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/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/licenses/publicdomain

 */

package java.util.concurrent;

import java.util.concurrent.locks.*;

/**

 * A cancellable asynchronous computation.  This class provides a base

 * implementation of {@link Future}, with methods to start and cancel

 * a computation, query to see if the computation is complete, and

 * retrieve the result of the computation.  The result can only be

 * retrieved when the computation has completed; the <tt>get</tt>

 * method will block if the computation has not yet completed.  Once

 * the computation has completed, the computation cannot be restarted

 * or cancelled.

 *

 * <p>A <tt>FutureTask</tt> can be used to wrap a {@link Callable} or

 * {@link java.lang.Runnable} object.  Because <tt>FutureTask</tt>

 * implements <tt>Runnable</tt>, a <tt>FutureTask</tt> can be

 * submitted to an {@link Executor} for execution.

 *

 * <p>In addition to serving as a standalone class, this class provides

 * <tt>protected</tt> functionality that may be useful when creating

 * customized task classes.

 *

 * @since 1.5

 * @author Doug Lea

 * @param <V> The result type returned by this FutureTask's <tt>get</tt> method

 */

public class FutureTask<V> implements RunnableFuture<V> {

    /** Synchronization control for FutureTask */

    private final Sync sync;

    /**

     * Creates a <tt>FutureTask</tt> that will upon running, execute the

     * given <tt>Callable</tt>.

     *

     * @param  callable the callable task

     * @throws NullPointerException if callable is null

     */

    public FutureTask(Callable<V> callable) {

        if (callable == null)

            throw new NullPointerException();

        sync = new Sync(callable);

    }

    /**

     * Creates a <tt>FutureTask</tt> that will upon running, execute the

     * given <tt>Runnable</tt>, and arrange that <tt>get</tt> will return the

     * given result on successful completion.

     *

     * @param  runnable the runnable task

     * @param result the result to return on successful completion. If

     * you don't need a particular result, consider using

     * constructions of the form:

     * <tt>Future&lt;?&gt; f = new FutureTask&lt;Object&gt;(runnable, null)</tt>

     * @throws NullPointerException if runnable is null

     */

    public FutureTask(Runnable runnable, V result) {

        sync = new Sync(Executors.callable(runnable, result));

    }

    public boolean isCancelled() {

        return sync.innerIsCancelled();

    }

    public boolean isDone() {

        return sync.innerIsDone();

    }

    public boolean cancel(boolean mayInterruptIfRunning) {

        return sync.innerCancel(mayInterruptIfRunning);

    }

    /**

     * @throws CancellationException {@inheritDoc}

     */

    public V get() throws InterruptedException, ExecutionException {

        return sync.innerGet();

    }

    /**

     * @throws CancellationException {@inheritDoc}

     */

    public V get(long timeout, TimeUnit unit)

        throws InterruptedException, ExecutionException, TimeoutException {

        return sync.innerGet(unit.toNanos(timeout));

    }

    /**

     * Protected method invoked when this task transitions to state

     * <tt>isDone</tt> (whether normally or via cancellation). The

     * default implementation does nothing.  Subclasses may override

     * this method to invoke completion callbacks or perform

     * bookkeeping. Note that you can query status inside the

     * implementation of this method to determine whether this task

     * has been cancelled.

     */

    protected void done() { }

    /**

     * Sets the result of this Future to the given value unless

     * this future has already been set or has been cancelled.

     * This method is invoked internally by the <tt>run</tt> method

     * upon successful completion of the computation.

     * @param v the value

     */

    protected void set(V v) {

        sync.innerSet(v);

    }

    /**

     * Causes this future to report an <tt>ExecutionException</tt>

     * with the given throwable as its cause, unless this Future has

     * already been set or has been cancelled.

     * This method is invoked internally by the <tt>run</tt> method

     * upon failure of the computation.

     * @param t the cause of failure

     */

    protected void setException(Throwable t) {

        sync.innerSetException(t);

    }

    // The following (duplicated) doc comment can be removed once

    //

    // 6270645: Javadoc comments should be inherited from most derived

    //          superinterface or superclass

    // is fixed.

    /**

     * Sets this Future to the result of its computation

     * unless it has been cancelled.

     */

    public void run() {

        sync.innerRun();

    }

    /**

     * Executes the computation without setting its result, and then

     * resets this Future to initial state, failing to do so if the

     * computation encounters an exception or is cancelled.  This is

     * designed for use with tasks that intrinsically execute more

     * than once.

