JDK动态代理实现原理

地址：http://rejoy.iteye.com/blog/1627405

jdk动态代理之所以只能代理接口是因为代理类本身已经extends了Proxy，而java是不允许多重继承的，但是允许实现多个接口，因此才有cglib的需要吧

jdk的代理是利用反射生成字节码，并生成对象，
cglib是直接修改目标类的字节码生成对象，所以性能+

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;  
   
 /** 
  * 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  

 /** 
  * 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  
     {  
     // 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);  
     // 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() {  
                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;  
   
 /** 
  * 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  

 // 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。。

附上核心类Proxy源码：

/*
 * Copyright (c) 2006, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.lang.reflect;

import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
import java.security.AccessController;
import java.security.Permission;
import java.security.PrivilegedAction;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.WeakHashMap;
import sun.misc.ProxyGenerator;
import sun.reflect.Reflection;
import sun.reflect.misc.ReflectUtil;
import sun.security.util.SecurityConstants;

/**
 * Proxy provides static methods for creating dynamic proxy
 * classes and instances, and it is also the superclass of all
 * dynamic proxy classes created by those methods.
 *
 * To create a proxy for some interface Foo:
 * 
 *     InvocationHandler handler = new MyInvocationHandler(...);
 *     Class proxyClass = Proxy.getProxyClass(
 *         Foo.class.getClassLoader(), new Class[] { Foo.class });
 *     Foo f = (Foo) proxyClass.
 *         getConstructor(new Class[] { InvocationHandler.class }).
 *         newInstance(new Object[] { handler });
 * 
 * or more simply:
 *  *     Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(),
 *                                          new Class[] { Foo.class },
 *                                          handler);
 * 
 *
 * A dynamic proxy class (simply referred to as a proxy
 * class below) is a class that implements a list of interfaces
 * specified at runtime when the class is created, with behavior as
 * described below.
 *
 * A proxy interface is such an interface that is implemented
 * by a proxy class.
 *
 * A proxy instance is an instance of a proxy class.
 *
 * Each proxy instance has an associated invocation handler
 * object, which implements the interface {@link InvocationHandler}.
 * A method invocation on a proxy instance through one of its proxy
 * interfaces will be dispatched to the {@link InvocationHandler#invoke
 * invoke} method of the instance's invocation handler, passing the proxy
 * instance, a java.lang.reflect.Method object identifying
 * the method that was invoked, and an array of type Object
 * containing the arguments.  The invocation handler processes the
 * encoded method invocation as appropriate and the result that it
 * returns will be returned as the result of the method invocation on
 * the proxy instance.
 *
 * 
A proxy class has the following properties:
 *
 * 

 * Proxy classes are public, final, and not abstract.
 *
 * 
The unqualified name of a proxy class is unspecified.  The space
 * of class names that begin with the string "$Proxy"
 * should be, however, reserved for proxy classes.
 *
 * 
A proxy class extends java.lang.reflect.Proxy.
 *
 * 
A proxy class implements exactly the interfaces specified at its
 * creation, in the same order.
 *
 * 
If a proxy class implements a non-public interface, then it will
 * be defined in the same package as that interface.  Otherwise, the
 * package of a proxy class is also unspecified.  Note that package
 * sealing will not prevent a proxy class from being successfully defined
 * in a particular package at runtime, and neither will classes already
 * defined by the same class loader and the same package with particular
 * signers.
 *
 * 
Since a proxy class implements all of the interfaces specified at
 * its creation, invoking getInterfaces on its
 * Class object will return an array containing the same
 * list of interfaces (in the order specified at its creation), invoking
 * getMethods on its Class object will return
 * an array of Method objects that include all of the
 * methods in those interfaces, and invoking getMethod will
 * find methods in the proxy interfaces as would be expected.
 *
 * 
The {@link Proxy#isProxyClass Proxy.isProxyClass} method will
 * return true if it is passed a proxy class-- a class returned by
 * Proxy.getProxyClass or the class of an object returned by
 * Proxy.newProxyInstance-- and false otherwise.
 *
 * 
The java.security.ProtectionDomain of a proxy class
 * is the same as that of system classes loaded by the bootstrap class
 * loader, such as java.lang.Object, because the code for a
 * proxy class is generated by trusted system code.  This protection
 * domain will typically be granted
 * java.security.AllPermission.
 *
 * 
Each proxy class has one public constructor that takes one argument,
 * an implementation of the interface {@link InvocationHandler}, to set
 * the invocation handler for a proxy instance.  Rather than having to use
 * the reflection API to access the public constructor, a proxy instance
 * can be also be created by calling the {@link Proxy#newProxyInstance
 * Proxy.newInstance} method, which combines the actions of calling
 * {@link Proxy#getProxyClass Proxy.getProxyClass} with invoking the
 * constructor with an invocation handler.
 * 
 *
 * A proxy instance has the following properties:
 *
 * 

