項目中經常有些任務需要異步(提交到線程池中)去執行，而主線程往往需要知道異步執行產生的結果，這時我們要怎麽做呢？用runnable是無法實現的，我們需要用callable實現。

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public
 
 class AddTask implements Callable<Integer> {

    private int a,b;

    public AddTask(int a, int b) {
        this.a = a;
        this.b = b;
    }

    @Override
    public Integer call() throws Exception {
        Integer result = a + b;
        return result;
    }

    public static void
 
 main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService executor = Executors.newSingleThreadExecutor();
        //JDK目前為止返回的都是FutureTask的實例  
        Future<Integer> future = executor.submit(new AddTask(1, 2));
        Integer result = future.get();// 只有當future的狀態是已完成時(future.isDone() = true),get()方法才會返回  
 

    }
}

Callable接口

Callable接口Runable接口可謂是兄弟關系，只不過Callable是帶返回值的。

public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

Future 接口

接口函數及含義 ：public interface Future<V>

boolean cancel(boolean mayInterruptIfRunning)

取消當前執行的任務，如果已經執行完畢或者已經被取消/由於某種原因不能被取消 則取消任務失敗。

參數mayInterruptIfRunning： 當任務正在執行，如果參數為true ，則嘗試中斷任務，否則讓任務繼續執行知道結束。

boolean isCancelled()

Returns {@code true} if this task was cancelled before it completed
* normally.

boolean isDone();

/**
 * Returns {@code true} if this task completed.
 *
 * Completion may be due to normal termination, an exception, or
 * cancellation -- in all of these cases, this method will return
 * {@code true}.
 *
 * @return {@code true} if this task completed
 */

V get() throws InterruptedException, ExecutionException;

/**
 * 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
 */

由註釋可以看出，當沒有執行完成時，需要等待任務執行完成了才會將計算結果返回。

V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;

Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.

如果等待的時間超過設置的時間則會報 TimeoutException異常

FutureTask

public class FutureTask<V> implements RunnableFuture<V>

由定義可以看出它實現了RunnableFuture接口，那麽這個接口又是什麽呢？看下面的接口定義，其實很簡單

public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

再回到FutureTask,它其實就是實現了Runnable和Future接口，FutureTask的執行是 狀態轉換的過程，源碼中有七種狀態如下：

  * 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;

當FutureTask剛剛被創建時，它的狀態是NEW，其它狀態查看源碼。

其它成員變量：

 /** 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;

callable是待執行的任務，FutureTask 的 run（）函數中執行callable中的任務。

outcome ： 是callable的執行結果，當正常執行完成後會將結果set到outcome中

runner：是執行callable 的線程

WaitNode : 是的受阻塞的線程鏈表，當cancel一個任務後，阻塞的線程會被喚醒。

構造函數：

public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

public FutureTask(Runnable runnable, V result) {
    this.callable = Executors.callable(runnable, result);
    this.state = NEW;       // ensure visibility of callable
}

從構造函數可以看出，不光可以通過callable構造FutureTask還可以通過Runnable接口轉化為callable來構造。關鍵函數為黃色標記部分，Executors中的實現源碼如下：

/**
     * A callable that runs given task and returns given result.
     */
    private static final class RunnableAdapter<T> implements Callable<T> {
        private final Runnable task;
        private final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }

這裏面不懂result到底有什麽意義，明明就是預先設置好的。

其它具體的方法說明這裏不再細說，裏面用到了很多sun.misc.Unsafe中的方法以及其他SDK底層接口，後續有時間再學習。下面貼出了整個源碼及說明

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;
    /** 用來存儲任務執行結果或者異常對象,根據任務state在get時候選擇返回執行結果還是拋出異常 */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** 當前運行Run方法的線程  */
    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
    }
    //判斷任務是否已取消(異常中斷、取消等)
    public boolean isCancelled() {
        return state >= CANCELLED;
    }
   /**
    判斷任務是否已結束（取消、異常、完成、NORMAL都等於結束）
    **
    public boolean isDone() {
        return state != NEW;
    }

    /**
   mayInterruptIfRunning用來決定任務的狀態。
                   true : 任務狀態= INTERRUPTING = 5。如果任務已經運行，則強行中斷。如果任務未運行，那麽則不會再運行
                   false：CANCELLED    = 4。如果任務已經運行，則允許運行完成（但不能通過get獲取結果）。如果任務未運行，那麽則不會再運行
    **/
    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;
        //如果任務未徹底完成，那麽則阻塞直至任務完成後喚醒該線程
        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() { }


    /**
    該方法在FutureTask裏只有run方法在任務完成後調用。
    主要保存任務執行結果到成員變量outcome 中，和切換任務執行狀態。
    由該方法可以得知：
    COMPLETING ： 任務已執行完成（也可能是異常完成），但還未設置結果到成員變量outcome中，也意味著還不能get
    NORMAL    ： 任務徹底執行完成
    **/
    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();
        }
    }

    /**
    由於實現了Runnable接口的緣故，該方法可由執行線程所調用。
    **/
    public void run() {
        //只有當任務狀態=new時才被運行繼續執行
        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 {
                    //調用Callable的Call方法
                    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);
        }
    }


    /**
   如果該任務在執行過程中不被取消或者異常結束，那麽該方法不記錄任務的執行結果，且不修改任務執行狀態。
   所以該方法可以重復執行N次。不過不能直接調用，因為是protected權限。
    **/
    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(); }
    }


    /**
    該方法在任務完成（包括異常完成、取消）後調用。刪除所有正在get獲取等待的節點且喚醒節點的線程。和調用done方法和置空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
    }


    /**
    阻塞等待任務執行完成（中斷、正常完成、超時）
    **/
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            /**
            這裏的if else的順序也是有講究的。
            1.先判斷線程是否中斷，中斷則從隊列中移除(也可能該線程不存在於隊列中)
            2.判斷當前任務是否執行完成，執行完成則不再阻塞，直接返回。
            3.如果任務狀態=COMPLETING，證明該任務處於已執行完成，正在切換任務執行狀態，CPU讓出片刻即可
            4.q==null，則證明還未創建節點，則創建節點
            5.q節點入隊
            6和7.阻塞
            **/

            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);
        }
    }

}

FutureTask簡單應用：

public class FutureMain {
    public static void main(String[] args)
            throws ExecutionException, InterruptedException {
        //構造FutureTask
        FutureTask<String> futureTask = new FutureTask<String>(new CallableClass("xxx"));
        ExecutorService executorService = Executors.newFixedThreadPool(1);
        //執行FutureTask，發送請求
        //在這裏開啟線程進行RealData的call()執行
        executorService.submit(futureTask);

        System.out.println("請求完畢。。。");
        try {
            //這裏可以進行其他額外的操作，這裏用sleep代替其他業務的處理
            Thread.sleep(200);
        }catch (InterruptedException e) {
            e.printStackTrace();
        }
        //獲取call()方法的返回值
        //如果此時call()方法沒有執行完成，則依然會等待
        System.out.println("真實數據:"+futureTask.get());
    }
}

如何給Future類 設置監聽器，查看參考鏈接中。

參考：

http://lixiaohui.iteye.com/blog/2319738

Java 並發編程——Callable+Future+FutureTask