cglib簡介

CGLIB(Code Generation Library)是一個(gè)開源項(xiàng)目慢哈！是一個(gè)強(qiáng)大的，高性能永票，高質(zhì)量的Code生成類庫卵贱，它廣泛的被許多AOP的框架使用滥沫，例如Spring AOP為他們提供方法的interception（攔截）。CGLIB包的底層是通過使用一個(gè)小而快的字節(jié)碼處理框架ASM键俱，來轉(zhuǎn)換字節(jié)碼并生成新的類兰绣。除了CGLIB包，腳本語言例如Groovy和BeanShell编振，也是使用ASM來生成java的字節(jié)碼缀辩。當(dāng)然不鼓勵(lì)直接使用ASM，因?yàn)樗竽惚仨殞?duì)JVM內(nèi)部結(jié)構(gòu)包括class文件的格式和指令集都很熟悉党觅。

image.png

CGLIB動(dòng)態(tài)代理實(shí)例

實(shí)現(xiàn)一個(gè)業(yè)務(wù)類雌澄，注意，這個(gè)業(yè)務(wù)類并沒有實(shí)現(xiàn)任何接口：

package com.jpeony.spring.proxy.cglib;
 
public class HelloService {
 
    public HelloService() {
        System.out.println("HelloService構(gòu)造");
    }
 
    /**
     * 該方法不能被子類覆蓋,Cglib是無法代理final修飾的方法的
     */
    final public String sayOthers(String name) {
        System.out.println("HelloService:sayOthers>>"+name);
        return null;
    }
 
    public void sayHello() {
        System.out.println("HelloService:sayHello");
    }
}

自定義MethodInterceptor：

package com.jpeony.spring.proxy.cglib;
 
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;
 
import java.lang.reflect.Method;
 
/**
 * 自定義MethodInterceptor
 */
public class MyMethodInterceptor implements MethodInterceptor{
 
    /**
     * sub：cglib生成的代理對(duì)象
     * method：被代理對(duì)象方法
     * objects：方法入?yún)?     * 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代理對(duì)象調(diào)用目標(biāo)方法：

package com.jpeony.spring.proxy.cglib;
 
import net.sf.cglib.core.DebuggingClassWriter;
import net.sf.cglib.proxy.Enhancer;
 
public class Client {
    public static void main(String[] args) {
        // 代理類class文件存入本地磁盤方便我們反編譯查看源碼
        System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "D:\\code");
        // 通過CGLIB動(dòng)態(tài)代理獲取代理對(duì)象的過程
        Enhancer enhancer = new Enhancer();
        // 設(shè)置enhancer對(duì)象的父類
        enhancer.setSuperclass(HelloService.class);
        // 設(shè)置enhancer的回調(diào)對(duì)象
        enhancer.setCallback(new MyMethodInterceptor());
        // 創(chuàng)建代理對(duì)象
        HelloService proxy= (HelloService)enhancer.create();
        // 通過代理對(duì)象調(diào)用目標(biāo)方法
        proxy.sayHello();
    }
}

Connected to the target VM, address: '127.0.0.1:65382', transport: 'socket'
CGLIB debugging enabled, writing to 'code'
HelloService構(gòu)造
======插入前置通知======
HelloService:sayHello
======插入后者通知======
Disconnected from the target VM, address: '127.0.0.1:65382', transport: 'socket'

Process finished with exit code 0

CGLIB動(dòng)態(tài)代理源碼分析

實(shí)現(xiàn)CGLIB動(dòng)態(tài)代理必須實(shí)現(xiàn)MethodInterceptor(方法攔截器)接口杯瞻，源碼如下:

/*
 * 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;
 
}

這個(gè)接口只有一個(gè)intercept()方法镐牺，這個(gè)方法有4個(gè)參數(shù)：

1）obj表示增強(qiáng)的對(duì)象，即實(shí)現(xiàn)這個(gè)接口類的一個(gè)對(duì)象魁莉；
2）method表示要被攔截的方法睬涧；
3）args表示要被攔截方法的參數(shù)；
4）proxy表示要觸發(fā)父類的方法對(duì)象旗唁；

在上面的Client代碼中畦浓，通過Enhancer.create()方法創(chuàng)建代理對(duì)象，create()方法的源碼：

/**
     * 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();
    }

該方法含義就是如果有必要就創(chuàng)建一個(gè)新類检疫，并且用指定的回調(diào)對(duì)象創(chuàng)建一個(gè)新的對(duì)象實(shí)例讶请，

使用的父類的參數(shù)的構(gòu)造方法來實(shí)例化父類的部分。核心內(nèi)容在createHelper()中屎媳，源碼如下:

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;
    }

通過newInstance()方法創(chuàng)建EnhancerKey對(duì)象夺溢，作為Enhancer父類AbstractClassGenerator.create()方法創(chuàng)建代理對(duì)象的參數(shù)。

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);
        }
    }

真正創(chuàng)建代理對(duì)象方法在nextInstance()方法中烛谊，該方法為抽象類AbstractClassGenerator的一個(gè)方法风响，簽名如下：
abstract protected Object nextInstance(Object instance) throws Exception;
在子類Enhancer中實(shí)現(xiàn)，實(shí)現(xiàn)源碼如下：