 * Given a proxy instance proxy and one of the
 * interfaces implemented by its proxy class Foo, the
 * following expression will return true:
 *  *     proxy instanceof Foo
 * 
 * and the following cast operation will succeed (rather than throwing
 * a ClassCastException):
 *  *     (Foo) proxy
 * 
 *
 * 
Each proxy instance has an associated invocation handler, the one
 * that was passed to its constructor.  The static
 * {@link Proxy#getInvocationHandler Proxy.getInvocationHandler} method
 * will return the invocation handler associated with the proxy instance
 * passed as its argument.
 *
 * 
An interface method invocation on a proxy instance will be
 * encoded and dispatched to the invocation handler's {@link
 * InvocationHandler#invoke invoke} method as described in the
 * documentation for that method.
 *
 * 
An invocation of the hashCode,
 * equals, or toString methods declared in
 * java.lang.Object on a proxy instance will be encoded and
 * dispatched to the invocation handler's invoke method in
 * the same manner as interface method invocations are encoded and
 * dispatched, as described above.  The declaring class of the
 * Method object passed to invoke will be
 * java.lang.Object.  Other public methods of a proxy
 * instance inherited from java.lang.Object are not
 * overridden by a proxy class, so invocations of those methods behave
 * like they do for instances of java.lang.Object.
 * 
 *
 * Methods Duplicated in Multiple Proxy Interfaces
 *
 * When two or more interfaces of a proxy class contain a method with
 * the same name and parameter signature, the order of the proxy class's
 * interfaces becomes significant.  When such a duplicate method
 * is invoked on a proxy instance, the Method object passed
 * to the invocation handler will not necessarily be the one whose
 * declaring class is assignable from the reference type of the interface
 * that the proxy's method was invoked through.  This limitation exists
 * because the corresponding method implementation in the generated proxy
 * class cannot determine which interface it was invoked through.
 * Therefore, when a duplicate method is invoked on a proxy instance,
 * the Method object for the method in the foremost interface
 * that contains the method (either directly or inherited through a
 * superinterface) in the proxy class's list of interfaces is passed to
 * the invocation handler's invoke method, regardless of the
 * reference type through which the method invocation occurred.
 *
 * 
If a proxy interface contains a method with the same name and
 * parameter signature as the hashCode, equals,
 * or toString methods of java.lang.Object,
 * when such a method is invoked on a proxy instance, the
 * Method object passed to the invocation handler will have
 * java.lang.Object as its declaring class.  In other words,
 * the public, non-final methods of java.lang.Object
 * logically precede all of the proxy interfaces for the determination of
 * which Method object to pass to the invocation handler.
 *
 * 
Note also that when a duplicate method is dispatched to an
 * invocation handler, the invoke method may only throw
 * checked exception types that are assignable to one of the exception
 * types in the throws clause of the method in all of
 * the proxy interfaces that it can be invoked through.  If the
 * invoke method throws a checked exception that is not
 * assignable to any of the exception types declared by the method in one
 * of the proxy interfaces that it can be invoked through, then an
 * unchecked UndeclaredThrowableException will be thrown by
 * the invocation on the proxy instance.  This restriction means that not
 * all of the exception types returned by invoking
 * getExceptionTypes on the Method object
 * passed to the invoke method can necessarily be thrown
 * successfully by the invoke method.
 *
 * @author	Peter Jones
 * @version	%I%, %E%
 * @see		InvocationHandler
 * @since	1.3
 */
public class Proxy implements java.io.Serializable {

    private static final long serialVersionUID = -2222568056686623797L;

