Java 并发编程：Callable和Future

Java 并发编程系列文章

Java 并发基础——线程安全性

Java 并发编程——Callable+Future+FutureTask

java 并发编程——Thread 源码重新学习

java并发编程——通过ReentrantLock,Condition实现银行存取款

Java并发编程——BlockingQueue

Java 并发编程——Executor框架和线程池原理

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项目中经常有些任务需要异步(提交到线程池中)去执行，而主线程往往需要知道异步执行产生的结果，这时我们要怎么做呢？用runnable是无法实现的，我们需要用callable实现。

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目前为止返回的都是FutureTask的实例
        Future<Integer> future = executor.submit(new AddTask(1, 2));
        Integer result = future.get();// 只有当future的状态是已完成时(future.isDone() = true),get()方法才会返回
    }
}

Callable接口

Callable接口Runable接口可谓是兄弟关系，只不过Callable是带返回值的。

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

接口函数及含义 ：public interface Future<V>

boolean  cancel(boolean mayInterruptIfRunning)

取消当前执行的任务，如果已经执行完毕或者已经被取消/由于某种原因不能被取消 则取消任务失败。

参数mayInterruptIfRunning： 当任务正在执行，如果参数为true ，则尝试中断任务，否则让任务继续执行知道结束。

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
 */

由注释可以看出，当没有执行完成时，需要等待任务执行完成了才会将计算结果返回。

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.

如果等待的时间超过设置的时间则会报 TimeoutException异常

FutureTask

public class FutureTask<V> implements RunnableFuture<V>

由定义可以看出它实现了RunnableFuture接口，那么这个接口又是什么呢？看下面的接口定义，其实很简单

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,它其实就是实现了Runnable和Future接口，FutureTask的执行是 状态转换的过程，源码中有七种状态如下：

  * 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;

当FutureTask刚刚被创建时，它的状态是NEW，其它状态查看源码。

其它成员变量：

 /** 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是待执行的任务，FutureTask 的 run（）函数中执行callable中的任务。

outcome ： 是callable的执行结果，当正常执行完成后会将结果set到outcome中

runner：是执行callable 的线程

WaitNode : 是的受阻塞的线程链表，当cancel一个任务后，阻塞的线程会被唤醒。

构造函数：

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
}

从构造函数可以看出，不光可以通过callable构造FutureTask还可以通过Runnable接口转化为callable来构造。关键函数为黄色标记部分，Executors中的实现源码如下：

/**
     * 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;
        }
    }

这里面不懂result到底有什么意义，明明就是预先设置好的。

其它具体的方法说明这里不再细说，里面用到了很多sun.misc.Unsafe中的方法以及其他SDK底层接口，后续有时间再学习。下面贴出了整个源码及说明

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;
    /** 用来存储任务执行结果或者异常对象,根据任务state在get时候选择返回执行结果还是抛出异常 */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** 当前运行Run方法的线程  */
    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
    }
    //判断任务是否已取消(异常中断、取消等)
    public boolean isCancelled() {
        return state >= CANCELLED;
    }
   /**
    判断任务是否已结束（取消、异常、完成、NORMAL都等于结束）
    **
    public boolean isDone() {
        return state != NEW;
    }

    /**
   mayInterruptIfRunning用来决定任务的状态。
                   true : 任务状态= INTERRUPTING = 5。如果任务已经运行，则强行中断。如果任务未运行，那么则不会再运行
                   false：CANCELLED    = 4。如果任务已经运行，则允许运行完成（但不能通过get获取结果）。如果任务未运行，那么则不会再运行
    **/
    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;
        //如果任务未彻底完成，那么则阻塞直至任务完成后唤醒该线程
        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() { }

    /**
    该方法在FutureTask里只有run方法在任务完成后调用。
    主要保存任务执行结果到成员变量outcome 中，和切换任务执行状态。
    由该方法可以得知：
    COMPLETING ： 任务已执行完成（也可能是异常完成），但还未设置结果到成员变量outcome中，也意味着还不能get
    NORMAL    ： 任务彻底执行完成
    **/
    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();
        }
    }

    /**
    由于实现了Runnable接口的缘故，该方法可由执行线程所调用。
    **/
    public void run() {
        //只有当任务状态=new时才被运行继续执行
        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 {
                    //调用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);
        }
    }

