001    /* java.lang.reflect.InvocationHandler - dynamically executes methods in
002       proxy instances
003       Copyright (C) 2001 Free Software Foundation, Inc.
004    
005    This file is part of GNU Classpath.
006    
007    GNU Classpath is free software; you can redistribute it and/or modify
008    it under the terms of the GNU General Public License as published by
009    the Free Software Foundation; either version 2, or (at your option)
010    any later version.
011    
012    GNU Classpath is distributed in the hope that it will be useful, but
013    WITHOUT ANY WARRANTY; without even the implied warranty of
014    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
015    General Public License for more details.
016    
017    You should have received a copy of the GNU General Public License
018    along with GNU Classpath; see the file COPYING.  If not, write to the
019    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
020    02110-1301 USA.
021    
022    Linking this library statically or dynamically with other modules is
023    making a combined work based on this library.  Thus, the terms and
024    conditions of the GNU General Public License cover the whole
025    combination.
026    
027    As a special exception, the copyright holders of this library give you
028    permission to link this library with independent modules to produce an
029    executable, regardless of the license terms of these independent
030    modules, and to copy and distribute the resulting executable under
031    terms of your choice, provided that you also meet, for each linked
032    independent module, the terms and conditions of the license of that
033    module.  An independent module is a module which is not derived from
034    or based on this library.  If you modify this library, you may extend
035    this exception to your version of the library, but you are not
036    obligated to do so.  If you do not wish to do so, delete this
037    exception statement from your version. */
038    
039    
040    package java.lang.reflect;
041    
042    /**
043     * This interface defines an invocation handler.  Suppose you are using
044     * reflection, and found a method that requires that its parameter
045     * be an object of a given interface.  You want to call this method,
046     * but have no idea what classes implement that interface.  So, you can
047     * create a {@link Proxy} instance, a convenient way to dynamically
048     * generate a class that meets all the necessary properties of that
049     * interface.  But in order for the proxy instance to do any good, it
050     * needs to know what to do when interface methods are invoked!  So,
051     * this interface is basically a cool wrapper that provides runtime
052     * code generation needed by proxy instances.
053     *
054     * <p>While this interface was designed for use by Proxy, it will also
055     * work on any object in general.</p>
056     *
057     * <p>Hints for implementing this class:</p>
058     * 
059     * <ul>
060     * <li>Don't forget that Object.equals, Object.hashCode, and
061     *     Object.toString will call this handler.  In particular,
062     *     a naive call to proxy.equals, proxy.hashCode, or proxy.toString
063     *     will put you in an infinite loop.  And remember that string
064     *     concatenation also invokes toString.</li>
065     * <li>Obey the contract of the Method object you are handling, or
066     *     the proxy instance will be forced to throw a
067     *     {@link NullPointerException}, {@link ClassCastException},
068     *     or {@link UndeclaredThrowableException}.</li>
069     * <li>Be prepared to wrap/unwrap primitives as necessary.</li>
070     * <li>The Method object may be owned by a different interface than
071     *     what was actually used as the qualifying type of the method
072     *     invocation in the Java source code. This means that it might
073     *     not always be safe to throw an exception listed as belonging
074     *     to the method's throws clause.</li>
075     * </ul>
076     *
077     * <p><small>For a fun time, create an InvocationHandler that handles the
078     * methods of a proxy instance of the InvocationHandler interface!</small></p>
079     *
080     * @see Proxy
081     * @see UndeclaredThrowableException
082     *
083     * @author Eric Blake (ebb9@email.byu.edu)
084     * @since 1.3
085     * @status updated to 1.4
086     */
087    public interface InvocationHandler
088    {
089      /**
090       * When a method is invoked on a proxy instance, it is wrapped and
091       * this method is called instead, so that you may decide at runtime
092       * how the original method should behave.
093       *
094       * @param proxy the instance that the wrapped method should be
095       *        invoked on.  When this method is called by a Proxy object,
096       *        `proxy' will be an instance of {@link Proxy}, and oddly enough,
097       *        <code>Proxy.getInvocationHandler(proxy)</code> will return
098       *        <code>this</code>!
099       * @param method the reflected method to invoke on the proxy.
100       *        When this method is called by a Proxy object, 'method'
101       *        will be the reflection object owned by the declaring
102       *        class or interface, which may be a supertype of the
103       *        interfaces the proxy directly implements.
104       * @param args the arguments passed to the original method, or
105       *        <code>null</code> if the method takes no arguments.
106       *        (But also be prepared to handle a 0-length array).
107       *        Arguments of primitive type, such as <code>boolean</code>
108       *        or <code>int</code>, are wrapped in the appropriate
109       *        class such as {@link Boolean} or {@link Integer}.
110       * @return whatever is necessary to return from the wrapped method.
111       *         If the wrapped method is <code>void</code>, the proxy
112       *         instance will ignore it.  If the wrapped method returns
113       *         a primitive, this must be the correct wrapper type whose value
114       *         is exactly assignable to the appropriate type (no widening
115       *         will be performed); a null object in this case causes a
116       *         {@link NullPointerException}.  In all remaining cases, if
117       *         the returned object is not assignment compatible to the
118       *         declared type of the original method, the proxy instance
119       *         will generate a {@link ClassCastException}.
120       * @throws Throwable this interface is listed as throwing anything,
121       *         but the implementation should only throw unchecked
122       *         exceptions and exceptions listed in the throws clause of
123       *         all methods being overridden by the proxy instance.  If
124       *         something is thrown that is not compatible with the throws
125       *         clause of all overridden methods, the proxy instance will
126       *         wrap the exception in an UndeclaredThrowableException.
127       *         Note that an exception listed in the throws clause of the
128       *         `method' parameter might not be declared in additional
129       *         interfaces also implemented by the proxy object.
130       *
131       * @see Proxy
132       * @see UndeclaredThrowableException
133       */
134      Object invoke(Object proxy, Method method, Object[] args)
135        throws Throwable;
136    
137    }