    /** prefix for all proxy class names */
    private final static String proxyClassNamePrefix = "$Proxy";

    /** parameter types of a proxy class constructor */
    private final static Class[] constructorParams =
	{ InvocationHandler.class };

    /** maps a class loader to the proxy class cache for that loader */
    private static Map loaderToCache = new WeakHashMap();

    /** marks that a particular proxy class is currently being generated */
    private static Object pendingGenerationMarker = new Object();

    /** next number to use for generation of unique proxy class names */
    private static long nextUniqueNumber = 0;
    private static Object nextUniqueNumberLock = new Object();

    /** set of all generated proxy classes, for isProxyClass implementation */
    private static Map proxyClasses =
	Collections.synchronizedMap(new WeakHashMap());

    /**
     * the invocation handler for this proxy instance.
     * @serial
     */
    protected InvocationHandler h;

    /**
     * Prohibits instantiation.
     */
    private Proxy() {
    }

    /**
     * Constructs a new Proxy instance from a subclass
     * (typically, a dynamic proxy class) with the specified value
     * for its invocation handler.
     *
     * @param   h the invocation handler for this proxy instance
     */
    protected Proxy(InvocationHandler h) {
        doNewInstanceCheck();
	this.h = h;
    }

    private static class ProxyAccessHelper {
        // The permission is implementation specific.
        static final Permission PROXY_PERMISSION =
            new ReflectPermission("proxyConstructorNewInstance");
        // These system properties are defined to provide a short-term
        // workaround if customers need to disable the new security checks.
        static final boolean allowNewInstance;
        static final boolean allowNullLoader;
        static {
            allowNewInstance = getBooleanProperty("sun.reflect.proxy.allowsNewInstance");
            allowNullLoader = getBooleanProperty("sun.reflect.proxy.allowsNullLoader");
        }
 
        private static boolean getBooleanProperty(final String key) {
            String s = AccessController.doPrivileged(new PrivilegedAction() {
                public String run() {
                    return System.getProperty(key);
                }
            });
            return Boolean.valueOf(s);
        }
 
        static boolean needsNewInstanceCheck(Class proxyClass) {
            if (!Proxy.isProxyClass(proxyClass) || allowNewInstance) {
                return false;
            }
 
            if (proxyClass.getName().startsWith(ReflectUtil.PROXY_PACKAGE + ".")) {
                // all proxy interfaces are public
                return false;
            }
            for (Class intf : proxyClass.getInterfaces()) {
                if (!Modifier.isPublic(intf.getModifiers())) {
                    return true;
                }
            }
            return false;
        }
    }
 
    /*
     * Access check on a proxy class that implements any non-public interface.
     *
     * @throws  SecurityException if a security manager exists, and
     *          the caller does not have the permission.
     */
    private void doNewInstanceCheck() {
        SecurityManager sm = System.getSecurityManager();
        Class proxyClass = this.getClass();
        if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(proxyClass)) {
            try {
                sm.checkPermission(ProxyAccessHelper.PROXY_PERMISSION);
            } catch (SecurityException e) {
                throw new SecurityException("Not allowed to construct a Proxy "
                        + "instance that implements a non-public interface", e);
            }
        }
    }

