原文地址：http://rejoy.iteye.com/blog/1627405
之前虽然会用JDK的动态代理，但是有些问题却一直没有搞明白。比如说：InvocationHandler的invoke方法是由谁来调用的，代理对象是怎么生成的，直到前几个星期才把这些问题全部搞明白了。
废话不多说了，先来看一下JDK的动态是怎么用的。
Java代码
package dynamic.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class MyInvocationHandler
implements InvocationHandler {
private Object target;
public MyInvocationHandler(Object target) {
super();
this.target = target; }
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable { System.out.println(
"------------------before------------------"); Object result = method.invoke(target, args); System.out.println(
"-------------------after------------------");
return result; }
public Object getProxy() {
return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), target.getClass().getInterfaces(),
this); } }
package dynamic.proxy;
public interface UserService {
public abstract void add(); }
package dynamic.proxy;
public class UserServiceImpl
implements UserService {
public void add() { System.out.println(
"--------------------add---------------"); } }
package dynamic.proxy;
import org.junit.Test;
public class ProxyTest {
@Test public void testProxy()
throws Throwable { UserService userService =
new UserServiceImpl(); MyInvocationHandler invocationHandler =
new MyInvocationHandler(userService); UserService proxy = (UserService) invocationHandler.getProxy(); proxy.add(); } }
package dynamic.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
/**
* 实现自己的InvocationHandler
* @author zyb
* @since 2012-8-9
*
*/
public class MyInvocationHandler implements InvocationHandler {
// 目标对象
private Object target;
/**
* 构造方法
* @param target 目标对象
*/
public MyInvocationHandler(Object target) {
super();
this.target = target;
}
/**
* 执行目标对象的方法
*/
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// 在目标对象的方法执行之前简单的打印一下
System.out.println("------------------before------------------");
// 执行目标对象的方法
Object result = method.invoke(target, args);
// 在目标对象的方法执行之后简单的打印一下
System.out.println("-------------------after------------------");
return result;
}
/**
* 获取目标对象的代理对象
* @return 代理对象
*/
public Object getProxy() {
return Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),
target.getClass().getInterfaces(), this);
}
}
package dynamic.proxy;
/**
* 目标对象实现的接口，用JDK来生成代理对象一定要实现一个接口
* @author zyb
* @since 2012-8-9
*
*/
public interface UserService {
/**
* 目标方法
*/
public abstract void add();
}
package dynamic.proxy;
/**
* 目标对象
* @author zyb
* @since 2012-8-9
*
*/
public class UserServiceImpl implements UserService {
/* (non-Javadoc)
* @see dynamic.proxy.UserService#add()
*/
public void add() {
System.out.println("--------------------add---------------");
}
}
package dynamic.proxy;
import org.junit.Test;
/**
* 动态代理测试类
* @author zyb
* @since 2012-8-9
*
*/
public class ProxyTest {
@Test
public void testProxy() throws Throwable {
// 实例化目标对象
UserService userService = new UserServiceImpl();
// 实例化InvocationHandler
MyInvocationHandler invocationHandler = new MyInvocationHandler(userService);
// 根据目标对象生成代理对象
UserService proxy = (UserService) invocationHandler.getProxy();
// 调用代理对象的方法
proxy.add();
}
}
执行结果如下：
------------------before------------------ --------------------add--------------- -------------------after------------------
用起来是很简单吧，其实这里基本上就是AOP的一个简单实现了，在目标对象的方法执行之前和执行之后进行了增强。Spring的AOP实现其实也是用了Proxy和InvocationHandler这两个东西的。
用起来是比较简单，但是如果能知道它背后做了些什么手脚，那就更好不过了。首先来看一下JDK是怎样生成代理对象的。既然生成代理对象是用的Proxy类的静态方newProxyInstance，那么我们就去它的源码里看一下它到底都做了些什么？
Java代码
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)
throws IllegalArgumentException {
if (h ==
null) {
throw new NullPointerException(); } Class cl = getProxyClass(loader, interfaces);
try { Constructor cons = cl.getConstructor(constructorParams);
return (Object) cons.newInstance(
new Object[] { h }); }
catch (NoSuchMethodException e) {
throw new InternalError(e.toString()); }
catch (IllegalAccessException e) {
throw new InternalError(e.toString()); }
catch (InstantiationException e) {
throw new InternalError(e.toString()); }
catch (InvocationTargetException e) {
throw new InternalError(e.toString()); } }
/**
* loader:类加载器
* interfaces:目标对象实现的接口
* h:InvocationHandler的实现类
*/
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
if (h == null) {
throw new NullPointerException();
}
/*
* Look up or generate the designated proxy class.
