/*
 * Copyright (C) 2000-2013 Heinz Max Kabutz
 *
 * See the NOTICE file distributed with this work for additional
 * information regarding copyright ownership.  Heinz Max Kabutz 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 eu.javaspecialists.tjsn.concurrency.util;

import java.util.*;
import java.util.concurrent.*;

/**
 * This executor invokes the submitted tasks in sequence.
 *
 * @author Doug Lea
 * @see java.util.concurrent.Executor
 */
final public class SerialExecutor implements Executor {
    private final Queue<Runnable> tasks = new ArrayDeque<>();
    private final Executor executor;
    private Runnable active;

    public SerialExecutor(Executor executor) {
        this.executor = executor;
    }

    public synchronized void execute(final Runnable r) {
        // add() should be used instead of offer()
        tasks.add(new Runnable() {
            public void run() {
                try {
                    r.run();
                } finally {
                    scheduleNext();
                }
            }
        });
        if (active == null) {
            scheduleNext();
        }
    }

    protected synchronized void scheduleNext() {
        if ((active = tasks.poll()) != null) {
            executor.execute(active);
        }
    }
}