一、前言

Java中常见的代理有JDK代理和Cglib代理，无论是AOP实现还是mybaits动态生成数据库操作类无一不是通过代理来搞的，下面讲解下原理以及比较

二、JDK代理

2.1 试验测试

接口类：

public interface UserService {

    public     abstract void add();
}

实现类：

public class UserServiceImpl implements UserService {

    @Override
    public void add() {
        // TODO Auto-generated method stub
        System.out.println("--------------------add----------------------");
    }
}

InvocationHandler类

public class MyInvocationHandler implements InvocationHandler {

    private Object target;

    public MyInvocationHandler(Object target) {

        super();
        this.target = target;

    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        PerformanceMonior.begin(target.getClass().getName()+"."+method.getName());
        //System.out.println("-----------------begin "+method.getName()+"-----------------");
        Object result = method.invoke(target, args);
        //System.out.println("-----------------end "+method.getName()+"-----------------");
        PerformanceMonior.end();
        return result;
    }

    public Object getProxy(){

        return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), target.getClass().getInterfaces(), this);
    }

}

测试类

public static void main(String[] args) {

  //生成的代理类保存到磁盘
  System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");  

  UserService service = new UserServiceImpl();
  MyInvocationHandler handler = new MyInvocationHandler(service);
  UserService proxy = (UserService) handler.getProxy();
  proxy.add();
}

UserServiceImpl被JDK代理后的类，在项目的com.sun.proxy下面生成$Proxy0.class类

package com.sun.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import proxy.JDK.UserService;

public final class $Proxy0
  extends Proxy
  implements UserService
{
  private static Method m1;
  private static Method m3;
  private static Method m0;
  private static Method m2;

  public $Proxy0(InvocationHandler paramInvocationHandler)
  {
    super(paramInvocationHandler);
  }

  public final void add()
  {
    try
    {
    //第一个参数是代理类本身，第二个是实现类的方法，第三个是参数
      this.h.invoke(this, m3, null);
      return;
    }
    catch (Error|RuntimeException localError)
    {
      throw localError;
    }
    catch (Throwable localThrowable)
    {
      throw new UndeclaredThrowableException(localThrowable);
    }
  }

 ...

  static
  {
    try
    {

      m3 = Class.forName("proxy.JDK.UserService").getMethod("add", new Class[0]);
    ...
      return;
    }
    catch (NoSuchMethodException localNoSuchMethodException)
    {
      throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
    }
    catch (ClassNotFoundException localClassNotFoundException)
    {
      throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
    }
  }
}

也就是说main函数里面的proxy实际就是$Proxy0的一个实例对象。
可知JDK动态代理是使用接口生成新的实现类，实现类里面则委托给InvocationHandler，InvocationHandler里面则调用被代理的类方法。

2.2 源码分析

如时序图首先调用了newProxyInstance方法：

public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h)
    throws IllegalArgumentException
{
    ...
    /*
     * 查找或者生成代理类的大class类.
     */
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * 
       使用代理类构造函数实例化一个对象
     */
    try {
        final Constructor<?> cons = cl.getConstructor(constructorParams);
        final InvocationHandler ih = h;
        if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
            // create proxy instance with doPrivilege as the proxy class may
            // implement non-public interfaces that requires a special permission
            return AccessController.doPrivileged(new PrivilegedAction<Object>() {
                public Object run() {
                    return newInstance(cons, ih);
                }
            });
        } else {
            return newInstance(cons, ih);
        }
    } catch (NoSuchMethodException e) {
        throw new InternalError(e.toString());
    }
}

然后看下关键函数getProxyClass0

    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }

        // 使用缓存，如果缓存中存在由实现类加载器创建的代理类，则直接返回，否者使用ProxyClassFactory创建代理类的Class
        return proxyClassCache.get(loader, interfaces);
    }

在看下ProxyClassFactory的apply函数：

public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

    Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
    ...

