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手动创徏一个新Class

dyerac in java... — Mon, 13 Aug 2007 10:52:00 GMT

要让Java�q�个面向“对象”的世界正常运作，创徏对象��是一��不可或�~�的操作�?br>
public class NewMain {
    public static void main(String[] args) {
        new Object();
    }
}

用javap反编译上面的代码�Q�我们可以得��C��面的指��o�Q�这里省��M��javac暗中创徏的构造函数�?br>
public class NewMain extends java.lang.Object{
    ...
public static void main(java.lang.String[]);
Code:
   0:   new     #3; //class java/lang/Object
   3:   invokespecial   #8; //Method java/lang/Object."":()V
   6:   return
}

从这�D�代码中�Q�我们可以清晰的看出创徏对象�Q�new�Q�和调用构造函敎ͼ�invokespecial�Q�两个过�E�。关于这个问题，我在�?a target=_blank>对象的生�?/font>》中曄��q�行�q�讨论�?br>
既然javac��一个new的动作被解释��Z��条指令，那在JVM的层面上�Q�我们当然就可以��它们分开。下面是一�D�|��什么实际用途的代码�Q�只是证明这个观点可行性�?br>
public class NewGenerator {
    public static void main(String[] args) throws Exception {
        String className = "New";
        ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
        cw.visit(Opcodes.V1_2, Opcodes.ACC_PUBLIC, className, null, "java/lang/Object", null);
        Method m = Method.getMethod("void main (String[])");
        GeneratorAdapter mg = new GeneratorAdapter(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC, m, null, null, cw);
        mg.newInstance(Type.getType(Object.class));
        Label label = mg.newLabel();
        mg.ifNonNull(label);
        mg.mark(label);
        mg.getStatic(Type.getType(System.class), "out", Type.getType(PrintStream.class));
        mg.push("new object is not null");
        mg.invokeVirtual(Type.getType(PrintStream.class), Method.getMethod("void println(java.lang.String)"));
        mg.pop();
        mg.returnValue();
        mg.endMethod();
        cw.visitEnd();

        OutputStream os = null;
        try {
            os = new FileOutputStream(className + ".class");
            os.write(cw.toByteArray());
        } finally {
            if (os != null) {
                os.close();
            }
        }
    }
}

�q�段代码生成的类是能够运行的�Q�有兴趣的可以自��p��一下。这�D�代码的作用是new��Z��个对象之后，如果�q�个对象非空的话�Q�就会��生一个输出：
    new object is not null

当然�Q�如果尝试用�q�个对象做一些其它的操作�Q�会有错误等待着我们�Q�因��个对象�ƈ不是一个完整的对象。在JVM规范中有相关的解释：new指��o�q�不能完整创建出一个新的对象，直到�Ҏ��初始化的对象调用了实例初始化�Ҏ��才会完成实例的创建。这�D�代码也正好�W�合�?a target=_blank>对象的生�?/font>》中的解释，它只是负责做出申请内存。当�Ӟ��在JVM中，它的实际工作要略多一些，如果�q�个对象的类没有加蝲�Q�就会加载相应的�c�R�?br>

dyerac in java... 2007-08-13 18:52 发表评论

动态代�?Dynamic Proxy�Q�Java Servlet

dyerac in java... — Tue, 24 Jul 2007 07:10:00 GMT

�q�篇文章讲的很好
基本上了解了动态代理的机制
��差写个代码实验一�?br>

------------------------------------------------------------------------------------------------------------------------

从JDK1.3开始，Java��引入了动态代理的概念。动态代理（Dynamic Proxy�Q�可以帮助你减少代码行数�Q�真正提高代码的可复用度。例如，你不必�ؓ所有的�cȝ��Ҏ��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�Ӟ��q�种便利是有条�g的。本文简单介�l�Java动态代理的原理�Q��ƈ实现一个被代理的Servlet创徏�Q�和调用的过�E��?

