sshd-core/src/main/java/org/apache/sshd/common/util/EventListenerUtils.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied. See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.sshd.common.util;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.util.EventListener;


/**
 * @author <a href="mailto:dev@mina.apache.org">Apache MINA SSHD Project</a>
 */
public final class EventListenerUtils {
    private EventListenerUtils() {
        throw new UnsupportedOperationException("No instance");
    }

    /**
     * Provides proxy wrapper around an {@link Iterable} container of listener
     * interface implementation. <b>Note:</b> a listener interface is one whose
     * invoked methods return <u>only</u> {@code void}.
     *
     * @param <T>          Generic listener type
     * @param listenerType The expected listener <u>interface</u>
     * @param listeners    An {@link Iterable} container of listeners to be invoked.
     *                     <p>
     *                     <b>Note(s):</b>
     *                     </p>
     *                     <ul>
     *                     <li><p>
     *                     The invocation order is same as the {@link Iterable} container
     *                     </p></li>
     *
     *                     <li><p>
     *                     If any of the invoked listener methods throws an exception, the
     *                     rest of the listener are <u>not</u> invoked and the exception is
     *                     propagated to the caller
     *                     </p></li>
     *
     *                     <li><p>
     *                     It is up to the <u>caller</u> to ensure that the container does
     *                     not change while the proxy is invoked
     *                     </p></li>
     *                     </ul>
     * @return A proxy wrapper implementing the same interface, but delegating
     * the calls to the container
     * @see #proxyWrapper(Class, ClassLoader, Iterable)
     */
    public static <T extends EventListener> T proxyWrapper(Class<T> listenerType, Iterable<? extends T> listeners) {
        return proxyWrapper(listenerType, listenerType.getClassLoader(), listeners);
    }

    /**
     * Provides proxy wrapper around an {@link Iterable} container of listener
     * interface implementation. <b>Note:</b> a listener interface is one whose
     * invoked methods return <u>only</u> {@code void}.
     *
     * @param <T>          Generic listener type
     * @param listenerType The expected listener <u>interface</u>
     * @param loader       The {@link ClassLoader} to use for the proxy
     * @param listeners    An {@link Iterable} container of listeners to be invoked.
     *                     <p>
     *                     <b>Note(s):</b>
     *                     </p>
     *                     <ul>
     *                     <li><p>
     *                     The invocation order is same as the {@link Iterable} container
     *                     </p></li>
     *
     *                     <li><p>
     *                     If any of the invoked listener methods throws an exception, the
     *                     rest of the listener are <u>not</u> invoked and the exception is
     *                     propagated to the caller
     *                     </p></li>
     *
     *                     <li><p>
     *                     It is up to the <u>caller</u> to ensure that the container does
     *                     not change while the proxy is invoked
     *                     </p></li>
     *                     </ul>
     * @return A proxy wrapper implementing the same interface, but delegating
     * the calls to the container
     * @throws IllegalArgumentException if <tt>listenerType</tt> is not an interface
     *                                  or a {@code null} container has been provided
     * @see #proxyWrapper(Class, ClassLoader, Iterable)
     */
    public static <T extends EventListener> T proxyWrapper(Class<T> listenerType, ClassLoader loader, final Iterable<? extends T> listeners) {
        ValidateUtils.checkNotNull(listenerType, "No listener type specified");
        ValidateUtils.checkTrue(listenerType.isInterface(), "Target proxy is not an interface: %s", listenerType.getSimpleName());
        ValidateUtils.checkNotNull(listeners, "No listeners container provided");

        Object wrapper = Proxy.newProxyInstance(loader, new Class<?>[]{listenerType}, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                Throwable err = null;
                for (T l : listeners) {
                    try {
                        method.invoke(l, args);
                    } catch (Throwable t) {
                        Throwable e = GenericUtils.peelException(t);
                        err = GenericUtils.accumulateException(err, e);
                    }
                }

                if (err != null) {
                    throw err;
                }

                return null;    // we assume always void return value...
            }
        });
        return listenerType.cast(wrapper);
    }
}