CURRENT.set(this);
                    try {
                        this.key = key;

                        if (attemptLoad) {
                            try {
                                gen = loader.loadClass(getClassName());
                            } catch (ClassNotFoundException e) {
                                // ignore
                            }
                        }
                        if (gen == null) {
                            byte[] b = strategy.generate(this);
                            String className = ClassNameReader.getClassName(new ClassReader(b));
                            getClassNameCache(loader).add(className);
                            gen = ReflectUtils.defineClass(className, b, loader);
                        }

                        if (useCache) {
                            cache2.put(key, new WeakReference(gen));
                        }
                        return firstInstance(gen);
                    } finally {
                        CURRENT.set(save);
                    }
                }
            }
            return firstInstance(gen);
        } catch (RuntimeException e) {
            throw e;
        } catch (Error e) {
            throw e;
        } catch (Exception e) {
            throw new CodeGenerationException(e);
        }
    }

真正创建代理对象方法在nextInstance()方法中，该方法为抽象类AbstractClassGenerator的一个方法，签名如下：

abstract protected Object nextInstance(Object instance) throws Exception;

在子类Enhancer中实现，实现源码如下：