1�Q?/span>代理模式�Q�Proxy Pattern�Q?/div>

在JDK1.3以前�Q�代理模式就已流行，所以得代理模式是生成一个和�cȝ��同接口的代理�c�，用户通过使用代理�c�L��装某个实现�c�R��如�?�Q�其目的是加强实现类的某个方法的功能�Q�而不必改变原有的源代码�?/div>

2�Q?/span>动态代理（Dynamic Proxy�Q?/div>

随着Proxy的流行，Sun把它�U�_��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，��是动态代理中的代理类是由java.lang.reflect.Proxy�c�d��q�行期时�Ҏ��接口定义�Q�采用Java反射功能动态生成的。和java.lang.reflect.InvocationHandler�l�合�Q�可以加强现有类的方法实现。如�?�Q�图中的自定义Handler实现InvocationHandler接口�Q�自定义Handler实例化时�Q�将实现�c�M��入自定义Handler对象。自定义Handler需要实现invoke�Ҏ��Q�该�Ҏ��可以使用Java反射调用实现�cȝ��实现的方法，同时当然可以实现其他功能�Q�例如在调用实现�c�L��法前后加入Log。而Proxy�c�L��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�q�个动态生成的实现�c�L��Q�实际上是调用了自定义Handler的invoke�Ҏ��?/div>

3�Q�动态代理Servlet

虽然Web Application Server的��品很多，但Servlet的处理原理是�怼�的：动态加载Servlet�Q�调用Servlet的init�Ҏ��Q�只被调用一�ơ）�Q��ƈ保存到Servlet容器�Q�Servlet使用�Ӟ��调用Servlet的service�Ҏ��。本文动态代理Servlet接口�Q��其init和service被调用时会在控制台打出方法调用前后信息�?/div>

首先实现2个Servlet�Q�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�?/div>

{

System.out.println(obj.getServletInfo()+":Init servlet starting..."); //增加控制台输出�?/div>

arg1.invoke(obj,arg2); //调用init�Ҏ��

System.out.println(obj.getServletInfo()+":Init servlet ending..."); //增加控制台输出�?/div>

}else if(arg1.getName().compareTo("service")==0){ //调用service�?/div>

System.out.println(obj.getServletInfo()+":service starting..."); //增加控制台输出�?/div>

arg1.invoke(obj,arg2); //调用service�Ҏ��?/div>

System.out.println(obj.getServletInfo()+":service ending..."); //增加控制台输出�?/div>

}

return null;

}

实现Servlet的调�?/div>

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�?span>

servletClass = loader.loadClass(name); //由Class loader 加蝲Servlet class�?/div>

theServlet = (Servlet) servletClass.newInstance(); //Servlet实例化�?/div>

WebAppContext.addServlet(name,theServlet); //��Servlet实例存入ServletContext�?/div>

InvocationHandler handler=new ServletHandler(theServlet); //自定义ServletHandler�Q�参见ServletHandler�c�R�?/div>

_theServlet=(Servlet)Proxy.newProxyInstance(theServlet.getClass().getClassLoader(),

new Class[]{Servlet.class},handler); //代理�c�d��例化�?/div>

_theServlet.init(config); //Servlet代理对象调用init�Ҏ��。参见ServletHandler的invoke�Ҏ��?/div>

}else{ //ServletContext里已存在�?/div>

InvocationHandler handler=new ServletHandler(theServlet); //自定义ServletHandler�Q�参见ServletHandler�c�R�?/div>

_theServlet=(Servlet)Proxy.newProxyInstance(theServlet.getClass().getClassLoader(),

new Class[]{Servlet.class},handler); //代理Servlet接口�Q�动态生成代理类�Q��ƈ实例化�?span>

}

return _theServlet; //�q�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��定义class loader,参见http://blog.csdn.net/tyrone1979/archive/2006/09/03/1164262.aspx�?/div>

最后编写测试类Main�Q�该�c�L��?0个用戯��问Servlet�Q?��问DefaultServlet�Q?��问UserServlet�?/div>

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位用戗��?/div>

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�U�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�?/div>

ServletWrapperImp wrapper=new ServletWrapperImp(config);

try {

Servlet defaultServlet=wrapper.getServlet(); //获得Servlet对象�Q�实际是Servlet的代理对�?/div>

defaultServlet.service(new HttpServletRequestWrapper(),

new HttpServletResponseWrapper()); 调用代理对象的service�Ҏ��Q�参见ServletHandler的invoke�Ҏ��?/div>

} catch (ServletException e) {

e.printStackTrace();

} catch(IOException e){

}

HttpServletRequestWrapper和HttpServletResponseWrapper实现HttpServletRequest�Q�和HttpServletResponse�?/div>

��试�l�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...