FutureTask源码阅读

FutureTask功能用法

类结构

源码中详细说明了FutureTask生命周期状态及变化

    /**
     * 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;

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实现细节

Future#get()方法的实现

FutureTask既然实现了Future接口，就先看下Future#get()函数的实现

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


    /**
     * 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);
        }
    }

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这个awaitDone函数，实现了在Callable对象未complete时当前线程等待的功能。成员变量waiters是一个等待在该FutureTask#get()上的线程的链表，WaitNode有当前Thread的变量

 /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

awaitDone首先将当前线程加入等待队列waiters，然后调用LockSupport#park阻塞自己，等待被唤醒再根据state状态返回，或者过nanos时间后返回。park是“忙碌等待”的一种优化，它不会浪费这么多的时间进行自旋。

可以看到，循环中设置了中断处理逻辑。LockSupport.park()支持中断影响，它不会抛出InterruptedException异常，只是默默的返回。但是，我们可以从Thread.interrupted()等方法获得终端标记。

当执行Callable的异步线程完成task后，会唤醒阻塞在awaitDone上的当前线程，

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);
        }
    }

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执行set()设置结果，并调用finishCompletion()清除和通知waiters上的等待线程。

/**
     * 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
    }

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中断处理代码和time-out时间到的代码中都调用到removeWaiter(WaitNode node)函数，该函数安全的删除waiters链表中的node结点，也就时当当前线程被中断或者time-out时，从等待队列中删除该线程。
看看该算法的实现：

 1     /**
 2      * Tries to unlink a timed-out or interrupted wait node to avoid
 3      * accumulating garbage.  Internal nodes are simply unspliced
 4      * without CAS since it is harmless if they are traversed anyway
 5      * by releasers.  To avoid effects of unsplicing from already
 6      * removed nodes, the list is retraversed in case of an apparent
 7      * race.  This is slow when there are a lot of nodes, but we don't
 8      * expect lists to be long enough to outweigh higher-overhead
 9      * schemes.
10      */
11     private void removeWaiter(WaitNode node) {
12         if (node != null) {
13             node.thread = null;
14             retry:
15             for (;;) {          // restart on removeWaiter race
16                 for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
17                     s = q.next;
18                     if (q.thread != null)
19                         pred = q;
20                     else if (pred != null) {
21                         pred.next = s;
22                         if (pred.thread == null) // check for race
23                             continue retry;
24                     }
25                     else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
26                                                           q, s))
27                         continue retry;
28                 }
29                 break;
30             }
31         }
32     }

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算法有特别的地方，它保证了线程安全，但是没有用任何锁或者CAS操作。waiter链表添加元素只在头部进行，多个线程同时对链表traverse删除结点，会导致一个显式的竞争，即在删除node结点的同时(进入第20行代码后)，另外一个线程在删除pre结点，此时会出现这样一种可能，删除pre结点的线程中s(删除结点的后继结点)还指向node，导致node被重新赋值给pred->next，就是node没有被删除。处理的办法时当前线程restart从头开始再删除一遍node(此时结点pre已经被删除，node的前趋结点变成了pre的前趋结点)，代码中第22-23行代码，对这种竞争进行检查并处理。除此之外，多个线程同时释放结点，不会产生竞争问题。25-26行代码是删除链表第一个node的处理逻辑。

Future#cancel()方法的实现

参数mayInterruptIfRunning决定，task运行时能否被中断。

    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;
    }

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Runnable#run()方法的实现

实现的代码很简单，看下他用什么机制实现的禁止并发调用该任务和禁止重启该任务

    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);
        }
    }

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run()方法没有显式的中断处理逻辑，他通过state状态来回应cancel()的中断。cancel()提出中断时，将state置为interrupting，此时调用set()和setException()不产生任何效果。

runAndReset()方法，这个函数为那些执行多次且不带返回值的任务设置。

    /**
     * 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;
    }

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总结

FutureTask是通过LockSupport来阻塞线程，唤醒线程。对于多线程访问FeatureTask的 waiters,state，都是采用Unsafe来操作，避免使用锁，改为直接原子操作对应的变量。FeatureTask是一个非常好的Unsafe和LockSupport例子。