CGLIB动态代理实现原理

短命女 2023-08-17 17:23 125阅读 0赞

# 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.proxy.cglib;
    
    import java.lang.reflect.Method;
    
    import net.sf.cglib.proxy.Enhancer;
    import net.sf.cglib.proxy.MethodInterceptor;
    import net.sf.cglib.proxy.MethodProxy;
    
    /**
     * @author pypua
     * @date 2019年8月14日 上午10:57:56
     * 
     */
    public class MyMethodInterceptor implements MethodInterceptor{
    
        
        //疑问？
            //好像并没有持有被代理对象的引用
            public Object getInstance(Class clazz) throws Exception{
                
                Enhancer enhancer = new Enhancer();
                //把父类设置为谁？
                //这一步就是告诉cglib，生成的子类需要继承哪个类
                enhancer.setSuperclass(clazz);
                //设置回调
                enhancer.setCallback(this);
                
                //第一步、生成源代码
                //第二步、编译成class文件
                //第三步、加载到JVM中，并返回被代理对象
                return enhancer.create();
            }
             /**
             * 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;

package com.proxy.cglib;
    
    
    /**
     * @author pypua
     * @date 2019年8月14日 上午11:02:51
     * 
     */
    public class Client {
    
        public static void main(String[] args) {
        //JDK的动态代理是通过接口来进行强制转换的
                //生成以后的代理对象，可以强制转换为接口
                
                //CGLib的动态代理是通过生成一个被代理对象的子类，然后重写父类的方法
                //生成以后的对象，可以强制转换为被代理对象（也就是用自己写的类）
                //子类引用赋值给父类
        try {
            HelloService obj = (HelloService)new MyMethodInterceptor().getInstance(HelloService.class);
            obj.sayHello();
            obj.sayOthers("sfsdfsd");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    }

运行结果:

![1055865-20190814150452518-933950977.png][]

# 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 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() {
            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：

![1055865-20190814143504812-2028400606.png][]

将其反编译后代码如下：

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。  
  
原文链接：https://blog.csdn.net/yhl\_jxy/article/details/80633194

转载于:https://www.cnblogs.com/pypua/p/11351772.html

[1055865-20190814150452518-933950977.png]: /images/20230808/3c1355705b3d4f41b71a29b743a70a7f.png
[1055865-20190814143504812-2028400606.png]: /images/20230808/1cde58e8a20a49e59ec0e1346e97541d.png