    /*
     * Choose a name for the proxy class to generate.
     */
    long num = nextUniqueNumber.getAndIncrement();
    String proxyName = proxyPkg + proxyClassNamePrefix + num;

    /*
     * 根据接口类生成代理类.
     */
    byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
        proxyName, interfaces);
    try {
        return defineClass0(loader, proxyName,
                            proxyClassFile, 0, proxyClassFile.length);
    } catch (ClassFormatError e) {
       ...
        throw new IllegalArgumentException(e.toString());
    }
}

三、Cglib代理

3.1 试验测试

测试代码

public void testCglibProxy() {

//生成代理类到本地
    System.setProperty(DebuggingClassWriter.DEBUG_LOCATION_PROPERTY, "/Users/zhuizhumengxiang/Downloads");


    UserServiceImpl service = new UserServiceImpl();
    CglibProxy cp = new CglibProxy();
    UserService proxy = (UserService) cp.getProxy(service.getClass());
    proxy.add();
    proxy.sub();
    proxy.hello("zlx");
    proxy.service("zlx");
    proxy.toString();
}

MethodInterceptor类：

public class CglibProxy implements MethodInterceptor {

    private Enhancer enhancer = new Enhancer();

    public Object getProxy(Class clazz) {

        enhancer.setSuperclass(clazz);
        enhancer.setCallback( this);
        //enhancer.setCallbackType(clazz);;
        return enhancer.create();
    }

    @Override
    public Object intercept(Object obj, Method method, Object[] args, MethodProxy proxy) throws Throwable {
        PerformanceMonior.begin(obj.getClass().getName()+"."+method.getName());
       Object result = proxy.invokeSuper(obj, args);
      //  Object result = method.invoke(obj, args);
        PerformanceMonior.end();

        return result;
    }
}

生成的代理类反编译后：

package proxy.JDK;

import java.lang.reflect.Method;
import org.springframework.cglib.proxy.Callback;
import org.springframework.cglib.proxy.Factory;
import org.springframework.cglib.proxy.MethodInterceptor;
import org.springframework.cglib.proxy.MethodProxy;

public class UserServiceImpl$$EnhancerByCGLIB$$a68ccf10
  extends UserServiceImpl
  implements Factory
{
  private boolean CGLIB$BOUND;
  private static final ThreadLocal CGLIB$THREAD_CALLBACKS;
  private static final Callback[] CGLIB$STATIC_CALLBACKS;
  private MethodInterceptor CGLIB$CALLBACK_0;
  private static final Method CGLIB$add$0$Method;
  private static final MethodProxy CGLIB$add$0$Proxy;
  private static final Object[] CGLIB$emptyArgs;
  private static final Method CGLIB$service$1$Method;
  private static final MethodProxy CGLIB$service$1$Proxy;
  private static final Method CGLIB$testAnnotaion$2$Method;
  private static final MethodProxy CGLIB$testAnnotaion$2$Proxy;
  private static final Method CGLIB$testAnnotaion2$3$Method;
  private static final MethodProxy CGLIB$testAnnotaion2$3$Proxy;
  private static final Method CGLIB$hello$4$Method;
  private static final MethodProxy CGLIB$hello$4$Proxy;
  private static final Method CGLIB$sub$5$Method;
  private static final MethodProxy CGLIB$sub$5$Proxy;
  private static final Method CGLIB$finalize$6$Method;
  private static final MethodProxy CGLIB$finalize$6$Proxy;
  private static final Method CGLIB$equals$7$Method;
  private static final MethodProxy CGLIB$equals$7$Proxy;
  private static final Method CGLIB$toString$8$Method;
  private static final MethodProxy CGLIB$toString$8$Proxy;
  private static final Method CGLIB$hashCode$9$Method;
  private static final MethodProxy CGLIB$hashCode$9$Proxy;
  private static final Method CGLIB$clone$10$Method;
  private static final MethodProxy CGLIB$clone$10$Proxy;

   ...

  final void CGLIB$add$0()
  {
    super.add();
  }

  public final void add()
  {
    MethodInterceptor tmp4_1 = this.CGLIB$CALLBACK_0;
    if (tmp4_1 == null)
    {
      tmp4_1;
      CGLIB$BIND_CALLBACKS(this);
    }
    if (this.CGLIB$CALLBACK_0 != null) {
      return;
    }
    super.add();
  }

  ...