*/
Class cl = getProxyClass(loader, interfaces);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
// 调用代理对象的构造方法（也就是$Proxy0(InvocationHandler h)）
Constructor cons = cl.getConstructor(constructorParams);
// 生成代理类的实例并把MyInvocationHandler的实例传给它的构造方法
return (Object) cons.newInstance(new Object[] { h });
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
} catch (IllegalAccessException e) {
throw new InternalError(e.toString());
} catch (InstantiationException e) {
throw new InternalError(e.toString());
} catch (InvocationTargetException e) {
throw new InternalError(e.toString());
}
}
我们再进去getProxyClass方法看一下
Java代码
public static Class<?> getProxyClass(ClassLoader loader, Class<?>... interfaces)
throws IllegalArgumentException {
if (interfaces.length >
65535) {
throw new IllegalArgumentException(
"interface limit exceeded"); } Class proxyClass =
null; String[] interfaceNames =
new String[interfaces.length]; Set interfaceSet =
new HashSet();
for (
int i =
0; i < interfaces.length; i++) { String interfaceName = interfaces[i].getName(); Class interfaceClass =
null;
try { interfaceClass = Class.forName(interfaceName,
false, loader); }
catch (ClassNotFoundException e) { }
if (interfaceClass != interfaces[i]) {
throw new IllegalArgumentException( interfaces[i] +
" is not visible from class loader"); } interfaceSet.add(interfaceClass); interfaceNames[i] = interfaceName; } Object key = Arrays.asList(interfaceNames); Map cache;
synchronized (loaderToCache) { cache = (Map) loaderToCache.get(loader);
if (cache ==
null) { cache =
new HashMap(); loaderToCache.put(loader, cache); } }
synchronized (cache) {
do { Object value = cache.get(key);
if (value
instanceof Reference) { proxyClass = (Class) ((Reference) value).get(); }
if (proxyClass !=
null) {
return proxyClass; }
else if (value == pendingGenerationMarker) {
try { cache.wait(); }
catch (InterruptedException e) { }
continue; }
else { cache.put(key, pendingGenerationMarker);
break; } }
while (
true); }
try {
byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces);
try { proxyClass = defineClass0(loader, proxyName, proxyClassFile,
0, proxyClassFile.length); }
catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString()); } } proxyClasses.put(proxyClass,
null); }
return proxyClass; }
public static Class<?> getProxyClass(ClassLoader loader,
Class<?>... interfaces)
throws IllegalArgumentException
{
// 如果目标类实现的接口数大于65535个则抛出异常（我XX，谁会写这么NB的代码啊？）
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// 声明代理对象所代表的Class对象（有点拗口）
Class proxyClass = null;
String[] interfaceNames = new String[interfaces.length];
Set interfaceSet = new HashSet(); // for detecting duplicates
// 遍历目标类所实现的接口
for (int i = 0; i < interfaces.length; i++) {
// 拿到目标类实现的接口的名称
String interfaceName = interfaces[i].getName();
Class interfaceClass = null;
try {
// 加载目标类实现的接口到内存中
interfaceClass = Class.forName(interfaceName, false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != interfaces[i]) {
throw new IllegalArgumentException(
interfaces[i] + " is not visible from class loader");
}
// 中间省略了一些无关紧要的代码 .......
// 把目标类实现的接口代表的Class对象放到Set中
interfaceSet.add(interfaceClass);
interfaceNames[i] = interfaceName;
}
// 把目标类实现的接口名称作为缓存（Map）中的key
Object key = Arrays.asList(interfaceNames);
Map cache;
synchronized (loaderToCache) {
// 从缓存中获取cache
cache = (Map) loaderToCache.get(loader);
if (cache == null) {
// 如果获取不到，则新建地个HashMap实例
cache = new HashMap();
// 把HashMap实例和当前加载器放到缓存中
loaderToCache.put(loader, cache);
}
}
synchronized (cache) {
do {
// 根据接口的名称从缓存中获取对象
Object value = cache.get(key);
if (value instanceof Reference) {
proxyClass = (Class) ((Reference) value).get();
}
if (proxyClass != null) {
// 如果代理对象的Class实例已经存在，则直接返回
return proxyClass;
} else if (value == pendingGenerationMarker) {
try {
cache.wait();
} catch (InterruptedException e) {
}
continue;
} else {
cache.put(key, pendingGenerationMarker);
break;
}
} while (true);
}
try {
// 中间省略了一些代码 .......
