动态代理

从JDK1.3开始，Java就引入了动态代理的概念。动态代理（Dynamic Proxy）可以帮助你减少代码行数，真正提高代码的可复用度。例如，你不必为所有的类的方法里面都写上相同的Log代码行，取而代之的是实用类的动态代理类。当然，这种便利是有条件的。本文简单介绍Java动态代理的原理，并实现一个被代理的Servlet创建，和调用的过程。

1．代理模式（Proxy Pattern）
在JDK1.3以前，代理模式就已流行，所以得代理模式是生成一个和类相同接口的代理类，用户通过使用代理类来封装某个实现类。如图1，其目的是加强实现类的某个方法的功能，而不必改变原有的源代码。


2．动态代理（Dynamic Proxy）
随着Proxy的流行，Sun把它纳入到JDK1.3实现了Java的动态代理。动态代理和普通的代理模式的区别，就是动态代理中的代理类是由java.lang.reflect.Proxy类在运行期时根据接口定义，采用Java反射功能动态生成的。和java.lang.reflect.InvocationHandler结合，可以加强现有类的方法实现。如图2，图中的自定义Handler实现InvocationHandler接口，自定义Handler实例化时，将实现类传入自定义Handler对象。自定义Handler需要实现invoke方法，该方法可以使用Java反射调用实现类的实现的方法，同时当然可以实现其他功能，例如在调用实现类方法前后加入Log。而Proxy类根据Handler和需要代理的接口动态生成一个接口实现类的对象。当用户调用这个动态生成的实现类时，实际上是调用了自定义Handler的invoke方法。 
 



3．动态代理Servlet
              虽然Web Application Server的产品很多，但Servlet的处理原理是相似的：动态加载Servlet，调用Servlet的init方法（只被调用一次），并保存到Servlet容器；Servlet使用时，调用Servlet的service方法。本文动态代理Servlet接口，使其init和service被调用时会在控制台打出方法调用前后信息。
首先实现2个Servlet，DefaultServlet和UserServlet

package org.colimas.servlet;

import javax.servlet.Servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;

public class DefaultServlet extends HttpServlet implements Servlet {
              public void init() throws ServletException {
                            super.init();
                            System.out.println(DefaultServlet.class.getName()+":Running init");
              }

              public String getServletInfo() {
                            return DefaultServlet.class.getName();
              }
}

package org.colimas.servlet;

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


public class UserServlet extends HttpServlet implements Servlet {

              private static final long serialVersionUID = -7016554795165038652L;
             
              public void init() throws ServletException {
                            super.init();
                            System.out.println(UserServlet.class.getName()+":Running init");
              }
              protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
                                         
                            System.out.println(UserServlet.class.getName()+":Do UserSErvlet Get");
              }
              public String getServletInfo() {
                            return UserServlet.class.getName();
              }            
             

}

然后实现InvocationHandler
package org.colimas.webapp;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

import javax.servlet.Servlet;

public class ServletHandler implements InvocationHandler {

              private Servlet obj;
             
              public ServletHandler(Servlet obj){
                            this.obj=obj;
              }
              public Object invoke(Object arg0, Method arg1, Object[] arg2)
                                          throws Throwable {
                           
                            if(arg1.getName().compareTo("init")==0) //调用init时
                            {
                                          System.out.println(obj.getServletInfo()+":Init servlet starting..."); //增加控制台输出。
                                          arg1.invoke(obj,arg2); //调用init方法
                                          System.out.println(obj.getServletInfo()+":Init servlet ending..."); //增加控制台输出。
                            }else if(arg1.getName().compareTo("service")==0){ //调用service时
                                          System.out.println(obj.getServletInfo()+":service starting..."); //增加控制台输出。

                                          arg1.invoke(obj,arg2); //调用service方法。
                                          System.out.println(obj.getServletInfo()+":service ending..."); //增加控制台输出。
                           
                            }
                            return null;
              }

}

实现Servlet的调用
package org.colimas.webapp;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Proxy;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;

public class ServletWrapperImp {
   
    private Class servletClass;
    private ServletConfig config;
    private String _servletname;
    private Servlet _theServlet;
              private ServletContext context;
    public ServletWrapperImp(ServletConfig config){
           this.config=config;
           this._servletname=this.config.getServletName();
           this.context=this.config.getServletContext();
    }
   
    public Servlet getServlet() throws ServletException{

                              destroy();
                              try {
                                            WebAppClassLoader loader=new WebAppClassLoader(this.getClass().getClassLoader()); //自定义class loader
                                            String name=getServletName(); //从ServletConfig中获得Servlet Name
                                            synchronized (context) {
                                                          Servlet theServlet=context.getServlet(name); //在ServletContext中查找Servlet
                                                          if(theServlet==null){         //如果ServletContext没有。              
                                                                        servletClass = loader.loadClass(name); //由Class loader 加载Servlet class。
                                                              theServlet = (Servlet) servletClass.newInstance(); //Servlet实例化。
                                                              WebAppContext.addServlet(name,theServlet); //将Servlet实例存入ServletContext。
                                                                        InvocationHandler handler=new ServletHandler(theServlet); //自定义ServletHandler，参见ServletHandler类。
                                                                        _theServlet=(Servlet)Proxy.newProxyInstance(theServlet.getClass().getClassLoader(),
                                                                                   new Class[]{Servlet.class},handler); //代理类实例化。
                                                                        _theServlet.init(config); //Servlet代理对象调用init方法。参见ServletHandler的invoke方法。
                                                                       
