Java example source code file (PendingFuture.java)

This example Java source code file (PendingFuture.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

asynchronouschannel, cancellationexception, cancelled, completionhandler, countdownlatch, executionexception, interruptedexception, ioexception, object, override, pendingfuture, threading, threads, throwable, timeoutexception

The PendingFuture.java Java example source code

/*
 * Copyright (c) 2008, 2009, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code 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
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package sun.nio.ch;

import java.nio.channels.*;
import java.util.concurrent.*;
import java.io.IOException;

/**
 * A Future for a pending I/O operation. A PendingFuture allows for the
 * attachment of an additional arbitrary context object and a timer task.
 */

final class PendingFuture<V,A> implements Future {
    private static final CancellationException CANCELLED =
        new CancellationException();

    private final AsynchronousChannel channel;
    private final CompletionHandler<V,? super A> handler;
    private final A attachment;

    // true if result (or exception) is available
    private volatile boolean haveResult;
    private volatile V result;
    private volatile Throwable exc;

    // latch for waiting (created lazily if needed)
    private CountDownLatch latch;

    // optional timer task that is cancelled when result becomes available
    private Future<?> timeoutTask;

    // optional context object
    private volatile Object context;

    PendingFuture(AsynchronousChannel channel,
                  CompletionHandler<V,? super A> handler,
                  A attachment,
                  Object context)
    {
        this.channel = channel;
        this.handler = handler;
        this.attachment = attachment;
        this.context = context;
    }

    PendingFuture(AsynchronousChannel channel,
                  CompletionHandler<V,? super A> handler,
                  A attachment)
    {
        this.channel = channel;
        this.handler = handler;
        this.attachment = attachment;
    }

    PendingFuture(AsynchronousChannel channel) {
        this(channel, null, null);
    }

    PendingFuture(AsynchronousChannel channel, Object context) {
        this(channel, null, null, context);
    }

    AsynchronousChannel channel() {
        return channel;
    }

    CompletionHandler<V,? super A> handler() {
        return handler;
    }

    A attachment() {
        return attachment;
    }

    void setContext(Object context) {
        this.context = context;
    }

    Object getContext() {
        return context;
    }

    void setTimeoutTask(Future<?> task) {
        synchronized (this) {
            if (haveResult) {
                task.cancel(false);
            } else {
                this.timeoutTask = task;
            }
        }
    }

    // creates latch if required; return true if caller needs to wait
    private boolean prepareForWait() {
        synchronized (this) {
            if (haveResult) {
                return false;
            } else {
                if (latch == null)
                    latch = new CountDownLatch(1);
                return true;
            }
        }
    }

    /**
     * Sets the result, or a no-op if the result or exception is already set.
     */
    void setResult(V res) {
        synchronized (this) {
            if (haveResult)
                return;
            result = res;
            haveResult = true;
            if (timeoutTask != null)
                timeoutTask.cancel(false);
            if (latch != null)
                latch.countDown();
        }
    }

    /**
     * Sets the result, or a no-op if the result or exception is already set.
     */
    void setFailure(Throwable x) {
        if (!(x instanceof IOException) && !(x instanceof SecurityException))
            x = new IOException(x);
        synchronized (this) {
            if (haveResult)
                return;
            exc = x;
            haveResult = true;
            if (timeoutTask != null)
                timeoutTask.cancel(false);
            if (latch != null)
                latch.countDown();
        }
    }

    /**
     * Sets the result
     */
    void setResult(V res, Throwable x) {
        if (x == null) {
            setResult(res);
        } else {
            setFailure(x);
        }
    }

    @Override
    public V get() throws ExecutionException, InterruptedException {
        if (!haveResult) {
            boolean needToWait = prepareForWait();
            if (needToWait)
                latch.await();
        }
        if (exc != null) {
            if (exc == CANCELLED)
                throw new CancellationException();
            throw new ExecutionException(exc);
        }
        return result;
    }

    @Override
    public V get(long timeout, TimeUnit unit)
        throws ExecutionException, InterruptedException, TimeoutException
    {
        if (!haveResult) {
            boolean needToWait = prepareForWait();
            if (needToWait)
                if (!latch.await(timeout, unit)) throw new TimeoutException();
        }
        if (exc != null) {
            if (exc == CANCELLED)
                throw new CancellationException();
            throw new ExecutionException(exc);
        }
        return result;
    }

    Throwable exception() {
        return (exc != CANCELLED) ? exc : null;
    }

    V value() {
        return result;
    }

    @Override
    public boolean isCancelled() {
        return (exc == CANCELLED);
    }

    @Override
    public boolean isDone() {
        return haveResult;
    }

    @Override
    public boolean cancel(boolean mayInterruptIfRunning) {
        synchronized (this) {
            if (haveResult)
                return false;    // already completed

            // notify channel
            if (channel() instanceof Cancellable)
                ((Cancellable)channel()).onCancel(this);

            // set result and cancel timer
            exc = CANCELLED;
            haveResult = true;
            if (timeoutTask != null)
                timeoutTask.cancel(false);
        }

        // close channel if forceful cancel
        if (mayInterruptIfRunning) {
            try {
                channel().close();
            } catch (IOException ignore) { }
        }

        // release waiters
        if (latch != null)
            latch.countDown();
        return true;
    }
}