【知识库】--FutureTask 异步原理（242）

1 任务提交给线程池后，直接get()--可能堵塞--park

    /**     * @throws CancellationException {@inheritDoc}     */    public V get() throws InterruptedException, ExecutionException {        int s = state;        if (s <= COMPLETING)            s = awaitDone(false, 0L);//可能堵塞        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);//调用native方法，将本线程part（因为this） 直到unpark唤醒        }    }

2 当线程池之行run()方法之后---触发因为该对象this而park住的线程----unpark

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);//运行结果            }        } 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);        }    }

    /**     * 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();//通知park住的线程        }    }

/**     * 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);//通知t线程                    }                    WaitNode next = q.next;                    if (next == null)                        break;                    q.next = null; // unlink to help gc                    q = next;                }                break;            }        }        done();        callable = null;        // to reduce footprint    }