CGLIB动态代理实现原理

一 CGLIB介绍

CGLIB(Code Generation Library)是一个开源项目!是一个强大的,高性能,高质量的Code生成类库,

它可以在运行期扩展Java类与实现Java接口。Hibernate用它来实现PO(Persistent Object 持久化对象)字节码的动态生成。

CGLIB是一个强大的高性能的代码生成包。它广泛的被许多AOP的框架使用,例如Spring AOP为他们提供

方法的interception(拦截)。CGLIB包的底层是通过使用一个小而快的字节码处理框架ASM,来转换字节码并生成新的类。

除了CGLIB包,脚本语言例如Groovy和BeanShell,也是使用ASM来生成java的字节码。当然不鼓励直接使用ASM,

因为它要求你必须对JVM内部结构包括class文件的格式和指令集都很熟悉。

二 CGLIB动态代理实例

实现一个业务类,注意,这个业务类并没有实现任何接口:

package com.lanhuigu.spring.proxy.cglib;

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:

package com.lanhuigu.spring.proxy.cglib;

import net.sf.cglib.proxy.MethodInterceptor;

import net.sf.cglib.proxy.MethodProxy;

import java.lang.reflect.Method;

/**

* 自定义MethodIntercepto

*/

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:

package com.lanhuigu.spring.proxy.cglib;

import net.sf.cglib.core.DebuggingClassWriter;

import net.sf.cglib.proxy.Enhancer;

public class Client {

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:

三 CGLIB动态代理源码分析

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 MethodIntercepto

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() {

preValidate();

Object key = KEY_FACTORY.newInstance((superclass != null) ? superclass.getName() : null,

ReflectUtils.getNames(interfaces),

filter == ALL_ZERO ? null : new WeakCacheKey<CallbackFilter>(filter),

callbackTypes,

useFactory,

interceptDuringConstruction,

serialVersionUID);

this.currentKey = key;

Object result = super.create(key);

return result;

}

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 {

ClassLoader loader = getClassLoader();

Map<ClassLoader, ClassLoaderData> cache = CACHE;

ClassLoaderData data = cache.get(loader);

if (data == null) {

synchronized (AbstractClassGenerator.class) {

cache = CACHE;

data = cache.get(loader);

if (data == null) {

Map<ClassLoader, ClassLoaderData> newCache = new WeakHashMap<ClassLoader, ClassLoaderData>(cache);

data = new ClassLoaderData(loader);

newCache.put(loader, data);

CACHE = newCache;

}

}

}

this.key = key;

Object obj = data.get(this, getUseCache());

if (obj instanceof Class) {

return firstInstance((Class) obj);

}

return nextInstance(obj);

} 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:

protected Object nextInstance(Object instance) {

EnhancerFactoryData data = (EnhancerFactoryData) instance;

if (classOnly) {

return data.generatedClass;

}

Class[] argumentTypes = this.argumentTypes;

Object[] arguments = this.arguments;

if (argumentTypes == null) {

argumentTypes = Constants.EMPTY_CLASS_ARRAY;

arguments = null;

}

return data.newInstance(argumentTypes, arguments, callbacks);

}

看看data.newInstance(argumentTypes, arguments, callbacks)方法,

第一个参数为代理对象的构成器类型,第二个为代理对象构造方法参数,第三个为对应回调对象。

最后根据这些参数,通过反射生成代理对象,源码如下:

/**

* Creates proxy instance for given argument types, and assigns the callbacks.

* Ideally, for each proxy class, just one set of argument types should be used,

* otherwise it would have to spend time on constructor lookup.

* Technically, it is a re-implementation of {@link Enhancer#createUsingReflection(Class)},

* with "cache {@link #setThreadCallbacks} and {@link #primaryConstructor}"

*

* @see #createUsingReflection(Class)

* @param argumentTypes constructor argument types

* @param arguments constructor arguments

* @param callbacks callbacks to set for the new instance

* @return newly created proxy

*/

public Object newInstance(Class[] argumentTypes, Object[] arguments, Callback[] callbacks) {

setThreadCallbacks(callbacks);

try {

// Explicit reference equality is added here just in case Arrays.equals does not have one

if (primaryConstructorArgTypes == argumentTypes ||

Arrays.equals(primaryConstructorArgTypes, argumentTypes)) {

// If we have relevant Constructor instance at hand, just call it

// This skips "get constructors" machinery

return ReflectUtils.newInstance(primaryConstructor, arguments);

}

// Take a slow path if observing unexpected argument types

return ReflectUtils.newInstance(generatedClass, argumentTypes, arguments);

