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authorupstream source tree <ports@midipix.org>2015-03-15 20:14:05 -0400
committerupstream source tree <ports@midipix.org>2015-03-15 20:14:05 -0400
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tree976dc5ab7fddf506dadce60ae936f43f58787092 /libjava/classpath/java/lang/reflect/InvocationHandler.java
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+/* 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. */
+
+
+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;
+
+}