    /**
     * Returns the java.lang.Class object for a proxy class
     * given a class loader and an array of interfaces.  The proxy class
     * will be defined by the specified class loader and will implement
     * all of the supplied interfaces.  If a proxy class for the same
     * permutation of interfaces has already been defined by the class
     * loader, then the existing proxy class will be returned; otherwise,
     * a proxy class for those interfaces will be generated dynamically
     * and defined by the class loader.
     *
     * 
There are several restrictions on the parameters that may be
     * passed to Proxy.getProxyClass:
     *
     * 

     * All of the Class objects in the
     * interfaces array must represent interfaces, not
     * classes or primitive types.
     *
     * 
No two elements in the interfaces array may
     * refer to identical Class objects.
     *
     * 
All of the interface types must be visible by name through the
     * specified class loader.  In other words, for class loader
     * cl and every interface i, the following
     * expression must be true:
     *      *     Class.forName(i.getName(), false, cl) == i
     * 
     *
     * 
All non-public interfaces must be in the same package;
     * otherwise, it would not be possible for the proxy class to
     * implement all of the interfaces, regardless of what package it is
     * defined in.
     *
     * 
For any set of member methods of the specified interfaces
     * that have the same signature:
     * 
     * If the return type of any of the methods is a primitive
     * type or void, then all of the methods must have that same
     * return type.
     * 
Otherwise, one of the methods must have a return type that
     * is assignable to all of the return types of the rest of the
     * methods.
     * 
     *
     * 
The resulting proxy class must not exceed any limits imposed
     * on classes by the virtual machine.  For example, the VM may limit
     * the number of interfaces that a class may implement to 65535; in
     * that case, the size of the interfaces array must not
     * exceed 65535.
     * 
     *
     * If any of these restrictions are violated,
     * Proxy.getProxyClass will throw an
     * IllegalArgumentException.  If the interfaces
     * array argument or any of its elements are null, a
     * NullPointerException will be thrown.
     *
     * 
Note that the order of the specified proxy interfaces is
     * significant: two requests for a proxy class with the same combination
     * of interfaces but in a different order will result in two distinct
     * proxy classes.
     *
     * @param	loader the class loader to define the proxy class
     * @param	interfaces the list of interfaces for the proxy class
     *		to implement
     * @return	a proxy class that is defined in the specified class loader
     *		and that implements the specified interfaces
     * @throws	IllegalArgumentException if any of the restrictions on the
     *		parameters that may be passed to getProxyClass
     *		are violated
     * @throws	NullPointerException if the interfaces array
     *		argument or any of its elements are null
     */
    public static Class getProxyClass(ClassLoader loader, 
                                         Class... interfaces)
	throws IllegalArgumentException
    {
        return getProxyClass0(loader, interfaces); // stack walk magic: do not refactor
    }

    private static void checkProxyLoader(ClassLoader ccl,
                                         ClassLoader loader)
    {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            if (loader == null && ccl != null) {
                if (!ProxyAccessHelper.allowNullLoader) {
                    sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
                }
            }
        }
    }
 
    /*
     * Generate a proxy class (caller-sensitive).
     *
     * To define a proxy class, it performs the access checks as in
     * Class.forName (VM will invoke ClassLoader.checkPackageAccess):
     * 1. "getClassLoader" permission check if loader == null
     * 2. checkPackageAccess on the interfaces it implements
     *
     * To get a constructor and new instance of a proxy class, it performs
     * the package access check on the interfaces it implements
     * as in Class.getConstructor.
     *
     * If an interface is non-public, the proxy class must be defined by
     * the defining loader of the interface.  If the caller's class loader
     * is not the same as the defining loader of the interface, the VM
     * will throw IllegalAccessError when the generated proxy class is
     * being defined via the defineClass0 method.
     */
    private static Class getProxyClass0(ClassLoader loader,
                                           Class... interfaces) {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            final int CALLER_FRAME = 3; // 0: Reflection, 1: getProxyClass0 2: Proxy 3: caller
            final Class caller = Reflection.getCallerClass(CALLER_FRAME);
            final ClassLoader ccl = caller.getClassLoader();
            checkProxyLoader(ccl, loader);
            ReflectUtil.checkProxyPackageAccess(ccl, interfaces);
        }
        if (interfaces.length > 65535) {
	    throw new IllegalArgumentException("interface limit exceeded");
	}