     * @return true if successfully run and reset

     */

    protected boolean runAndReset() {

        return sync.innerRunAndReset();

    }

    /**

     * Synchronization control for FutureTask. Note that this must be

     * a non-static inner class in order to invoke the protected

     * <tt>done</tt> method. For clarity, all inner class support

     * methods are same as outer, prefixed with "inner".

     *

     * Uses AQS sync state to represent run status

     */

    private final class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = -7828117401763700385L;

        /** State value representing that task is running */

        private static final int RUNNING   = 1;

        /** State value representing that task ran */

        private static final int RAN       = 2;

        /** State value representing that task was cancelled */

        private static final int CANCELLED = 4;

        /** The underlying callable */

        private final Callable<V> callable;

        /** The result to return from get() */

        private V result;

        /** The exception to throw from get() */

        private Throwable exception;

        /**

         * The thread running task. When nulled after set/cancel, this

         * indicates that the results are accessible.  Must be

         * volatile, to ensure visibility upon completion.

         */

        private volatile Thread runner;

        Sync(Callable<V> callable) {

            this.callable = callable;

        }

        private boolean ranOrCancelled(int state) {

            return (state & (RAN | CANCELLED)) != 0;

        }

        /**

         * Implements AQS base acquire to succeed if ran or cancelled

         */

        protected int tryAcquireShared(int ignore) {

            return innerIsDone()? 1 : -1;

        }

        /**

         * Implements AQS base release to always signal after setting

         * final done status by nulling runner thread.

         */

        protected boolean tryReleaseShared(int ignore) {

            runner = null;

            return true;

        }

        boolean innerIsCancelled() {

            return getState() == CANCELLED;

        }

        boolean innerIsDone() {

            return ranOrCancelled(getState()) && runner == null;

        }

        V innerGet() throws InterruptedException, ExecutionException {

            acquireSharedInterruptibly(0);

            if (getState() == CANCELLED)

                throw new CancellationException();

            if (exception != null)

                throw new ExecutionException(exception);

            return result;

        }

        V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {

            if (!tryAcquireSharedNanos(0, nanosTimeout))

                throw new TimeoutException();

            if (getState() == CANCELLED)

                throw new CancellationException();

            if (exception != null)

                throw new ExecutionException(exception);

            return result;

        }

        void innerSet(V v) {

            for (;;) {

                int s = getState();

                if (s == RAN)

                    return;

                if (s == CANCELLED) {

                    // aggressively release to set runner to null,

                    // in case we are racing with a cancel request

                    // that will try to interrupt runner

                    releaseShared(0);

                    return;

                }

                if (compareAndSetState(s, RAN)) {

                    result = v;

                    releaseShared(0);

                    done();

                    return;

                }

            }

        }

        void innerSetException(Throwable t) {

            for (;;) {

                int s = getState();

                if (s == RAN)

                    return;

                if (s == CANCELLED) {

                    // aggressively release to set runner to null,

                    // in case we are racing with a cancel request

                    // that will try to interrupt runner

                    releaseShared(0);

                    return;

                }

                if (compareAndSetState(s, RAN)) {

                    exception = t;

                    result = null;

                    releaseShared(0);

                    done();

                    return;

                }

            }

        }

        boolean innerCancel(boolean mayInterruptIfRunning) {

            for (;;) {

                int s = getState();

                if (ranOrCancelled(s))

                    return false;

                if (compareAndSetState(s, CANCELLED))

                    break;

            }

            if (mayInterruptIfRunning) {

                Thread r = runner;

                if (r != null)

                    r.interrupt();

            }

            releaseShared(0);

            done();

            return true;

        }

        void innerRun() {

            if (!compareAndSetState(0, RUNNING))

                return;

            try {

                runner = Thread.currentThread();

                if (getState() == RUNNING) // recheck after setting thread

                    innerSet(callable.call());

                else

                    releaseShared(0); // cancel

            } catch (Throwable ex) {

                innerSetException(ex);

            }

        }

        boolean innerRunAndReset() {

            if (!compareAndSetState(0, RUNNING))

                return false;

            try {

                runner = Thread.currentThread();

                if (getState() == RUNNING)

                    callable.call(); // don't set result

                runner = null;

                return compareAndSetState(RUNNING, 0);

            } catch (Throwable ex) {

                innerSetException(ex);

                return false;

            }

        }

    }

}