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A versatile Java(TM) library that implements many useful container and utility classes.
There is a newer version: 1.1.12

/*
 * de.unkrig.commons - A general-purpose Java class library
 *
 * Copyright (c) 2015, Arno Unkrig
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the
 * following conditions are met:
 *
 *    1. Redistributions of source code must retain the above copyright notice, this list of conditions and the
 *       following disclaimer.
 *    2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the
 *       following disclaimer in the documentation and/or other materials provided with the distribution.
 *    3. The name of the author may not be used to endorse or promote products derived from this software without
 *       specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
 * THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

package de.unkrig.commons.util.concurrent;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.TimeUnit;

import de.unkrig.commons.lang.protocol.Consumer;
import de.unkrig.commons.lang.protocol.ConsumerUtil;
import de.unkrig.commons.lang.protocol.ConsumerWhichThrows;
import de.unkrig.commons.lang.protocol.ProxyConsumerWhichThrows;
import de.unkrig.commons.nullanalysis.NotNullByDefault;

/**
 * Executes multiple tasks asynchronously, but forwards their output to a delegate {@link Consumer} (or {@link
 * ConsumerWhichThrows}) in the order the tasks were submitted, i.e. the last subject written by one task
 * appears before the output of all subsequently submitted tasks.
 *
 * @param   The type of the objects that pose the "output" of the tasks
 * @param  The exception type that the tasks may throw
 */
@NotNullByDefault(false) public
class ObjectSequentializer {

    private final SquadExecutor squadExecutor;

    /** The consumer to which the next task must send its output. */
    private ConsumerWhichThrows nextTarget;

    /** @see ObjectSequentializer */
    public
    ObjectSequentializer(ConsumerWhichThrows delegate, ExecutorService squadExecutor) {

        // The QuadExecutor will allow us to await the completion of all submitted tasks.
        this.squadExecutor = new SquadExecutor(squadExecutor);

        // The first task will write directly to the delegate.
        this.nextTarget = delegate;
    }

    /**
     * Subjects written to the task's subject will be written to the {@link
     * #ObjectSequentializer(ConsumerWhichThrows, ExecutorService) delegate} after all subjects written by the
     * previously submitted tasks, and before the subjects of all tasks submitted afterwards.
     * 
     *   Notice that when a task submits another task (the "subtask"), then the output of the subtask will appear
     *   after the output of the task (and the output of all other tasks submitted in the meantime).
     *   If you want to have the output of subtasks to be in the middle of the task's output, then you should
     *   create a second {@link ObjectSequentializer} and await its completion.
     * 
     */
    public synchronized void
    submit(final ConsumerWhichThrows, ? extends Exception> task) {

        final ConsumerWhichThrows previousTarget = this.nextTarget;

        final List buffer = new ArrayList();

        final ProxyConsumerWhichThrows
        pc = new ProxyConsumerWhichThrows(ConsumerUtil.addToCollection(buffer), 0);

        // The next task(s) will write to the proxy consumer.
        this.nextTarget = pc;

        this.squadExecutor.submit(new Callable() {

            @Override public Void
            call() throws Exception {

                // Run the task.
                task.consume(previousTarget);

                synchronized (pc) {

                    // Copy the buffered products of the FOLLOWING tasks to the output.
                    for (T t : buffer) previousTarget.consume(t);

                    // Redirect FUTURE output of the FOLLOWING tasks directly to MY output.
                    pc.setDelegate(previousTarget);
                }

                return null;
            }
        });
    }

    /**
     * Returns when all tasks that were previously submitted with {@link #submit(ConsumerWhichThrows)} have completed.
     *
     * @throws CancellationException One of the tasks was cancelled
     * @throws ExecutionException    One of the tasks threw an exception
     * @throws InterruptedException  The current thread was interrupted while waiting
     */
    public void
    awaitCompletion() throws InterruptedException, ExecutionException, CancellationException {

        this.squadExecutor.awaitCompletion();
    }

    /**
     * Returns when all tasks that were previously submitted with {@link #submit(Consumer)
     * Object)}, {@link #submit(Runnable, Object)} and {@link #execute(Runnable)} have completed, or when then
     * {@code timeout} expires.
     *
     * @throws CancellationException One of the tasks was cancelled
     * @throws ExecutionException    One of the tasks threw an exception
     * @throws InterruptedException  The current thread was interrupted while waiting
     */
    void
    awaitCompletion(long timeout, TimeUnit unit)
    throws CancellationException, ExecutionException, InterruptedException {

        this.squadExecutor.awaitCompletion(timeout, unit);
    }
}