                                                          }else{ //ServletContext里已存在。
                                                                        InvocationHandler handler=new ServletHandler(theServlet); //自定义ServletHandler，参见ServletHandler类。
                                                                        _theServlet=(Servlet)Proxy.newProxyInstance(theServlet.getClass().getClassLoader(),
                                                                                   new Class[]{Servlet.class},handler);                 //代理Servlet接口，动态生成代理类，并实例化。                                  
                                                          }
                                            }
                                            return _theServlet; //返回Servlet代理对象
                              } catch( ClassNotFoundException ex1 ) {
                                
                              } catch( InstantiationException ex ) {
                                 
                              }catch(IllegalAccessException ex2){
                                           
                              }
                      return null;
    }
    public void destroy() {
        if (_theServlet != null) {
                      _theServlet.destroy();
        }
    }   
   
    protected String getServletName(){
           return _servletname;
    }
}

其中的ServletConfig保存Servlet相关信息。ServletContext保存所有的Servlet对象。WebAppClassLoader为自定义class loader,参见http://blog.csdn.net/tyrone1979/archive/2006/09/03/1164262.aspx。

最后编写测试类Main，该类模拟10个用户访问Servlet，5人访问DefaultServlet，5人访问UserServlet。
package org.colimas.main;

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;

import org.colimas.webapp.HttpServletRequestWrapper;
import org.colimas.webapp.HttpServletResponseWrapper;
import org.colimas.webapp.ServletConfigImpl;
import org.colimas.webapp.ServletWrapper;
import org.colimas.webapp.ServletWrapperImp;
import org.colimas.webapp.WebAppContext;

public class Main {

              private ThreadGroup _threadGroup;
              private Thread[] _threads;
              String defaultServletName="org.colimas.servlet.DefaultServlet";
              String userServletName="org.colimas.servlet.UserServlet";
              WebAppContext context=WebAppContext.newInstance();     
             
              public void doStart(){
             
                            _threadGroup=new ThreadGroup("SERVLETS");
                            int i=0;
                            _threads=new ServletThread[10]; //模拟10位用户。
                            for(i=0;i<5;i++){
                                          _threads[i]=new ServletThread(_threadGroup,new Integer(i).toString(),
                                                                      defaultServletName);
                                          _threads[i].start();
                            }
                            for(i=5;i<10;i++){
                                          _threads[i]=new ServletThread(_threadGroup,new Integer(i).toString(),
                                                                      userServletName);
                                          _threads[i].start();
                            }                          
                           
              }
              /**
               * @param args
               */
              public static void main(String[] args) {
                            Main _main=new Main();
                            _main.doStart();
                           
              }
              //模拟用户线程
              private class ServletThread extends Thread{
                           
                            private String servletName;
                           
                            public ServletThread(ThreadGroup group,String threadname,String servletname){
                                          super(group,threadname);
                                          servletName=servletname;
                            }
                            //调用Servlet的service.
                            public void run() { //用户调用Servlet
                                          ServletConfig config=new ServletConfigImpl(servletName,context); //调用的Servlet信息。
                                          ServletWrapperImp wrapper=new ServletWrapperImp(config);
                                          try {
                                                        Servlet defaultServlet=wrapper.getServlet(); //获得Servlet对象，实际是Servlet的代理对象
                                                        defaultServlet.service(new HttpServletRequestWrapper(),
                                                                                    new HttpServletResponseWrapper()); 调用代理对象的service方法，参见ServletHandler的invoke方法。
                                          } catch (ServletException e) {
                                                                                                                e.printStackTrace();
                                          } catch(IOException e){
                                                       
                                          }
                            }
                           
              }
}
HttpServletRequestWrapper和HttpServletResponseWrapper实现HttpServletRequest，和HttpServletResponse。
测试结果如下
org.colimas.servlet.DefaultServlet:Init servlet starting...
org.colimas.servlet.DefaultServlet:Running init
org.colimas.servlet.DefaultServlet:Init servlet ending...
org.colimas.servlet.UserServlet:Init servlet starting...
org.colimas.servlet.UserServlet:Running init
org.colimas.servlet.UserServlet:Init servlet ending...
org.colimas.servlet.DefaultServlet:service starting...
org.colimas.servlet.DefaultServlet:service ending...
org.colimas.servlet.DefaultServlet:service starting...
org.colimas.servlet.DefaultServlet:service ending...
org.colimas.servlet.UserServlet:service starting...
org.colimas.servlet.UserServlet:Do UserSErvlet Get
org.colimas.servlet.UserServlet:service ending...
org.colimas.servlet.UserServlet:service starting...
org.colimas.servlet.UserServlet:Do UserSErvlet Get
org.colimas.servlet.UserServlet:service ending...
org.colimas.servlet.UserServlet:service starting...
org.colimas.servlet.UserServlet:Do UserSErvlet Get
org.colimas.servlet.UserServlet:service ending...
org.colimas.servlet.UserServlet:service starting...
org.colimas.servlet.DefaultServlet:service starting...
org.colimas.servlet.UserServlet:service starting...
org.colimas.servlet.DefaultServlet:service starting...
org.colimas.servlet.DefaultServlet:service starting...
org.colimas.servlet.UserServlet:Do UserSErvlet Get
org.colimas.servlet.DefaultServlet:service ending...
org.colimas.servlet.UserServlet:Do UserSErvlet Get
org.colimas.servlet.DefaultServlet:service ending...
org.colimas.servlet.DefaultServlet:service ending...
org.colimas.servlet.UserServlet:service ending...
org.colimas.servlet.UserServlet:service ending...

2个Servlet第一次Load时初始化，被调用init，之后保存到ServletContext中。第二次直接从ServletContext获得，执行service。红字表示代理类里增加的输出结果。

4．动态代理的限制
              JDK的动态代理并不能随心所欲的代理所有的类。Proxy.newProxyInstance方法的第二个参数只能是接口数组， 也就是Proxy只能代理接口。