/* 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
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Linking this library statically or dynamically with other modules is
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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package java.lang.reflect;
/**
* 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 {@link 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:
*
* - Don't forget that Object.equals, Object.hashCode, and
* Object.toString will call this handler. In particular,
* a naive call to proxy.equals, proxy.hashCode, or proxy.toString
* will put you in an infinite loop. And remember that string
* concatenation also invokes toString.
* - Obey the contract of the Method object you are handling, or
* the proxy instance will be forced to throw a
* {@link NullPointerException}, {@link ClassCastException},
* or {@link UndeclaredThrowableException}.
* - Be prepared to wrap/unwrap primitives as necessary.
* - The Method object may be owned by a different interface than
* what was actually used as the qualifying type of the method
* invocation in the Java source code. This means that it might
* not always be safe to throw an exception listed as belonging
* to the method's throws clause.
*
*
* For a fun time, create an InvocationHandler that handles the
* methods of a proxy instance of the InvocationHandler interface!
*
* @see Proxy
* @see UndeclaredThrowableException
*
* @author Eric Blake
* @since 1.3
* @status updated to 1.4
*/
public interface InvocationHandler
{
/**
* 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.
*
* @param 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 {@link Proxy}, and oddly enough,
* Proxy.getInvocationHandler(proxy) will return
* this!
* @param 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.
* @param 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 {@link Boolean} or {@link Integer}.
* @return 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
* {@link 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 {@link ClassCastException}.
* @throws 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.
*
* @see Proxy
* @see UndeclaredThrowableException
*/
Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}