二、動態代理

Java中動態代理的實現，關鍵就是這兩個東西：Proxy、InvocationHandler，下面從InvocationHandler介面中的invoke方法入手，簡單說明一下Java如何實現動態代理的。 
首先，invoke方法的完整形式如下：

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {  
	method.invoke(obj, args);  
	return null;  
}

A method invocation on a proxy instance through one of its proxy interfaces will be dispatched to the invoke method of the instance's invocation handler, 
passing the proxy instance,a java.lang.reflect.Method object identifying the method that was invoked, and an array of type Object containing the arguments.  
 

為了方便說明，這裡寫一個簡單的例子來實現動態代理。

 //抽象角色（動態代理只能代理介面）  
public interface Subject {  
      
    public void request();  
}

 //真實角色：實現了Subject的request()方法  
public class RealSubject implements Subject{  
      
    public void request(){  
        System.out.println("From real subject.");  
    }  
} 
 

    //實現了InvocationHandler  
    public class DynamicSubject implements InvocationHandler  
    {  
        private Object obj;//這是動態代理的好處，被封裝的物件是Object型別，接受任意型別的物件  
      
        public DynamicSubject()  
        {  
        }  
      
        public DynamicSubject(Object obj)  
        {  
            this.obj = obj;  
        }  
      
        //這個方法不是我們顯示的去呼叫  
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable  
        {  
            System.out.println("before calling " + method);  
      
            method.invoke(obj, args);  
      
            System.out.println("after calling " + method);  
      
            return null;  
        }  
      
    }  

    //客戶端：生成代理例項，並呼叫了request()方法  
    public class Client {  
      
        public static void main(String[] args) throws Throwable{  
            // TODO Auto-generated method stub  
      
            Subject rs=new RealSubject();//這裡指定被代理類  
            InvocationHandler ds=new DynamicSubject(rs);  
            Class<?> cls=rs.getClass();  
              
            //以下是一次性生成代理  
              
            Subject subject=(Subject) Proxy.newProxyInstance(  
                    cls.getClassLoader(),cls.getInterfaces(), ds);  
              
            //這裡可以通過執行結果證明subject是Proxy的一個例項，這個例項實現了Subject介面  
            System.out.println(subject instanceof Proxy);  
              
            //這裡可以看出subject的Class類是$Proxy0,這個$Proxy0類繼承了Proxy，實現了Subject介面  
            System.out.println("subject的Class類是："+subject.getClass().toString());  
              
            System.out.print("subject中的屬性有：");  
              
            Field[] field=subject.getClass().getDeclaredFields();  
            for(Field f:field){  
                System.out.print(f.getName()+", ");  
            }  
              
            System.out.print("\n"+"subject中的方法有：");  
              
            Method[] method=subject.getClass().getDeclaredMethods();  
              
            for(Method m:method){  
                System.out.print(m.getName()+", ");  
            }  
              
            System.out.println("\n"+"subject的父類是："+subject.getClass().getSuperclass());  
              
            System.out.print("\n"+"subject實現的介面是：");  
              
            Class<?>[] interfaces=subject.getClass().getInterfaces();  
              
            for(Class<?> i:interfaces){  
                System.out.print(i.getName()+", ");  
            }  
      
            System.out.println("\n\n"+"執行結果為：");  
            subject.request();  
        }  
    }  

    執行結果如下：此處省略了包名，***代替  
    true  
    subject的Class類是：class $Proxy0  
    subject中的屬性有：m1, m3, m0, m2,   
    subject中的方法有：request, hashCode, equals, toString,   
    subject的父類是：class java.lang.reflect.Proxy  
    subject實現的介面是：cn.edu.ustc.dynamicproxy.Subject,   
      
    執行結果為：  
    before calling public abstract void ***.Subject.request()  
    From real subject.  
    after calling public abstract void ***.Subject.request()  

從以上程式碼和結果可以看出，我們並沒有顯示的呼叫invoke()方法，但是這個方法確實執行了。下面就整個的過程進行分析一下：
從Client中的程式碼看，可以從newProxyInstance這個方法作為突破口，我們先來看一下Proxy類中newProxyInstance方法的原始碼：

    public static Object newProxyInstance(ClassLoader loader,  
            Class<?>[] interfaces,  
            InvocationHandler h)  
    throws IllegalArgumentException  
    {  
        if (h == null) {  
            throw new NullPointerException();  
        }  
      
        /* 
         * Look up or generate the designated proxy class. 
         */  
        Class cl = getProxyClass(loader, interfaces);  
      
        /* 
         * Invoke its constructor with the designated invocation handler. 
         */  
        try {  
               /* 
                * Proxy原始碼開始有這樣的定義： 
                * private final static Class[] constructorParams = { InvocationHandler.class }; 
                * cons即是形參為InvocationHandler型別的構造方法 
               */  
            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());  
        }  
    }  

    class Proxy{  
        InvocationHandler h=null;  
        protected Proxy(InvocationHandler h) {  
            this.h = h;  
        }  
        ...  
    }  

    public final class $Proxy0 extends Proxy implements Subject {  
        private static Method m1;  
        private static Method m0;  
        private static Method m3;  
        private static Method m2;  
      
        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("***.RealSubject").getMethod("request",  
                        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());  
            }  
        } //static  
      
        public $Proxy0(InvocationHandler invocationhandler) {  
            super(invocationhandler);  
        }  
      
        @Override  
        public final boolean equals(Object obj) {  
            try {  
                return ((Boolean) super.h.invoke(this, m1, new Object[] { obj })) .booleanValue();  
            } catch (Throwable throwable) {  
                throw new UndeclaredThrowableException(throwable);  
            }  
        }  
      
        @Override  
        public final int hashCode() {  
            try {  
                return ((Integer) super.h.invoke(this, m0, null)).intValue();  
            } catch (Throwable throwable) {  
                throw new UndeclaredThrowableException(throwable);  
            }  
        }  
      
        public final void request() {  
            try {  
                super.h.invoke(this, m3, null);  
                return;  
            } catch (Error e) {  
            } catch (Throwable throwable) {  
                throw new UndeclaredThrowableException(throwable);  
            }  
        }  
      
        @Override  
        public final String toString() {  
            try {  
                return (String) super.h.invoke(this, m2, null);  
            } catch (Throwable throwable) {  
                throw new UndeclaredThrowableException(throwable);  
            }  
        }  
    }