// 这里就是动态生成代理对象的最关键的地方
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces);
try {
// 根据代理类的字节码生成代理类的实例
proxyClass = defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
// add to set of all generated proxy classes, for isProxyClass
proxyClasses.put(proxyClass, null);
}
// 中间省略了一些代码 .......
return proxyClass;
}
进去ProxyGenerator类的静态方法generateProxyClass，这里是真正生成代理类class字节码的地方。
Java代码
public static byte[] generateProxyClass(
final String name, Class[] interfaces) { ProxyGenerator gen =
new ProxyGenerator(name, interfaces);
final byte[] classFile = gen.generateClassFile();
if (saveGeneratedFiles) { java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<Void>() {
public Void run() {
try { FileOutputStream file =
new FileOutputStream(dotToSlash(name) +
".class"); file.write(classFile); file.close();
return null; }
catch (IOException e) {
throw new InternalError(
"I/O exception saving generated file: " + e); } } }); }
return classFile; }
public static byte[] generateProxyClass(final String name,
Class[] interfaces)
{
ProxyGenerator gen = new ProxyGenerator(name, interfaces);
// 这里动态生成代理类的字节码，由于比较复杂就不进去看了
final byte[] classFile = gen.generateClassFile();
// 如果saveGeneratedFiles的值为true，则会把所生成的代理类的字节码保存到硬盘上
if (saveGeneratedFiles) {
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<Void>() {
public Void run() {
try {
FileOutputStream file =
new FileOutputStream(dotToSlash(name) + ".class");
file.write(classFile);
file.close();
return null;
} catch (IOException e) {
throw new InternalError(
"I/O exception saving generated file: " + e);
}
}
});
}
// 返回代理类的字节码
return classFile;
}
现在，JDK是怎样动态生成代理类的字节的原理已经一目了然了。
好了，再来解决另外一个问题，那就是由谁来调用InvocationHandler的invoke方法的。要解决这个问题就要看一下JDK到底为我们生成了一个什么东西。用以下代码可以获取到JDK为我们生成的字节码并写到硬盘中。
Java代码
package dynamic.proxy;
import java.io.FileOutputStream;
import java.io.IOException;
import sun.misc.ProxyGenerator;
public class ProxyGeneratorUtils {
public static void writeProxyClassToHardDisk(String path) {
byte[] classFile = ProxyGenerator.generateProxyClass(
"$Proxy11", UserServiceImpl.
class.getInterfaces()); FileOutputStream out =
null;
try { out =
new FileOutputStream(path); out.write(classFile); out.flush(); }
catch (Exception e) { e.printStackTrace(); }
finally {
try { out.close(); }
catch (IOException e) { e.printStackTrace(); } } } }
package dynamic.proxy;
import org.junit.Test;
public class ProxyTest {
@Test public void testProxy()
throws Throwable { UserService userService =
new UserServiceImpl(); MyInvocationHandler invocationHandler =
new MyInvocationHandler(userService); UserService proxy = (UserService) invocationHandler.getProxy(); proxy.add(); }
@Test public void testGenerateProxyClass() { ProxyGeneratorUtils.writeProxyClassToHardDisk(
"F:/$Proxy11.class"); } }
package dynamic.proxy;
import java.io.FileOutputStream;
import java.io.IOException;
import sun.misc.ProxyGenerator;
/**
* 代理类的生成工具
* @author zyb
* @since 2012-8-9
*/
public class ProxyGeneratorUtils {
/**
* 把代理类的字节码写到硬盘上
* @param path 保存路径
*/
public static void writeProxyClassToHardDisk(String path) {
// 第一种方法，这种方式在刚才分析ProxyGenerator时已经知道了
// System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", true);
// 第二种方法
// 获取代理类的字节码
byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy11", UserServiceImpl.class.getInterfaces());
FileOutputStream out = null;
try {
out = new FileOutputStream(path);
out.write(classFile);
out.flush();
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
package dynamic.proxy;
import org.junit.Test;
/**
* 动态代理测试类
* @author zyb
* @since 2012-8-9
*
*/
public class ProxyTest {
@Test
public void testProxy() throws Throwable {
// 实例化目标对象
UserService userService = new UserServiceImpl();
// 实例化InvocationHandler
MyInvocationHandler invocationHandler = new MyInvocationHandler(userService);
// 根据目标对象生成代理对象
UserService proxy = (UserService) invocationHandler.getProxy();
// 调用代理对象的方法
proxy.add();
}
@Test
public void testGenerateProxyClass() {