    /**
   如果该任务在执行过程中不被取消或者异常结束，那么该方法不记录任务的执行结果，且不修改任务执行状态。
   所以该方法可以重复执行N次。不过不能直接调用，因为是protected权限。
    **/
    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(); }
    }

    /**
    该方法在任务完成（包括异常完成、取消）后调用。删除所有正在get获取等待的节点且唤醒节点的线程。和调用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
    }

    /**
    阻塞等待任务执行完成（中断、正常完成、超时）
    **/
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            /**
            这里的if else的顺序也是有讲究的。
            1.先判断线程是否中断，中断则从队列中移除(也可能该线程不存在于队列中)
            2.判断当前任务是否执行完成，执行完成则不再阻塞，直接返回。
            3.如果任务状态=COMPLETING，证明该任务处于已执行完成，正在切换任务执行状态，CPU让出片刻即可
            4.q==null，则证明还未创建节点，则创建节点
            5.q节点入队
            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简单应用：

public class FutureTaskTest {

    public static void main(String[] args) {
        test();
    }

    private static void test() {
        Task task = new Task();
        FutureTask futureTask = new FutureTask(task);
//step3:将FutureTask提交给Thread执行
        Thread thread1 = new Thread(futureTask);
        thread1.setName("task thread 1");
        thread1.start();

//step4:获取执行结果，由于get()方法可能会阻塞当前调用线程，如果子任务执行时间不确定，最好在子线程中获取执行结果
        try {
            // boolean result = (boolean) futureTask.get();
            boolean result = (boolean) futureTask.get(5, TimeUnit.SECONDS);
            System.out.println("result:" + result);
        } catch (InterruptedException e) {
            System.out.println("守护线程阻塞被打断...");
            e.printStackTrace();
        } catch (ExecutionException e) {
            System.out.println("执行任务时出错...");
            e.printStackTrace();
        } catch (TimeoutException e) {
            System.out.println("执行超时...");
            futureTask.cancel(true);
            e.printStackTrace();
        } catch (CancellationException e) {
            //如果线程已经cancel了，再执行get操作会抛出这个异常
            System.out.println("future已经cancel了...");
            e.printStackTrace();
        }
    }

    private static final long SLEEP_TIME = 100;

    static class Task implements Callable<Boolean> {

        @Override
        public Boolean call() throws Exception {
            try {
                for (int i = 0; i < 10; i++) {
                    System.out.println("curr threadName=" + Thread.currentThread().getName() + " i=" + i);
                    //模拟耗时操作
                    Thread.sleep(SLEEP_TIME);
                }
            } catch (InterruptedException e) {
                System.out.println(" is interrupted when calculating, will stop...");
                return false; // 注意这里如果不return的话，线程还会继续执行，所以任务超时后在这里处理结果然后返回
            }
            return true;
        }
    }
}

1. 上述代码的执行结果为：

curr threadName=task thread 1 i=0
curr threadName=task thread 1 i=1
curr threadName=task thread 1 i=2
curr threadName=task thread 1 i=3
curr threadName=task thread 1 i=4
result:true

上述结果可以看出，get方法为阻塞执行，需要等到任务执行才会有返回值。

2. 当把SLEEP_TIME改为1500时，get方法回超时，进入timeout的异常处理分支，其结果如下。

curr threadName=task thread 1 i=0
curr threadName=task thread 1 i=1
curr threadName=task thread 1 i=2
curr threadName=task thread 1 i=3
执行超时...
 is interrupted when calculating, will stop...
java.util.concurrent.TimeoutException
    at java.util.concurrent.FutureTask.get(FutureTask.java:205)
    at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.test(FutureTaskTest.java:30)
    at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.main(FutureTaskTest.java:16)

可以看出超时后，任务并不会继续执行，因为cancel方法传了true。（这里能够被cancel，是因为runable处于sleep状态，如果是一直执行的任务则无法被interrupt）

3. 当调用cancel传值为false时，执行结果如下：

curr threadName=task thread 1 i=0
curr threadName=task thread 1 i=1
curr threadName=task thread 1 i=2
curr threadName=task thread 1 i=3
执行超时...
java.util.concurrent.TimeoutException
	at java.util.concurrent.FutureTask.get(FutureTask.java:205)
	at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.test(FutureTaskTest.java:30)
	at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.main(FutureTaskTest.java:16)
curr threadName=task thread 1 i=4

虽然执行了cancel，但是任务并没有被中断。

 

参考：

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