  public UserServiceImpl$$EnhancerByCGLIB$$a68ccf10()
  {
    CGLIB$BIND_CALLBACKS(this);
  }

  public static void CGLIB$SET_THREAD_CALLBACKS(Callback[] paramArrayOfCallback)
  {
    CGLIB$THREAD_CALLBACKS.set(paramArrayOfCallback);
  }

  public static void CGLIB$SET_STATIC_CALLBACKS(Callback[] paramArrayOfCallback)
  {
    CGLIB$STATIC_CALLBACKS = paramArrayOfCallback;
  }

  private static final void CGLIB$BIND_CALLBACKS(Object paramObject)
  {
    a68ccf10 locala68ccf10 = (a68ccf10)paramObject;
    if (!locala68ccf10.CGLIB$BOUND)
    {
      locala68ccf10.CGLIB$BOUND = true;
      Object tmp23_20 = CGLIB$THREAD_CALLBACKS.get();
      if (tmp23_20 == null)
      {
        tmp23_20;
        CGLIB$STATIC_CALLBACKS;
      }
      locala68ccf10.CGLIB$CALLBACK_0 = (tmp31_28 == null ? tmp31_28 : (MethodInterceptor)((Callback[])tmp23_20)[0]);
    }
  }

  public Object newInstance(Callback[] paramArrayOfCallback)
  {
    CGLIB$SET_THREAD_CALLBACKS(paramArrayOfCallback);
    CGLIB$SET_THREAD_CALLBACKS(null);
    return new a68ccf10();
  }

  public Object newInstance(Callback paramCallback)
  {
    CGLIB$SET_THREAD_CALLBACKS(new Callback[] { paramCallback });
    CGLIB$SET_THREAD_CALLBACKS(null);
    return new a68ccf10();
  }


  public Callback getCallback(int paramInt)
  {
    CGLIB$BIND_CALLBACKS(this);
    switch (paramInt)
    {
    case 0: 
      break;
    }
    return null;
  }

  public void setCallback(int paramInt, Callback paramCallback)
  {
    switch (paramInt)
    {
    case 0: 
      this.CGLIB$CALLBACK_0 = ((MethodInterceptor)paramCallback);
      break;
    }
  }

  public Callback[] getCallbacks()
  {
    CGLIB$BIND_CALLBACKS(this);
    return new Callback[] { this.CGLIB$CALLBACK_0 };
  }

  public void setCallbacks(Callback[] paramArrayOfCallback)
  {
    this.CGLIB$CALLBACK_0 = ((MethodInterceptor)paramArrayOfCallback[0]);
  }

  static {}
}

Cglib是通过直接继承被代理类，并委托为回调函数来做具体的事情：

从代理类里面可知道对于原来的add函数，代理类里面对应了两个函数分布是add 和CGLIB$add$0
其中后者是在方法拦截器里面调用的的，前者则是我们使用代理类时候调用的函数。当我们代码调用add时候，会具体调用到方法拦截器的intercept方法，该方法内则通过proxy.invokeSuper调用CGLIB$add$0

3.2 源码分析

如图首先设置被代理类，然后设置自己写的方法拦截器，然后创建创建代理类的Class对象，并调用代理类的CGLIB$SET_THREAD_CALLBACKS方法设置回调。

四、总结

对应JDK动态代理机制是委托机制，具体说动态实现接口类，在动态生成的实现类里面委托为hanlder去调用原始实现类方法。

比如接口类为Abo,实现类为AboImpl,AboImpl的代理类为$ProxyAoImpl ，那么$ProxyAoImpl 能赋值给Abo?能够赋值给AboImpl？

$ProxyAoImpl 是能够赋值给Abo的，因为前者间接实现了后者，但是$ProxyAoImpl 不能赋值给AboImpl因为他们没有继承或者实现关系。所以回顾下自己项目中Rpc里面autowired时候都是对bo类进行的，而不是对boimpl，并且我们的boimpl类一般都是配置了事务切面被代理过的。

对应Cglib则使用的继承机制，具体说被代理类和代理类是继承关系，所以代理类是可以赋值给被代理类的,如果被代理类有接口，那么代理类也可以赋值给接口。

另外JDK代理只能对接口进行代理，Cglib则是对实现类进行代理。