ProxyGeneratorUtils.writeProxyClassToHardDisk("F:/$Proxy11.class");
}
}
通过以上代码，就可以在F盘上生成一个$Proxy.class文件了，现在用反编译工具来看一下这个class文件里面的内容。
Java代码
import dynamic.proxy.UserService;
import java.lang.reflect.*;
public final class $Proxy11
extends Proxy
implements UserService {
public $Proxy11(InvocationHandler invocationhandler) {
super(invocationhandler); }
public final boolean equals(Object obj) {
try {
return ((Boolean)
super.h.invoke(
this, m1,
new Object[] { obj })).booleanValue(); }
catch(Error _ex) { }
catch(Throwable throwable) {
throw new UndeclaredThrowableException(throwable); } }
public final void add() {
try {
super.h.invoke(
this, m3,
null);
return; }
catch(Error _ex) { }
catch(Throwable throwable) {
throw new UndeclaredThrowableException(throwable); } }
public final int hashCode() {
try {
return ((Integer)
super.h.invoke(
this, m0,
null)).intValue(); }
catch(Error _ex) { }
catch(Throwable throwable) {
throw new UndeclaredThrowableException(throwable); } }
public final String toString() {
try {
return (String)
super.h.invoke(
this, m2,
null); }
catch(Error _ex) { }
catch(Throwable throwable) {
throw new UndeclaredThrowableException(throwable); } }
private static Method m1;
private static Method m3;
private static Method m0;
private static Method m2;
static {
try { m1 = Class.forName(
"java.lang.Object").getMethod(
"equals",
new Class[] { Class.forName(
"java.lang.Object") }); m3 = Class.forName(
"dynamic.proxy.UserService").getMethod(
"add",
new Class[
0]); m0 = Class.forName(
"java.lang.Object").getMethod(
"hashCode",
new Class[
0]); m2 = Class.forName(
"java.lang.Object").getMethod(
"toString",
new Class[
0]); }
catch(NoSuchMethodException nosuchmethodexception) {
throw new NoSuchMethodError(nosuchmethodexception.getMessage()); }
catch(ClassNotFoundException classnotfoundexception) {
throw new NoClassDefFoundError(classnotfoundexception.getMessage()); } } }
// Decompiled by DJ v3.11.11.95 Copyright 2009 Atanas Neshkov Date: 2012/8/9 20:11:32
// Home Page: http://members.fortunecity.com/neshkov/dj.html http://www.neshkov.com/dj.html - Check often for new version!
// Decompiler options: packimports(3)
import dynamic.proxy.UserService;
import java.lang.reflect.*;
public final class $Proxy11 extends Proxy
implements UserService
{
// 构造方法，参数就是刚才传过来的MyInvocationHandler类的实例
public $Proxy11(InvocationHandler invocationhandler)
{
super(invocationhandler);
}
public final boolean equals(Object obj)
{
try
{
return ((Boolean)super.h.invoke(this, m1, new Object[] {
obj
})).booleanValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
/**
* 这个方法是关键部分
*/
public final void add()
{
try
{
// 实际上就是调用MyInvocationHandler的public Object invoke(Object proxy, Method method, Object[] args)方法，第二个问题就解决了
super.h.invoke(this, m3, null);
return;
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final int hashCode()
{
try
{
return ((Integer)super.h.invoke(this, m0, null)).intValue();
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
public final String toString()
{
try
{
return (String)super.h.invoke(this, m2, null);
}
catch(Error _ex) { }
catch(Throwable throwable)
{
throw new UndeclaredThrowableException(throwable);
}
}
private static Method m1;
private static Method m3;
private static Method m0;
private static Method m2;
// 在静态代码块中获取了4个方法：Object中的equals方法、UserService中的add方法、Object中的hashCode方法、Object中toString方法
static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] {
Class.forName("java.lang.Object")
});
m3 = Class.forName("dynamic.proxy.UserService").getMethod("add", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
}
catch(NoSuchMethodException nosuchmethodexception)
{
throw new NoSuchMethodError(nosuchmethodexception.getMessage());
}
catch(ClassNotFoundException classnotfoundexception)
{
throw new NoClassDefFoundError(classnotfoundexception.getMessage());
}
}
}
好了，到目前为止，前面 的两个问题都已经知道回事了，现在再用JDK动态代理的时候就不只会用而已了，真正的达到了“知其然，知其所以然”的目的。。。
就写到这了，累死了。。
