簡(jiǎn)單討論JVM的class加載機(jī)制,給出兩個(gè)反射的例子代碼并分析工作原理,并給出了sun的動(dòng)態(tài)代理實(shí)現(xiàn)原理——代碼生成
JavaVM,反射與動(dòng)態(tài)代理
Java程序的工作機(jī)制:Java對(duì)象都以單獨(dú)的class文件存在,java虛擬機(jī)將其載入并執(zhí)行其虛擬機(jī)指令。
Java虛擬機(jī)查找這些java對(duì)象:
java虛擬機(jī)根據(jù)class path來(lái)查找java對(duì)象,而虛擬機(jī)的class path又分為三層:
bootstrap:sun.boot.class.path
extension: java.ext.dirs
application: java.class.path
三個(gè)class path各有對(duì)應(yīng)的classloader。由上而下形成父子關(guān)系
當(dāng)程序中調(diào)用new指令,或者ClassLoader.load方法時(shí)。其順序如下:
1. 首先查看application的classloader中是否已有對(duì)應(yīng)的class緩存,如果有則返回,并根據(jù)class分配內(nèi)存。如果沒(méi)有,接下一步。
2. 首先查看extension的classloader中是否已有對(duì)應(yīng)的class緩存,如果有則返回,并根據(jù)class分配內(nèi)存。如果沒(méi)有,接下一步。
3. 首先查看bootstrap的classloader中是否已有對(duì)應(yīng)的class緩存,如果有則返回,并根據(jù)class分配內(nèi)存。如果沒(méi)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,并返回。如果沒(méi)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,并返回。如果沒(méi)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,并返回。如果沒(méi)有,則拋出ClassNotFound的exception。
Java虛擬機(jī)加載這些java對(duì)象:
每個(gè)java虛擬機(jī)都在其啟動(dòng)時(shí)產(chǎn)生一個(gè)唯一的class heap,并把所有的class instance都分配在其中。其中每個(gè)類(lèi)實(shí)例的信息又分兩部分,fields域和methods域。每個(gè)類(lèi)實(shí)例各自擁有fields,但同一個(gè)類(lèi)的不同實(shí)例共享methods
反射
JVM對(duì)反射的處理
簡(jiǎn)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 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. } } } |
復(fù)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 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(jī)把每個(gè)methods當(dāng)作一個(gè)執(zhí)行單元。該執(zhí)行單元帶有兩種簽名:類(lèi)簽名和屬性簽名(public,static等)。 反射的第一步,驗(yàn)證簽名的合法性。驗(yàn)證通過(guò)后,順序執(zhí)行該method中的指令,當(dāng)需要訪問(wèn)類(lèi)實(shí)例的fields和傳入?yún)?shù)時(shí),由虛擬機(jī)注入。
動(dòng)態(tài)代理
Sun對(duì)動(dòng)態(tài)代理的說(shuō)明:
一個(gè)簡(jiǎn)單例子代碼:
動(dòng)態(tài)代理的內(nèi)部實(shí)現(xiàn)——代碼生成:
研究JDK源代碼,發(fā)現(xiàn)在Proxy的sun實(shí)現(xiàn)中調(diào)用了sun.misc.ProxyGenerator類(lèi)的generateProxyClass( proxyName, interfaces)方法,其返回值為byte[]和class文件的內(nèi)存類(lèi)型一致。于是做如下試驗(yàn):
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(yùn)行該類(lèi),到class文件夾下,利用反編譯技術(shù),發(fā)現(xiàn)原來(lái)其采用了代碼生產(chǎn)技術(shù):
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()); } } } |
from: http://www.cnblogs.com/fengye/archive/2007/02/18/652389.html