Interface InvocationHandler

Known Implementing Classes:
CompositeDataInvocationHandler, EventHandler, MBeanServerInvocationHandler, RemoteObjectInvocationHandler

public interface InvocationHandler

This interface defines an invocation handler. Suppose you are using reflection, and found a method that requires that its parameter be an object of a given interface. You want to call this method, but have no idea what classes implement that interface. So, you can create a Proxy instance, a convenient way to dynamically generate a class that meets all the necessary properties of that interface. But in order for the proxy instance to do any good, it needs to know what to do when interface methods are invoked! So, this interface is basically a cool wrapper that provides runtime code generation needed by proxy instances.

While this interface was designed for use by Proxy, it will also work on any object in general.

Hints for implementing this class:

For a fun time, create an InvocationHandler that handles the methods of a proxy instance of the InvocationHandler interface!

See Also:
Proxy, UndeclaredThrowableException

Method Summary

invoke(Object proxy, Method method, Object[] args)
When a method is invoked on a proxy instance, it is wrapped and this method is called instead, so that you may decide at runtime how the original method should behave.

Method Details


public Object invoke(Object proxy,
                     Method method,
                     Object[] args)
            throws Throwable
When a method is invoked on a proxy instance, it is wrapped and this method is called instead, so that you may decide at runtime how the original method should behave.
proxy - the instance that the wrapped method should be invoked on. When this method is called by a Proxy object, `proxy' will be an instance of Proxy, and oddly enough, Proxy.getInvocationHandler(proxy) will return this!
method - the reflected method to invoke on the proxy. When this method is called by a Proxy object, 'method' will be the reflection object owned by the declaring class or interface, which may be a supertype of the interfaces the proxy directly implements.
args - the arguments passed to the original method, or null if the method takes no arguments. (But also be prepared to handle a 0-length array). Arguments of primitive type, such as boolean or int, are wrapped in the appropriate class such as Boolean or Integer.
whatever is necessary to return from the wrapped method. If the wrapped method is void, the proxy instance will ignore it. If the wrapped method returns a primitive, this must be the correct wrapper type whose value is exactly assignable to the appropriate type (no widening will be performed); a null object in this case causes a NullPointerException. In all remaining cases, if the returned object is not assignment compatible to the declared type of the original method, the proxy instance will generate a ClassCastException.
Throwable - this interface is listed as throwing anything, but the implementation should only throw unchecked exceptions and exceptions listed in the throws clause of all methods being overridden by the proxy instance. If something is thrown that is not compatible with the throws clause of all overridden methods, the proxy instance will wrap the exception in an UndeclaredThrowableException. Note that an exception listed in the throws clause of the `method' parameter might not be declared in additional interfaces also implemented by the proxy object.

java.lang.reflect.InvocationHandler - dynamically executes methods in proxy instances Copyright (C) 2001 Free Software Foundation, Inc. This file is part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; see the file COPYING. If not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version.