Callable介面類似於Runnable，從名字就可以看出來了，但是Runnable不會返回結果，並且無法丟擲返回結果的異常，而Callable功能更強大一些，被執行緒執行後，可以返回值，這個返回值可以被Future拿到。FutureTask實現了兩個介面，Runnable和Future，所以它既可以作為Runnable被執行緒執行，又可以作為Future得到Callable的返回值，那麼這個組合的使用有什麼好處呢？假設有一個很耗時的返回值需要計算，並且這個返回值不是立刻需要的話，那麼就可以使用這個組合，用另一個執行緒去計算返回值，而當前執行緒在使用這個返回值之前可以做其它的操作，等到需要這個返回值時，再通過Future得到。

Future特性

需要明確一點：java.util.concurrent.Future 是一個非同步回撥介面。

Future介面原始碼

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 */

/*
 *
 * 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/publicdomain/zero/1.0/
 */

package java.util.concurrent;

/**
 * A <tt>Future</tt> represents the result of an asynchronous
 * computation.  Methods are provided to check if the computation is
 * complete, to wait for its completion, and to retrieve the result of
 * the computation.  The result can only be retrieved using method
 * <tt>get</tt> when the computation has completed, blocking if
 * necessary until it is ready.  Cancellation is performed by the
 * <tt>cancel</tt> method.  Additional methods are provided to
 * determine if the task completed normally or was cancelled. Once a
 * computation has completed, the computation cannot be cancelled.
 * If you would like to use a <tt>Future</tt> for the sake
 * of cancellability but not provide a usable result, you can
 * declare types of the form {@code Future<?>} and
 * return <tt>null</tt> as a result of the underlying task.
 *
 * <p>
 * <b>Sample Usage</b> (Note that the following classes are all
 * made-up.) <p>
 *  <pre> {@code
 * interface ArchiveSearcher { String search(String target); }
 * class App {
 *   ExecutorService executor = ...
 *   ArchiveSearcher searcher = ...
 *   void showSearch(final String target)
 *       throws InterruptedException {
 *     Future<String> future
 *       = executor.submit(new Callable<String>() {
 *         public String call() {
 *             return searcher.search(target);
 *         }});
 *     displayOtherThings(); // do other things while searching
 *     try {
 *       displayText(future.get()); // use future
 *     } catch (ExecutionException ex) { cleanup(); return; }
 *   }
 * }}</pre>
 *
 * The {@link FutureTask} class is an implementation of <tt>Future</tt> that
 * implements <tt>Runnable</tt>, and so may be executed by an <tt>Executor</tt>.
 * For example, the above construction with <tt>submit</tt> could be replaced by:
 *  <pre> {@code
 *     FutureTask<String> future =
 *       new FutureTask<String>(new Callable<String>() {
 *         public String call() {
 *           return searcher.search(target);
 *       }});
 *     executor.execute(future);}</pre>
 *
 * <p>Memory consistency effects: Actions taken by the asynchronous computation
 * <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
 * actions following the corresponding {@code Future.get()} in another thread.
 *
 * @see FutureTask
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this Future's <tt>get</tt> method
 */
public interface Future<V> {

    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when <tt>cancel</tt> is called,
     * this task should never run.  If the task has already started,
     * then the <tt>mayInterruptIfRunning</tt> parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return <tt>true</tt>.  Subsequent calls to {@link #isCancelled}
     * will always return <tt>true</tt> if this method returned <tt>true</tt>.
     *
     * @param mayInterruptIfRunning <tt>true</tt> if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return <tt>false</tt> if the task could not be cancelled,
     * typically because it has already completed normally;
     * <tt>true</tt> otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns <tt>true</tt> if this task was cancelled before it completed
     * normally.
     *
     * @return <tt>true</tt> if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns <tt>true</tt> if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * <tt>true</tt>.
     *
     * @return <tt>true</tt> if this task completed
     */
    boolean isDone();

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}
 

Callable返回Future示例

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
/**
 * Callable的Future用法
 * @package .CallableDemo
 * @date   2017年4月5日  下午2:53:18
 * @author pengjunlin
 * @comment   
 * @update
 */
public class CallableFuture {

	/**
	 * @param args
	 * @throws Exception
	 * @throws InterruptedException
	 */
	public static void main(String[] args) throws InterruptedException,
			Exception {
		// TODO Auto-generated method stub
		ExecutorService exec = Executors.newCachedThreadPool();
		// Future是一個介面，該介面用來返回非同步的結果。
		Future<String> st = exec.submit(new TaskCallable());

		/* 同步結果，並且設定超時時間 */
		System.out.println(st.get(10000, TimeUnit.MILLISECONDS));
		System.out.println("finished");

	}

}

class TaskCallable implements Callable<String> {

	public String call() throws Exception {
		// TODO Auto-generated method stub
		Thread.sleep(1000);
		return "callstatus=OK";
	}

}
 

FutureTask

FutureTask實現了java.util.concurrent.RunnableFuture<V>介面，實際上實現了Runnable和 Future<V>兩個介面。

FutureTask原始碼

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 */

/*
 *
 * 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/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;

