JavaTM 2 Platform Std. Ed. v1.6.0
java.lang.reflect
Class Proxy
java.lang.Object
java.lang.reflect.Proxy
- All Implemented Interfaces:
- Serializable
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 InvocationHandler .
A method invocation on a proxy instance through one of its proxy
interfaces will be dispatched to the 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
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
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 Proxy.newInstance method, which combines the actions of calling
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
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
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.
- Since:
- 1.3
- See Also:
InvocationHandler ,
Serialized Form
Field Summary |
protected InvocationHandler |
h
the invocation handler for this proxy instance. |
Constructor Summary |
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. |
Method Summary |
static InvocationHandler |
getInvocationHandler(Object proxy)
Returns the invocation handler for the specified proxy instance. |
static Class<?> |
getProxyClass(ClassLoader loader,
Class<?>... interfaces)
Returns the java.lang.Class object for a proxy class
given a class loader and an array of interfaces. |
static boolean |
isProxyClass(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. |
static Object |
newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
Returns an instance of a proxy class for the specified interfaces
that dispatches method invocations to the specified invocation
handler. |
Methods inherited from class java.lang.Object |
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
h
protected InvocationHandler h
- the invocation handler for this proxy instance.
Proxy
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.
- Parameters:
h - the invocation handler for this proxy instance
getProxyClass
public static Class<?> getProxyClass(ClassLoader loader,
Class<?>... interfaces)
throws IllegalArgumentException
- 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 :
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.
- Parameters:
loader - the class loader to define the proxy classinterfaces - the list of interfaces for the proxy class
to implement
- Returns:
- 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
NullPointerException - if the interfaces array
argument or any of its elements are null
newProxyInstance
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
- 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.
- Parameters:
loader - the class loader to define the proxy classinterfaces - the list of interfaces for the proxy class
to implementh - the invocation handler to dispatch method invocations to
- Returns:
- 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
NullPointerException - if the interfaces array
argument or any of its elements are null , or
if the invocation handler, h , is
null
isProxyClass
public static boolean isProxyClass(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 .
- Parameters:
cl - the class to test
- Returns:
true if the class is a proxy class and
false otherwise
- Throws:
NullPointerException - if cl is null
getInvocationHandler
public static InvocationHandler getInvocationHandler(Object proxy)
throws IllegalArgumentException
- Returns the invocation handler for the specified proxy instance.
- Parameters:
proxy - the proxy instance to return the invocation handler for
- Returns:
- the invocation handler for the proxy instance
- Throws:
IllegalArgumentException - if the argument is not a
proxy instance
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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