java.lang.Object java.lang.reflect.Proxy
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:
A proxy instance has the following properties:
proxy instanceof Fooand the following cast operation will succeed (rather than throwing a ClassCastException):
(Foo) proxy
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.
Field Summary | |
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protected InvocationHandler |
h
the invocation handler for this proxy instance. |
Constructor Summary | |
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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 | |
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static InvocationHandler |
getInvocationHandler
(
Object
proxy) Returns the invocation handler for the specified proxy instance. |
static Class |
getProxyClass
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 |
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clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait |
Field Detail |
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protected InvocationHandler h
Constructor Detail |
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protected Proxy(InvocationHandler h)
Method Detail |
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public static ClassgetProxyClass(ClassLoader loader, Class... interfaces)[] interfaces)throws IllegalArgumentException
There are several restrictions on the parameters that may be passed to Proxy.getProxyClass:
Class.forName(i.getName(), false, cl) == i
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.
public static Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h) throws IllegalArgumentException
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.
public static boolean isProxyClass(Class cl)
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.
public static InvocationHandler getInvocationHandler(Object proxy) throws IllegalArgumentException