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jlechner |
/* java.lang.reflect.InvocationHandler - dynamically executes methods in
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proxy instances
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Copyright (C) 2001 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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02110-1301 USA.
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package java.lang.reflect;
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/**
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* This interface defines an invocation handler. Suppose you are using
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* reflection, and found a method that requires that its parameter
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* be an object of a given interface. You want to call this method,
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* but have no idea what classes implement that interface. So, you can
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* create a {@link Proxy} instance, a convenient way to dynamically
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* generate a class that meets all the necessary properties of that
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* interface. But in order for the proxy instance to do any good, it
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* needs to know what to do when interface methods are invoked! So,
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* this interface is basically a cool wrapper that provides runtime
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* code generation needed by proxy instances.
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*
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* <p>While this interface was designed for use by Proxy, it will also
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* work on any object in general.</p>
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*
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* <p>Hints for implementing this class:</p>
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*
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* <ul>
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* <li>Don't forget that Object.equals, Object.hashCode, and
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* Object.toString will call this handler. In particular,
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* a naive call to proxy.equals, proxy.hashCode, or proxy.toString
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* will put you in an infinite loop. And remember that string
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* concatenation also invokes toString.</li>
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* <li>Obey the contract of the Method object you are handling, or
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* the proxy instance will be forced to throw a
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* {@link NullPointerException}, {@link ClassCastException},
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* or {@link UndeclaredThrowableException}.</li>
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* <li>Be prepared to wrap/unwrap primitives as necessary.</li>
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* <li>The Method object may be owned by a different interface than
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* what was actually used as the qualifying type of the method
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* invocation in the Java source code. This means that it might
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* not always be safe to throw an exception listed as belonging
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* to the method's throws clause.</li>
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* </ul>
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*
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* <p><small>For a fun time, create an InvocationHandler that handles the
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* methods of a proxy instance of the InvocationHandler interface!</small></p>
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*
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* @see Proxy
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* @see UndeclaredThrowableException
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*
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* @author Eric Blake (ebb9@email.byu.edu)
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* @since 1.3
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* @status updated to 1.4
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*/
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public interface InvocationHandler
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{
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/**
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* When a method is invoked on a proxy instance, it is wrapped and
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* this method is called instead, so that you may decide at runtime
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* how the original method should behave.
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*
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* @param proxy the instance that the wrapped method should be
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* invoked on. When this method is called by a Proxy object,
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* `proxy' will be an instance of {@link Proxy}, and oddly enough,
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* <code>Proxy.getInvocationHandler(proxy)</code> will return
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* <code>this</code>!
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* @param method the reflected method to invoke on the proxy.
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* When this method is called by a Proxy object, 'method'
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* will be the reflection object owned by the declaring
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* class or interface, which may be a supertype of the
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* interfaces the proxy directly implements.
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* @param args the arguments passed to the original method, or
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* <code>null</code> if the method takes no arguments.
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* (But also be prepared to handle a 0-length array).
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* Arguments of primitive type, such as <code>boolean</code>
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* or <code>int</code>, are wrapped in the appropriate
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* class such as {@link Boolean} or {@link Integer}.
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* @return whatever is necessary to return from the wrapped method.
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* If the wrapped method is <code>void</code>, the proxy
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* instance will ignore it. If the wrapped method returns
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* a primitive, this must be the correct wrapper type whose value
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* is exactly assignable to the appropriate type (no widening
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* will be performed); a null object in this case causes a
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* {@link NullPointerException}. In all remaining cases, if
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* the returned object is not assignment compatible to the
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* declared type of the original method, the proxy instance
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* will generate a {@link ClassCastException}.
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* @throws Throwable this interface is listed as throwing anything,
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* but the implementation should only throw unchecked
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* exceptions and exceptions listed in the throws clause of
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* all methods being overridden by the proxy instance. If
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* something is thrown that is not compatible with the throws
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* clause of all overridden methods, the proxy instance will
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* wrap the exception in an UndeclaredThrowableException.
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* Note that an exception listed in the throws clause of the
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* `method' parameter might not be declared in additional
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* interfaces also implemented by the proxy object.
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*
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* @see Proxy
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* @see UndeclaredThrowableException
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*/
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Object invoke(Object proxy, Method method, Object[] args)
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throws Throwable;
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}
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