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Proxy.javaAPI DocJava SE 5 API24840Fri Aug 26 14:57:06 BST 2005java.lang.reflect

Proxy

public class Proxy extends Object implements Serializable
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
1.20, 04/04/20
see
InvocationHandler
since
1.3

Fields Summary
private static final long
serialVersionUID
private static final String
proxyClassNamePrefix
prefix for all proxy class names
private static final Class[]
constructorParams
parameter types of a proxy class constructor
private static Map
loaderToCache
maps a class loader to the proxy class cache for that loader
private static Object
pendingGenerationMarker
marks that a particular proxy class is currently being generated
private static long
nextUniqueNumber
next number to use for generation of unique proxy class names
private static Object
nextUniqueNumberLock
private static Map
proxyClasses
set of all generated proxy classes, for isProxyClass implementation
protected InvocationHandler
h
the invocation handler for this proxy instance.
Constructors Summary
private Proxy()
Prohibits instantiation.
protected Proxy(InvocationHandler h)
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

	this.h = h;
Methods Summary
private static native java.lang.ClassdefineClass0(java.lang.ClassLoader loader, java.lang.String name, byte[] b, int off, int len)

public static java.lang.reflect.InvocationHandlergetInvocationHandler(java.lang.Object proxy)
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
	/*
	 * 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;
    
public static java.lang.ClassgetProxyClass(java.lang.ClassLoader loader, java.lang.Class interfaces)
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
	Class proxyClass = null;

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

	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");
	    }

	    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;
    
public static booleanisProxyClass(java.lang.Class cl)
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
	if (cl == null) {
	    throw new NullPointerException();
	}

	return proxyClasses.containsKey(cl);
    
public static java.lang.ObjectnewProxyInstance(java.lang.ClassLoader loader, java.lang.Class[] interfaces, java.lang.reflect.InvocationHandler h)
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
	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 {
	    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());
	}