} finally {

// clear thread callbacks to allow them to be gc'd

setThreadCallbacks(null);

}

}

最后生成代理对象:

 

将其反编译后代码如下:

最后生成代理对象:

package com.lanhuigu.spring.proxy.cglib;

import java.lang.reflect.Method;

import net.sf.cglib.core.ReflectUtils;

import net.sf.cglib.core.Signature;

import net.sf.cglib.proxy.*;

public class HelloService$$EnhancerByCGLIB$$4da4ebaf extends HelloService

implements Factory

{

private boolean CGLIB$BOUND;

public static Object CGLIB$FACTORY_DATA;

private static final ThreadLocal CGLIB$THREAD_CALLBACKS;

private static final Callback CGLIB$STATIC_CALLBACKS[];

private MethodInterceptor CGLIB$CALLBACK_0; // 拦截器

private static Object CGLIB$CALLBACK_FILTER;

private static final Method CGLIB$sayHello$0$Method; // 被代理方法

private static final MethodProxy CGLIB$sayHello$0$Proxy; // 代理方法

private static final Object CGLIB$emptyArgs[];

private static final Method CGLIB$equals$1$Method;

private static final MethodProxy CGLIB$equals$1$Proxy;

private static final Method CGLIB$toString$2$Method;

private static final MethodProxy CGLIB$toString$2$Proxy;

private static final Method CGLIB$hashCode$3$Method;

private static final MethodProxy CGLIB$hashCode$3$Proxy;

private static final Method CGLIB$clone$4$Method;

private static final MethodProxy CGLIB$clone$4$Proxy;

static void CGLIB$STATICHOOK1()

{

Method amethod[];

Method amethod1[];

CGLIB$THREAD_CALLBACKS = new ThreadLocal();

CGLIB$emptyArgs = new Object[0];

// 代理类

Class class1 = Class.forName("com.lanhuigu.spring.proxy.cglib.HelloService$$EnhancerByCGLIB$$4da4ebaf");

// 被代理类

Class class2;

amethod = ReflectUtils.findMethods(new String[] {

"equals", "(Ljava/lang/Object;)Z", "toString", "()Ljava/lang/String;", "hashCode", "()I", "clone", "()Ljava/lang/Object;"

}, (class2 = Class.forName("java.lang.Object")).getDeclaredMethods());

Method[] = amethod;

CGLIB$equals$1$Method = amethod[0];

CGLIB$equals$1$Proxy = MethodProxy.create(class2, class1, "(Ljava/lang/Object;)Z", "equals", "CGLIB$equals$1");

CGLIB$toString$2$Method = amethod[1];

CGLIB$toString$2$Proxy = MethodProxy.create(class2, class1, "()Ljava/lang/String;", "toString", "CGLIB$toString$2");

CGLIB$hashCode$3$Method = amethod[2];

CGLIB$hashCode$3$Proxy = MethodProxy.create(class2, class1, "()I", "hashCode", "CGLIB$hashCode$3");

CGLIB$clone$4$Method = amethod[3];

CGLIB$clone$4$Proxy = MethodProxy.create(class2, class1, "()Ljava/lang/Object;", "clone", "CGLIB$clone$4");

amethod1 = ReflectUtils.findMethods(new String[] {

"sayHello", "()V"

}, (class2 = Class.forName("com.lanhuigu.spring.proxy.cglib.HelloService")).getDeclaredMethods());

Method[] 1 = amethod1;

CGLIB$sayHello$0$Method = amethod1[0];

CGLIB$sayHello$0$Proxy = MethodProxy.create(class2, class1, "()V", "sayHello", "CGLIB$sayHello$0");

}

final void CGLIB$sayHello$0()

{

super.sayHello();

}

public final void sayHello()

{

MethodInterceptor var10000 = this.CGLIB$CALLBACK_0;

if(this.CGLIB$CALLBACK_0 == null) {

CGLIB$BIND_CALLBACKS(this);

var10000 = this.CGLIB$CALLBACK_0;

}

if(var10000 != null) {

// 调用拦截器

var10000.intercept(this, CGLIB$setPerson$0$Method, CGLIB$emptyArgs, CGLIB$setPerson$0$Proxy);

} else {

super.sayHello();

}

}

......

......

}

从代理对象反编译源码可以知道,代理对象继承于HelloService,拦截器调用intercept()方法,

intercept()方法由自定义MyMethodInterceptor实现,所以,最后调用MyMethodInterceptor中

的intercept()方法,从而完成了由代理对象访问到目标对象的动态代理实现。

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