	Class proxyClass = null;

	/* collect interface names to use as key for proxy class cache */
	String[] interfaceNames = new String[interfaces.length];

	Set interfaceSet = new HashSet();	// for detecting duplicates

	for (int i = 0; i < interfaces.length; i++) {
	    /*
	     * Verify that the class loader resolves the name of this
	     * interface to the same Class object.
	     */
	    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");
	    }

	    /*
	     * Verify that the Class object actually represents an
	     * interface.
	     */
	    if (!interfaceClass.isInterface()) {
		throw new IllegalArgumentException(
		    interfaceClass.getName() + " is not an interface");
	    }

	    /*
	     * Verify that this interface is not a duplicate.
	     */
	    if (interfaceSet.contains(interfaceClass)) {
		throw new IllegalArgumentException(
		    "repeated interface: " + interfaceClass.getName());
	    }
	    interfaceSet.add(interfaceClass);

	    interfaceNames[i] = interfaceName;
	}

	/*
	 * Using string representations of the proxy interfaces as
	 * keys in the proxy class cache (instead of their Class
	 * objects) is sufficient because we require the proxy
	 * interfaces to be resolvable by name through the supplied
	 * class loader, and it has the advantage that using a string
	 * representation of a class makes for an implicit weak
	 * reference to the class.
	 */
	Object key = Arrays.asList(interfaceNames);

	/*
	 * Find or create the proxy class cache for the class loader.
	 */
	Map cache;
	synchronized (loaderToCache) {
	    cache = (Map) loaderToCache.get(loader);
	    if (cache == null) {
		cache = new HashMap();
		loaderToCache.put(loader, cache);
	    }
	    /*
	     * This mapping will remain valid for the duration of this
	     * method, without further synchronization, because the mapping
	     * will only be removed if the class loader becomes unreachable.
	     */
	}

	/*
	 * Look up the list of interfaces in the proxy class cache using
	 * the key.  This lookup will result in one of three possible
	 * kinds of values:
	 *     null, if there is currently no proxy class for the list of
	 *         interfaces in the class loader,
	 *     the pendingGenerationMarker object, if a proxy class for the
	 *         list of interfaces is currently being generated,
	 *     or a weak reference to a Class object, if a proxy class for
	 *         the list of interfaces has already been generated.
	 */
	synchronized (cache) {
	    /*
	     * Note that we need not worry about reaping the cache for
	     * entries with cleared weak references because if a proxy class
	     * has been garbage collected, its class loader will have been
	     * garbage collected as well, so the entire cache will be reaped
	     * from the loaderToCache map.
	     */
	    do {
		Object value = cache.get(key);
		if (value instanceof Reference) {
		    proxyClass = (Class) ((Reference) value).get();
		}
		if (proxyClass != null) {
		    // proxy class already generated: return it
		    return proxyClass;
		} else if (value == pendingGenerationMarker) {
		    // proxy class being generated: wait for it
		    try {
			cache.wait();
		    } catch (InterruptedException e) {
			/*
			 * The class generation that we are waiting for should
			 * take a small, bounded time, so we can safely ignore
			 * thread interrupts here.
			 */
		    }
		    continue;
		} else {
		    /*
		     * No proxy class for this list of interfaces has been
		     * generated or is being generated, so we will go and
		     * generate it now.  Mark it as pending generation.
		     */
		    cache.put(key, pendingGenerationMarker);
		    break;
		}
	    } while (true);
	}

	try {
	    String proxyPkg = null;	// package to define proxy class in

	    /*
	     * Record the package of a non-public proxy interface so that the
	     * proxy class will be defined in the same package.  Verify that
	     * all non-public proxy interfaces are in the same package.
	     */
	    for (int i = 0; i < interfaces.length; i++) {
		int flags = interfaces[i].getModifiers();
		if (!Modifier.isPublic(flags)) {
		    String name = interfaces[i].getName();
		    int n = name.lastIndexOf('.');
		    String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
		    if (proxyPkg == null) {
			proxyPkg = pkg;
		    } else if (!pkg.equals(proxyPkg)) {
			throw new IllegalArgumentException(
			    "non-public interfaces from different packages");
		    }
		}
	    }

            if (proxyPkg == null) {
                // if no non-public proxy interfaces, use com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
	    }

