动态代理详解

动态代理使用：

public interface Subject   {     public void doSomething();   }   public class RealSubject implements Subject   {     public void doSomething()     {       System.out.println( "call doSomething()" );     }   }   public class ProxyHandler implements InvocationHandler   {     private Object proxied;          public ProxyHandler( Object proxied )     {       this.proxied = proxied;     }          public Object invoke( Object proxy, Method method, Object[] args ) throws Throwable     {       //在转调具体目标对象之前，可以执行一些功能处理    //转调具体目标对象的方法    return method.invoke( proxied, args);          //在转调具体目标对象之后，可以执行一些功能处理  }    }

import java.lang.reflect.InvocationHandler;   import java.lang.reflect.Method;   import java.lang.reflect.Proxy;   import sun.misc.ProxyGenerator;   import java.io.*;   public class DynamicProxy   {     public static void main( String args[] )     {       RealSubject real = new RealSubject();       Subject proxySubject = (Subject)Proxy.newProxyInstance(Subject.class.getClassLoader(),      new Class[]{Subject.class},      new ProxyHandler(real));             proxySubject.doSomething();       //write proxySubject class binary data to file       createProxyClassFile();     }          public static void createProxyClassFile()     {       String name = "ProxySubject";       byte[] data = ProxyGenerator.generateProxyClass( name, new Class[] { Subject.class } );       try      {         FileOutputStream out = new FileOutputStream( name + ".class" );         out.write( data );         out.close();       }       catch( Exception e )       {         e.printStackTrace();       }     }   }

动态代理内部实现：

首先来看看Proxy类的内部代码的主要静态变量的实现：

// 映射表：用于维护类装载器对象到其对应的代理类缓存private static Map loaderToCache = new WeakHashMap(); // 标记：用于标记一个动态代理类正在被创建中private static Object pendingGenerationMarker = new Object(); // 同步表：记录已经被创建的动态代理类类型，主要被方法 isProxyClass 进行相关的判断private static Map proxyClasses = Collections.synchronizedMap(new WeakHashMap()); // 关联的调用处理器引用protected InvocationHandler h;

Proxy的构造方法：

// 由于 Proxy 内部从不直接调用构造函数，所以 private 类型意味着禁止任何调用private Proxy() {} // 由于 Proxy 内部从不直接调用构造函数，所以 protected 意味着只有子类可以调用protected Proxy(InvocationHandler h) {this.h = h;} 

Proxy的静态方法newProxyInstance：

public static Object newProxyInstance(ClassLoader loader, Class<?>[]interfaces,InvocationHandler h) throws IllegalArgumentException {     // 检查 h 不为空，否则抛异常    if (h == null) {         throw new NullPointerException();     }     // 获得与指定类装载器和一组接口相关的代理类类型对象    Class cl = getProxyClass(loader, interfaces);     // 通过反射获取构造函数对象并生成代理类实例    try {         Constructor cons = cl.getConstructor(constructorParams);         return (Object) cons.newInstance(new Object[] { h });     } catch (NoSuchMethodException e) { throw new InternalError(e.toString());     } catch (IllegalAccessException e) { throw new InternalError(e.toString());     } catch (InstantiationException e) { throw new InternalError(e.toString());     } catch (InvocationTargetException e) { throw new InternalError(e.toString());     } }

类Proxy的getProxyClass方法调用ProxyGenerator的 generateProxyClass方法产生ProxySubject.class的二进制数据：

public static byte[] generateProxyClass(final String name, Class[] interfaces)

我们可以import sun.misc.ProxyGenerator，调用 generateProxyClass方法产生binary data，然后写入文件，最后通过反编译工具来查看内部实现原理。 反编译后的ProxySubject.java, (Proxy的静态方法newProxyInstance产生的class文件反编译后的java文件)

import java.lang.reflect.*;   public final class ProxySubject extends Proxy       implements Subject   {       private static Method m1;       private static Method m0;       private static Method m3;       private static Method m2;       public ProxySubject(InvocationHandler invocationhandler)       {           super(invocationhandler);       }       public final boolean equals(Object obj)       {           try          {               return ((Boolean)super.h.invoke(this, m1, new Object[] {                   obj               })).booleanValue();           }           catch(Error _ex) { }           catch(Throwable throwable)           {               throw new UndeclaredThrowableException(throwable);           }       }       public final int hashCode()       {           try          {               return ((Integer)super.h.invoke(this, m0, null)).intValue();           }           catch(Error _ex) { }           catch(Throwable throwable)           {               throw new UndeclaredThrowableException(throwable);           }       }       public final void doSomething()       {           try          {               super.h.invoke(this, m3, null);               return;           }           catch(Error _ex) { }           catch(Throwable throwable)           {               throw new UndeclaredThrowableException(throwable);           }       }       public final String toString()       {           try          {               return (String)super.h.invoke(this, m2, null);           }           catch(Error _ex) { }           catch(Throwable throwable)           {               throw new UndeclaredThrowableException(throwable);           }       }       static        {           try          {               m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {                   Class.forName("java.lang.Object")               });               m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);               m3 = Class.forName("Subject").getMethod("doSomething", new Class[0]);               m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);           }           catch(NoSuchMethodException nosuchmethodexception)           {               throw new NoSuchMethodError(nosuchmethodexception.getMessage());           }           catch(ClassNotFoundException classnotfoundexception)           {               throw new NoClassDefFoundError(classnotfoundexception.getMessage());           }       }   }