JavaVM,反射与动态代理

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JavaVM,反射与动态代理

 

Java程序的工作机制:Java对象都以单独的class文件存在,java虚拟机将其载入并执行其虚拟机指令。

 

Java虚拟机查找这些java对象:

java虚拟机根据class path来查找java对象,而虚拟机的class path又分为三层:

bootstrap:sun.boot.class.path

extension: java.ext.dirs

application: java.class.path

三个class path各有对应的classloader。由上而下形成父子关系

当程序中调用new指令,或者ClassLoader.load方法时。其顺序如下:

1.       首先查看application的classloader中是否已有对应的class缓存,如果有则返回,并根据class分配内存。如果没有,接下一步。

2.       首先查看extension的classloader中是否已有对应的class缓存,如果有则返回,并根据class分配内存。如果没有,接下一步。

3.       首先查看bootstrap的classloader中是否已有对应的class缓存,如果有则返回,并根据class分配内存。如果没有,接下一步。

4.       由bootstrap的classloader在其class path中试图加载该class,如果有,则将该class放入cache中,并返回。如果没有,接下一步。

5.       由extension的classloader在其class path中试图加载该class,如果有,则将该class放入cache中,并返回。如果没有,接下一步。

6.       由application的classloader在其class path中试图加载该class,如果有,则将该class放入cache中,并返回。如果没有,则抛出ClassNotFound的exception。

 

Java虚拟机加载这些java对象:

每个java虚拟机都在其启动时产生一个唯一的class heap,并把所有的class instance都分配在其中。其中每个类实例的信息又分两部分,fields域和methods域。每个类实例各自拥有fields,但同一个类的不同实例共享methods

 

反射

JVM对反射的处理

简单例子代码:

import java.lang.reflect.InvocationHandler;

import java.lang.reflect.Method;

import java.lang.reflect.InvocationTargetException;

import java.io.IOException;

 

public class Main {

    public static void main(String[] args){

        TempImpl t1 = new TempImpl("temp1");

        try {

            Method t1Talk = t1.getClass().getMethod("Talk", new Class[0]) ;

            t1Talk.invoke(t1, null);

        } catch (NoSuchMethodException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        } catch (IllegalAccessException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        } catch (InvocationTargetException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        }

        try {

            System.in.read();

        } catch (IOException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        }

    }

}

复杂例子代码:

import java.lang.reflect.InvocationHandler;

import java.lang.reflect.Method;

import java.lang.reflect.InvocationTargetException;

import java.io.IOException;

 

public class Main {

    public static void main(String[] args){

        TempImpl t1 = new TempImpl("temp1");

        TempImpl t2 = new TempImpl("temp2");

        Temp2 temp2 = new Temp2();

        try {

            Method t1Talk = t1.getClass().getMethod("Talk", new Class[0]) ;

            Method t2Talk = t2.getClass().getMethod("Talk", new Class[0]) ;

            t1Talk.invoke(t2, null);

            t2Talk.invoke(t1, null);

            if(t1Talk.equals(t2Talk)){

                System.out.println("equals");

            }

           else{

                System.out.println("not equals");

            }

            if(t1Talk==t2Talk){

                System.out.println("ref equals");

            }

           else{

                System.out.println("ref not equals");

            }

            t2Talk.invoke(temp2, null);

        } catch (NoSuchMethodException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        } catch (IllegalAccessException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        } catch (InvocationTargetException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        }

        try {

            System.in.read();

        } catch (IOException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        }

    }

}

 

分析:java虚拟机把每个methods当作一个执行单元。该执行单元带有两种签名:类签名和属性签名(public,static等)。 反射的第一步,验证签名的合法性。验证通过后,顺序执行该method中的指令,当需要访问类实例的fields和传入参数时,由虚拟机注入。

 

动态代理

Sun对动态代理的说明:

一个简单例子代码:

动态代理的内部实现——代码生成:

研究JDK源代码,发现在Proxy的sun实现中调用了sun.misc.ProxyGenerator类的generateProxyClass( proxyName, interfaces)方法,其返回值为byte[]和class文件的内存类型一致。于是做如下试验:

public class  ProxyClassFile{

       public static void main(String[] args){

              String proxyName = "TempProxy";

        TempImpl t = new TempImpl("proxy");

              Class[] interfaces =t.getClass().getInterfaces();

             

              byte[] proxyClassFile = ProxyGenerator.generateProxyClass(

                  proxyName, interfaces);

        File f = new File("classes/TempProxy.class");

        try {

            FileOutputStream fos = new FileOutputStream(f);

            fos.write(proxyClassFile);

            fos.flush();

            fos.close();

        } catch (FileNotFoundException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        } catch (IOException e) {

            e.printStackTrace();  //To change body of catch statement use File | Settings | File Templates.

        }

       }

}

运行该类,到class文件夹下,利用反编译技术,发现原来其采用了代码生产技术:

 

public interface Temp{

       public void Talk();

       public void Run();

}

import java.lang.reflect.*;

 

public final class TempProxy extends Proxy

    implements Temp{

 

    private static Method m4;

    private static Method m2;

    private static Method m0;

    private static Method m3;

    private static Method m1;

 

    public TempProxy(InvocationHandler invocationhandler)   {

        super(invocationhandler);

    }

 

    public final void Run()    {

        try {

            h.invoke(this, m4, null);

            return;

        }

        catch(Error _ex) { }

        catch(Throwable throwable)  {

            throw new UndeclaredThrowableException(throwable);

        }

    }

 

    public final String toString(){

        try{

            return (String)h.invoke(this, m2, null);

        }

        catch(Error _ex) { }

        catch(Throwable throwable)  {

            throw new UndeclaredThrowableException(throwable);

        }

        return "";

    }

 

    public final int hashCode() {

        try {

            return ((Integer)h.invoke(this, m0, null)).intValue();

        }

        catch(Error _ex) { }

        catch(Throwable throwable){

            throw new UndeclaredThrowableException(throwable);

        }

        return 123;

    }

 

    public final void Talk(){

        try{

            h.invoke(this, m3, null);

            return;

        }

        catch(Error _ex) { }

        catch(Throwable throwable) {

            throw new UndeclaredThrowableException(throwable);

        }

    }

 

    public final boolean equals(Object obj) {

        try  {

            return ((Boolean)h.invoke(this, m1, new Object[] {

                obj

            })).booleanValue();

        }

        catch(Error _ex) { }

        catch(Throwable throwable) {

            throw new UndeclaredThrowableException(throwable);

        }

        return false;

    }

 

    static{

        try{

     m4 = Class.forName("Temp").getMethod("Run", new Class[0]);

     m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);

     m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);

     m3 = Class.forName("Temp").getMethod("Talk", new Class[0]);

     m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {

                Class.forName("java.lang.Object")

            });

        }

        catch(NoSuchMethodException nosuchmethodexception) {

            throw new NoSuchMethodError(nosuchmethodexception.getMessage());

        }

        catch(ClassNotFoundException classnotfoundexception) {

            throw new NoClassDefFoundError(classnotfoundexception.getMessage());

        }

    }

}

 

 

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