| 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
InvocationHandler.invokeinvoke 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 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.newProxyInstanceProxy.newInstance method, which combines the actions of calling
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
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
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 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:
1.8, 00/02/02
See Also: InvocationHandler
since:
JDK1.3 |
