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JVM,反射與動態代理

來源:互聯網  2007-02-01 20:11:06  評論

JavaVM,反射與動態代理

Java程序的工作機制:Java對象都以單獨的class文件存在,java虛擬機將其載入並執行其虛擬機指令。

Java虛擬機查找這些java對象:

java虛擬機根據class path來查找java對象,而虛擬機的class path又分爲三層:

bootstrap:sun.boot.class.path

extension: java.ext.dirs

application: java.class.path

三個class path各有對應的classloader。由上而下形成父子關系

當程序中調用new指令,或者ClassLoader.load方法時。其順序如下:

1. 首先查看application的classloader中是否已有對應的class緩存,如果有則返回,並根據class分配內存。如果沒有,接下一步。

2. 首先查看extension的classloader中是否已有對應的class緩存,如果有則返回,並根據class分配內存。如果沒有,接下一步。

3. 首先查看bootstrap的classloader中是否已有對應的class緩存,如果有則返回,並根據class分配內存。如果沒有,接下一步。

4. 由bootstrap的classloader在其class path中試圖加載該class,如果有,則將該class放入cache中,並返回。如果沒有,接下一步。

5. 由extension的classloader在其class path中試圖加載該class,如果有,則將該class放入cache中,並返回。如果沒有,接下一步。

6. 由application的classloader在其class path中試圖加載該class,如果有,則將該class放入cache中,並返回。如果沒有,則抛出ClassNotFound的exception。

Java虛擬機加載這些java對象:

每個java虛擬機都在其啓動時産生一個唯一的class heap,並把所有的class instance都分配在其中。其中每個類實例的信息又分兩部分,fields域和methods域。每個類實例各自擁有fields,但同一個類的不同實例共享methods

反射

JVM對反射的處理

簡單例子代碼:

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.

}

}

}

複雜例子代碼:

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虛擬機把每個methods當作一個執行單元。該執行單元帶有兩種簽名:類簽名和屬性簽名(public,static等)。 反射的第一步,驗證簽名的合法性。驗證通過後,順序執行該method中的指令,當需要訪問類實例的fields和傳入參數時,由虛擬機注入。

動態代理

Sun對動態代理的說明:

一個簡單例子代碼:

動態代理的內部實現——代碼生成:

研究JDK源代碼,發現在Proxy的sun實現中調用了sun.misc.ProxyGenerator類的generateProxyClass( proxyName, interfaces)方法,其返回值爲byte[]和class文件的內存類型一致。于是做如下試驗:

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.

}

}

}

運行該類,到class文件夾下,利用反編譯技術,發現原來其采用了代碼生産技術:

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());

}

}

}

JavaVM,反射與動態代理 Java程序的工作機制:Java對象都以單獨的class文件存在,java虛擬機將其載入並執行其虛擬機指令。 Java虛擬機查找這些java對象: java虛擬機根據class path來查找java對象,而虛擬機的class path又分爲三層: bootstrap:sun.boot.class.path extension: java.ext.dirs application: java.class.path 三個class path各有對應的classloader。由上而下形成父子關系 當程序中調用new指令,或者ClassLoader.load方法時。其順序如下: 1. 首先查看application的classloader中是否已有對應的class緩存,如果有則返回,並根據class分配內存。如果沒有,接下一步。 2. 首先查看extension的classloader中是否已有對應的class緩存,如果有則返回,並根據class分配內存。如果沒有,接下一步。 3. 首先查看bootstrap的classloader中是否已有對應的class緩存,如果有則返回,並根據class分配內存。如果沒有,接下一步。 4. 由bootstrap的classloader在其class path中試圖加載該class,如果有,則將該class放入cache中,並返回。如果沒有,接下一步。 5. 由extension的classloader在其class path中試圖加載該class,如果有,則將該class放入cache中,並返回。如果沒有,接下一步。 6. 由application的classloader在其class path中試圖加載該class,如果有,則將該class放入cache中,並返回。如果沒有,則抛出ClassNotFound的exception。 Java虛擬機加載這些java對象: 每個java虛擬機都在其啓動時産生一個唯一的class heap,並把所有的class instance都分配在其中。其中每個類實例的信息又分兩部分,fields域和methods域。每個類實例各自擁有fields,但同一個類的不同實例共享methods 反射 JVM對反射的處理 簡單例子代碼: 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. } } } 複雜例子代碼: 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虛擬機把每個methods當作一個執行單元。該執行單元帶有兩種簽名:類簽名和屬性簽名(public,static等)。 反射的第一步,驗證簽名的合法性。驗證通過後,順序執行該method中的指令,當需要訪問類實例的fields和傳入參數時,由虛擬機注入。 動態代理 Sun對動態代理的說明: 一個簡單例子代碼: 動態代理的內部實現——代碼生成: 研究JDK源代碼,發現在Proxy的sun實現中調用了sun.misc.ProxyGenerator類的generateProxyClass( proxyName, interfaces)方法,其返回值爲byte[]和class文件的內存類型一致。于是做如下試驗: 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. } } } 運行該類,到class文件夾下,利用反編譯技術,發現原來其采用了代碼生産技術: 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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