	    {
		/*
		 * Choose a name for the proxy class to generate.
		 */
		long num;
		synchronized (nextUniqueNumberLock) {
		    num = nextUniqueNumber++;
		}
		String proxyName = proxyPkg + proxyClassNamePrefix + num;
		/*
		 * Verify that the class loader hasn't already
		 * defined a class with the chosen name.
		 */

		/*
		 * Generate the specified proxy class.
		 */
		byte[] proxyClassFile =	ProxyGenerator.generateProxyClass(
		    proxyName, interfaces);
		try {
		    proxyClass = defineClass0(loader, proxyName,
			proxyClassFile, 0, proxyClassFile.length);
		} catch (ClassFormatError e) {
		    /*
		     * A ClassFormatError here means that (barring bugs in the
		     * proxy class generation code) there was some other
		     * invalid aspect of the arguments supplied to the proxy
		     * class creation (such as virtual machine limitations
		     * exceeded).
		     */
		    throw new IllegalArgumentException(e.toString());
		}
	    }
	    // add to set of all generated proxy classes, for isProxyClass
	    proxyClasses.put(proxyClass, null);

	} finally {
	    /*
	     * We must clean up the "pending generation" state of the proxy
	     * class cache entry somehow.  If a proxy class was successfully
	     * generated, store it in the cache (with a weak reference);
	     * otherwise, remove the reserved entry.  In all cases, notify
	     * all waiters on reserved entries in this cache.
	     */
	    synchronized (cache) {
		if (proxyClass != null) {
		    cache.put(key, new WeakReference(proxyClass));
		} else {
		    cache.remove(key);
		}
		cache.notifyAll();
	    }
	}
	return proxyClass;
    }

    /**
     * Returns an instance of a proxy class for the specified interfaces
     * that dispatches method invocations to the specified invocation
     * handler.  This method is equivalent to:
     * 
     *     Proxy.getProxyClass(loader, interfaces).
     *         getConstructor(new Class[] { InvocationHandler.class }).
     *         newInstance(new Object[] { handler });
     * 
     *
     * Proxy.newProxyInstance throws
     * IllegalArgumentException for the same reasons that
     * Proxy.getProxyClass does.
     *
     * @param	loader the class loader to define the proxy class
     * @param	interfaces the list of interfaces for the proxy class
     *		to implement
     * @param   h the invocation handler to dispatch method invocations to
     * @return	a proxy instance with the specified invocation handler of a
     *		proxy class that is defined by the specified class loader
     *		and that implements the specified interfaces
     * @throws	IllegalArgumentException if any of the restrictions on the
     *		parameters that may be passed to getProxyClass
     *		are violated
     * @throws	NullPointerException if the interfaces array
     *		argument or any of its elements are null, or
     *		if the invocation handler, h, is
     *		null
     */
    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 = getProxyClass0(loader, interfaces); // stack walk magic: do not refactor

	/*
	 * Invoke its constructor with the designated invocation handler.
	 */
	try {
            final Constructor cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            SecurityManager sm = System.getSecurityManager();
            if (sm != null && ProxyAccessHelper.needsNewInstanceCheck(cl)) {
                // create proxy instance with doPrivilege as the proxy class may
                // implement non-public interfaces that requires a special permission
                return AccessController.doPrivileged(new PrivilegedAction

JDK动态代理实现原理

Methods Duplicated in Multiple Proxy Interfaces

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