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)方法丹禀，

第一個(gè)參數(shù)為代理對(duì)象的構(gòu)成器類型状勤，第二個(gè)為代理對(duì)象構(gòu)造方法參數(shù)，第三個(gè)為對(duì)應(yīng)回調(diào)對(duì)象双泪。最后根據(jù)這些參數(shù)持搜，通過反射生成代理對(duì)象，源碼如下：

/**
         * 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);
            }
 
        }

image.png

將其反編譯后代碼如下：

package com.jpeony.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$CALLBACK_0; // 攔截器
    private static Object CGLIB$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];
        // 代理類
        Class class1 = Class.forName("com.jpeony.spring.proxy.cglib.HelloService$$EnhancerByCGLIB$$4da4ebaf");
        // 被代理類
        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$4");
        amethod1 = ReflectUtils.findMethods(new String[] {
            "sayHello", "()V"
        }, (class2 = Class.forName("com.jpeony.spring.proxy.cglib.HelloService")).getDeclaredMethods());
        Method[] 1 = amethod1;
        CGLIB$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) {
         // 調(diào)用攔截器
         var10000.intercept(this, CGLIB$setPerson$0$Method, CGLIB$emptyArgs, CGLIB$setPerson$0$Proxy);
      } else {
         super.sayHello();
      }
    }
    ......
    ......
}

從代理對(duì)象反編譯源碼可以知道焙矛，代理對(duì)象繼承于HelloService朵诫，攔截器調(diào)用intercept()方法，

intercept()方法由自定義MyMethodInterceptor實(shí)現(xiàn)薄扁，所以剪返，最后調(diào)用MyMethodInterceptor中的intercept()方法废累，從而完成了由代理對(duì)象訪問到目標(biāo)對(duì)象的動(dòng)態(tài)代理實(shí)現(xiàn)。

常用的API

Enhancer

Enhancer可能是CGLIB中最常用的一個(gè)類脱盲，和Java1.3動(dòng)態(tài)代理中引入的Proxy類差不多邑滨，和Proxy不同的是，Enhancer既能夠代理普通的class钱反，也能夠代理接口掖看。Enhancer創(chuàng)建一個(gè)被代理對(duì)象的子類并且攔截所有的方法調(diào)用（包括從Object中繼承的toString和hashCode方法）Enhancer不能夠攔截final方法，例如Object.getClass()方法面哥，這是由于Java final方法語義決定的哎壳。基于同樣的道理尚卫，Enhancer也不能對(duì)fianl類進(jìn)行代理操作归榕。

public class SampleClass {
    public String test(String input){
        return "hello world";
    }
}

下面我們將以這個(gè)類作為主要的測(cè)試類，來測(cè)試調(diào)用各種方法

@Test
public void testFixedValue(){
    Enhancer enhancer = new Enhancer();
    enhancer.setSuperclass(SampleClass.class);
    enhancer.setCallback(new FixedValue() {
        @Override
        public Object loadObject() throws Exception {
            return "Hello cglib";
        }
    });
    SampleClass proxy = (SampleClass) enhancer.create();
    System.out.println(proxy.test(null)); //攔截test吱涉，輸出Hello cglib
    System.out.println(proxy.toString()); 
    System.out.println(proxy.getClass());
    System.out.println(proxy.hashCode());
}

程序的輸出為：

Hello cglib
Hello cglib
class com.zeus.cglib.SampleClass$$EnhancerByCGLIB$$e3ea9b7

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Number

    at com.zeus.cglib.SampleClass$$EnhancerByCGLIB$$e3ea9b7.hashCode(<generated>)
    ...

上述代碼中刹泄，F(xiàn)ixedValue用來對(duì)所有攔截的方法返回相同的值，從輸出我們可以看出來怎爵，Enhancer對(duì)非final方法test()特石、toString()、hashCode()進(jìn)行了攔截鳖链，沒有對(duì)getClass進(jìn)行攔截姆蘸。由于hashCode()方法需要返回一個(gè)Number，但是我們返回的是一個(gè)String芙委，這解釋了上面的程序中為什么會(huì)拋出異常逞敷。
Enhancer.setSuperclass用來設(shè)置父類型，從toString方法可以看出题山，使用CGLIB生成的類為被代理類的一個(gè)子類兰粉，形如SampleClasse3ea9b7

有些時(shí)候我們可能只想對(duì)特定的方法進(jìn)行攔截故痊，對(duì)其他的方法直接放行顶瞳，不做任何操作，使用Enhancer處理這種需求同樣很簡單,只需要一個(gè)CallbackFilter即可：

@Test
public void testCallbackFilter() throws Exception{
    Enhancer enhancer = new Enhancer();
    CallbackHelper callbackHelper = new CallbackHelper(SampleClass.class, new Class[0]) {
        @Override
        protected Object getCallback(Method method) {
            if(method.getDeclaringClass() != Object.class && method.getReturnType() == String.class){
                return new FixedValue() {
                    @Override
                    public Object loadObject() throws Exception {
                        return "Hello cglib";
                    }
                };
            }else{
                return NoOp.INSTANCE;
            }
        }
    };
    enhancer.setSuperclass(SampleClass.class);
    enhancer.setCallbackFilter(callbackHelper);
    enhancer.setCallbacks(callbackHelper.getCallbacks());
    SampleClass proxy = (SampleClass) enhancer.create();
    Assert.assertEquals("Hello cglib", proxy.test(null));
    Assert.assertNotEquals("Hello cglib",proxy.toString());
    System.out.println(proxy.hashCode());
}