/**
 * 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 {@code get}
 * methods will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled (unless the computation is invoked using
 * {@link #runAndReset}).
 *
 * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 * {@link Runnable} object.  Because {@code FutureTask} implements
 * {@code Runnable}, a {@code FutureTask} can be submitted to an
 * {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * {@code protected} 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 {@code get} methods
 */
public class FutureTask<V> implements RunnableFuture<V> {
    /*
     * Revision notes: This differs from previous versions of this
     * class that relied on AbstractQueuedSynchronizer, mainly to
     * avoid surprising users about retaining interrupt status during
     * cancellation races. Sync control in the current design relies
     * on a "state" field updated via CAS to track completion, along
     * with a simple Treiber stack to hold waiting threads.
     *
     * Style note: As usual, we bypass overhead of using
     * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
     */

    /**
     * The run state of this task, initially NEW.  The run state
     * transitions to a terminal state only in methods set,
     * setException, and cancel.  During completion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (only while interrupting the runner to satisfy a
     * cancel(true)). Transitions from these intermediate to final
     * states use cheaper ordered/lazy writes because values are unique
     * and cannot be further modified.
     *
     * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

    /** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} 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:
     * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
     * @throws NullPointerException if the runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

    public boolean isCancelled() {
        return state >= CANCELLED;
    }

    public boolean isDone() {
        return state != NEW;
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        if (state != NEW)
            return false;
        if (mayInterruptIfRunning) {
            if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
                return false;
            Thread t = runner;
            if (t != null)
                t.interrupt();
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
        }
        else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
            return false;
        finishCompletion();
        return true;
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }

    /**
     * Protected method invoked when this task transitions to state
     * {@code isDone} (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.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon successful completion of the computation.
     *
     * @param v the value
     */
    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

    /**
     * Causes this future to report an {@link ExecutionException}
     * with the given throwable as its cause, unless this future has
     * already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon failure of the computation.
     *
     * @param t the cause of failure
     */
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

    public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

    /**
     * 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() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don't set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }

    /**
     * Ensures that any interrupt from a possible cancel(true) is only
     * delivered to a task while in run or runAndReset.
     */
    private void handlePossibleCancellationInterrupt(int s) {
        // It is possible for our interrupter to stall before getting a
        // chance to interrupt us.  Let's spin-wait patiently.
        if (s == INTERRUPTING)
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending interrupt

        // assert state == INTERRUPTED;

        // We want to clear any interrupt we may have received from
        // cancel(true).  However, it is permissible to use interrupts
        // as an independent mechanism for a task to communicate with
        // its caller, and there is no way to clear only the
        // cancellation interrupt.
        //
        // Thread.interrupted();
    }

    /**
     * Simple linked list nodes to record waiting threads in a Treiber
     * stack.  See other classes such as Phaser and SynchronousQueue
     * for more detailed explanation.
     */
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }

    /**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }

        done();

        callable = null;        // to reduce footprint
    }

    /**
     * Awaits completion or aborts on interrupt or timeout.
     *
     * @param timed true if use timed waits
     * @param nanos time to wait, if timed
     * @return state upon completion
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    /**
     * Tries to unlink a timed-out or interrupted wait node to avoid
     * accumulating garbage.  Internal nodes are simply unspliced
     * without CAS since it is harmless if they are traversed anyway
     * by releasers.  To avoid effects of unsplicing from already
     * removed nodes, the list is retraversed in case of an apparent
     * race.  This is slow when there are a lot of nodes, but we don't
     * expect lists to be long enough to outweigh higher-overhead
     * schemes.
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
                    else if (pred != null) {
                        pred.next = s;
                        if (pred.thread == null) // check for race
                            continue retry;
                    }
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long stateOffset;
    private static final long runnerOffset;
    private static final long waitersOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            stateOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("state"));
            runnerOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("runner"));
            waitersOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

}

Callable返回FutureTask示例

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
/**
 * Callable的MyFutureTask用法
 * 
 * @package .MyFutureTask
 * @date   2017年4月5日  下午2:56:50
 * @author pengjunlin
 * @comment   
 * @update
 */
public class MyFutureTask {
	
	public static void main(String[] args) {
		
		Callable<int[]> primeCallable = new PrimeCallable(1000);
		
		FutureTask<int[]> primeTask = new FutureTask<int[]>(primeCallable);
		
		Thread t = new Thread(primeTask);
		
		t.start();
		
		try {
			// 假設現在做其他事情 
			Thread.sleep(5000);

			// 回來看看質數找好了嗎
			if (primeTask.isDone()) {
				int[] primes = primeTask.get();
				for (int prime : primes) {
					System.out.print(prime + " ");
				}
				System.out.println();
			}
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
}

class PrimeCallable implements Callable<int[]> {
	private int max;

	public PrimeCallable(int max) {
		this.max = max;
	}

	public int[] call() throws Exception {
		int[] prime = new int[max + 1];

		List<Integer> list = new ArrayList<Integer>();

		for (int i = 2; i <= max; i++)
			prime[i] = 1;

		for (int i = 2; i * i <= max; i++) { // 這裡可以改進
			if (prime[i] == 1) {
				for (int j = 2 * i; j <= max; j++) {
					if (j % i == 0)
						prime[j] = 0;
				}
			}
		}

		for (int i = 2; i < max; i++) {
			if (prime[i] == 1) {
				list.add(i);
			}
		}

		int[] p = new int[list.size()];
		for (int i = 0; i < p.length; i++) {
			p[i] = list.get(i).intValue();
		}

		return p;
	}

}