FutureTask源码分析

FutureTask介绍

FutureTask是一种可取消的异步计算任务。它实现了Future接口，代表了异步任务的返回结果。从而FutureTask可以启动和取消异步计算任务、查询异步计算任务是否完成和获取异步计算任务的返回结果。只有到异步计算任务结束时才能获取返回结果，当异步计算任务还未结束时调用get方法会使线程阻塞。一旦异步计算任务完成，计算任务不能重新启动或者取消，除非调用了runAndReset。

FutureTask实现了RunnableFuture，RunnableFuture结合了Future和Runnable。

FutureTask原理分析

在ThreadPoolExecutor分析中我们没有看它的父类AbstractExecutorService，其中有一个方法submit，返回一个Future，说明该方法可以获取异步任务的返回结果。该方法有三个重载，可以接收Runnable和Callable，Callable是可以返回结果的一个Runnable，而Callable就是FutureTask的一个重要的变量。

@FunctionalInterface
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;
}

FutureTask的一些变量和状态

/**
 * 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 */
//一个可以返回结果的任务
private Callable<V> callable;
/** The result to return or exception to throw from get() */
//包装返回结果或者异常，没有被volatile修饰，状态保护读写安全
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
//运行线程
private volatile Thread runner;
/** Treiber stack of waiting threads */
//单链表，是一个线程的栈的结构
private volatile WaitNode waiters;

FutureTask有7中状态，介绍一下状态之间的转换：
NEW -> COMPLETING -> NORMAL：任务正常执行；
NEW -> COMPLETING -> EXCEPTIONAL：任务发生异常；
NEW -> CANCELLED：任务被取消；
NEW -> INTERRUPTING -> INTERRUPTED：任务被中断；

run方法

public void run() {
    //如果state不为NEW，说明任务已经在执行或者取消
    //如果设置运行线程失败，说明任务已经有运行线程抢在前面
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        //NEW状态才可以执行
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                //执行任务
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                //设置异常信息
                setException(ex);
            }
            if (ran)
                //设置任务运行结果
                set(result);
        }
    } finally {
        // runner must be non-null until state is settled to
        // prevent concurrent calls to run()
        //将运行线程清空，在state被更改之前要保证runner非空，这样能包装run方法不被多次执行
        runner = null;
        // state must be re-read after nulling runner to prevent
        // leaked interrupts
        int s = state;
        //中断处理
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

set、setException和handlePossibleCancellationInterrupt

protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}

当执行时发生异常，调用setException，首先将state设置为COMPLETING，设置成功后将outcome设置为异常，然后将state设置为EXCEPTIONAL。

protected void set(V v) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = v;
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
        finishCompletion();
    }
}

当callable执行成功并返回，调用set，首先将state设置为COMPLETING，设置成功后将结果设置为outcome，然后设置state为NORMAL。

finally中如果state为中断，调用handlePossibleCancellationInterrupt：

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();
}

如果状态一直是INTERRUPTING，稍稍等待。

finishCompletion和get

在上面set和setException中最后都调用了finishCompletion方法：

private void finishCompletion() {
    // assert state > COMPLETING;
    //该方法必须在state > COMPLETING时调用
    //从头到尾唤醒WaitNode中阻塞的线程
    for (WaitNode q; (q = waiters) != null;) {
        //设置栈顶为空
        if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
            for (;;) {
                Thread t = q.thread;
                //唤醒线程
                if (t != null) {
                    q.thread = null;
                    LockSupport.unpark(t);
                }
                WaitNode next = q.next;
                //如果next为空，break
                if (next == null)
                    break;
                q.next = null; // unlink to help gc
                q = next;
            }
            break;
        }
    }

    done();

    callable = null;        // to reduce footprint
}

在调用get方法时，如果任务还在执行，线程会阻塞，FutureTask会将阻塞的线程放入waiters单链表。等待任务结束时被唤醒，我们继续看get方法：

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
        //如果任务还在执行，阻塞当前线程，放入waiters单链表
        s = awaitDone(false, 0L);
    return report(s);
}

awaitDone

private int awaitDone(boolean timed, long nanos)
    throws InterruptedException {
    final long deadline = timed ? System.nanoTime() + nanos : 0L;
    WaitNode q = null;
    boolean queued = false;
    for (;;) {
        //如果线程被中断，移除当前node，抛出异常
        if (Thread.interrupted()) {
            removeWaiter(q);
            throw new InterruptedException();
        }

        int s = state;
        //如果任务完成或者被取消，直接返回
        if (s > COMPLETING) {
            if (q != null)
                q.thread = null;
            return s;
        }
        //如果任务正在执行，线程等待一下
        else if (s == COMPLETING) // cannot time out yet
            Thread.yield();
        //如果q为空，新建一个node
        else if (q == null)
            q = new WaitNode();
        //如果还未入列，尝试将新建的node放入链表
        else if (!queued)
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                 q.next = waiters, q);
        //如果设置了超时且超时了
        else if (timed) {
            nanos = deadline - System.nanoTime();
            if (nanos <= 0L) {
                //超时，移除node
                removeWaiter(q);
                return state;
            }
            //阻塞线程
            LockSupport.parkNanos(this, nanos);
        }
        //阻塞当前线程
        else
            LockSupport.park(this);
    }
}

removeWaiter

private void removeWaiter(WaitNode node) {
    if (node != null) {
        //设置节点的线程为空，做删除标记
        node.thread = null;
        retry:
        for (;;) {          // restart on removeWaiter race
            for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                s = q.next;
                //thread不为空，continue
                if (q.thread != null)
                    pred = q;
                //thread为空且pred不为空
                else if (pred != null) {
                    //删除q
                    pred.next = s;
                    //检查一下pred的thread，如果被其他线程修改，retry outer loop
                    if (pred.thread == null) // check for race
                        continue retry;
                }
                //thread为空且pred为空说明q为栈顶，将q.next设置为栈顶，失败则retry
                else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                      q, s))
                    continue retry;
            }
            break;
        }
    }
}

report方法

get方法最后调用了report方法：

private V report(int s) throws ExecutionException {
    Object x = outcome;
    //NORMAL表示任务执行正常，返回结果
    if (s == NORMAL)
        return (V)x;
    //任务被取消，抛出异常
    if (s >= CANCELLED)
        throw new CancellationException();
    //其他情况只有可能发生异常，抛出该异常
    throw new ExecutionException((Throwable)x);
}

cancel方法

最后看一下cancel方法：

public boolean cancel(boolean mayInterruptIfRunning) {
    //当state不为NEW说明任务已经开始，不能被取消，返回false
    //当设置state失败时，返回false
    if (!(state == NEW &&
          UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
              mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
        return false;
    try {    // in case call to interrupt throws exception
        if (mayInterruptIfRunning) {
            try {
                Thread t = runner;
                //中断线程
                if (t != null)
                    t.interrupt();
            } finally { // final state
                //设置任务为INTERRUPTED
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
            }
        }
    } finally {
        finishCompletion();
    }
    return true;
}