java Proxy源码解读

为了搞清楚动态代理中是如何使用java 反射的，特别的查看了reflect类中的Proxy的源码。下面将源码贴出，里面会写上自己对代码的理解。

/*

 * @(#)Proxy.java 1.22 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */


package java.lang.reflect;


import java.lang.ref.Reference;
import java.lang.ref.WeakReference;
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;


/**
 * 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 1.22, 05/11/17
 * @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();

/**这里的weakHashMap是map的一个弱引用，其主要是为了垃圾回收器能够对不使用的对象进行回收**/
    /** 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

      * 该handler为其代理的主要处理对象。其中代理的方法也是交给该接口中的invoke来进行处理的

     */
    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) {
this.h = h;
    }


    /**
     * 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
    {
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

      //获得所有的接口，将接口转成class，并且放入set集合中
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,
proxyPkg = ""; // use the unnamed 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 = getProxyClass(loader, interfaces);


/*
* Invoke its constructor with the designated invocation handler.
*/

try {

//反射的用途，通过构造函数创建该对象的实例

   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());
}
    }


    /**
     * Returns true if and only if the specified class was dynamically
     * generated to be a proxy class using the getProxyClass
     * method or the newProxyInstance method.
     *
     *

The reliability of this method is important for the ability
     * to use it to make security decisions, so its implementation should
     * not just test if the class in question extends Proxy.
     *
     * @param cl the class to test
     * @return  true if the class is a proxy class and
     * false otherwise
     * @throws NullPointerException if cl is null
     */
    public static boolean isProxyClass(Class cl) {
if (cl == null) {
   throw new NullPointerException();
}


return proxyClasses.containsKey(cl);
    }


    /**
     * Returns the invocation handler for the specified proxy instance.
     *
     * @param proxy the proxy instance to return the invocation handler for
     * @return the invocation handler for the proxy instance
     * @throws IllegalArgumentException if the argument is not a
     * proxy instance
     */
    public static InvocationHandler getInvocationHandler(Object proxy)
throws IllegalArgumentException
    {
/*
* Verify that the object is actually a proxy instance.
*/
if (!isProxyClass(proxy.getClass())) {
   throw new IllegalArgumentException("not a proxy instance");
}


Proxy p = (Proxy) proxy;
return p.h;
    }


    private static native Class defineClass0(ClassLoader loader, String name,
    byte[] b, int off, int len);
}