gcc/libjava/java/lang/reflect/InvocationHandler.java

/* 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.

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along with GNU Classpath; see the file COPYING.  If not, write to the
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02111-1307 USA.

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
executable, regardless of the license terms of these independent
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independent module, the terms and conditions of the license of that
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this exception to your version of the library, but you are not
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exception statement from your version. */


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.
 *
 * <p>While this interface was designed for use by Proxy, it will also
 * work on any object in general.</p>
 *
 * <p>Hints for implementing this class:</p>
 * 
 * <ul>
 * <li>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.</li>
 * <li>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}.</li>
 * <li>Be prepared to wrap/unwrap primitives as necessary.</li>
 * <li>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.</li>
 * </ul>
 *
 * <p><small>For a fun time, create an InvocationHandler that handles the
 * methods of a proxy instance of the InvocationHandler interface!</small></p>
 *
 * @see Proxy
 * @see UndeclaredThrowableException
 *
 * @author Eric Blake (ebb9@email.byu.edu)
 * @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,
   *        <code>Proxy.getInvocationHandler(proxy)</code> will return
   *        <code>this</code>!
   * @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
   *        <code>null</code> if the method takes no arguments.
   *        (But also be prepared to handle a 0-length array).
   *        Arguments of primitive type, such as <code>boolean</code>
   *        or <code>int</code>, 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 <code>void</code>, 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;

}