FutureTask 源码学习

FutureTask主要利用 LockSupport 实现阻塞和唤醒

/** 要执行的内容 */
private Callable<V> callable;
/** 执行结果 */
private Object outcome;
/** 当前执行的线程 */
private volatile Thread runner;
/** 阻塞等待的链表，所有阻塞线程都保存在这里 */
private volatile WaitNode waiters;

WaitNode 只是简单保存了一个线程和 next 节点

static final class WaitNode {
    volatile Thread thread;
    volatile WaitNode next;
    WaitNode() { thread = Thread.currentThread(); }
}

状态有这几种：

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;

get()，如果没有设置超时时间，就是永久阻塞等待；设置了超时时间，就会
阻塞一定时间，都是通过 LockSupport.park()

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

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)
            // 把当前阻塞的 Node 放到链表里
            // 这种 CAS 是用的内存偏移量
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                 q.next = waiters, q);
        else if (timed) {
            nanos = deadline - System.nanoTime();
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            // this 这个参数是 blocker，为了打印堆栈的时候能看到被阻塞的是什么
            LockSupport.parkNanos(this, nanos);
        }
        else
            LockSupport.park(this);
    }
}

另一边是执行的run()方法

public void run() {
    if (state != NEW ||
        // 赋值当前线程为变量 runner
        !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);
    }
}

// 遍历waiters，一个个唤醒阻塞的线程
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;

                    // 唤醒等待的线程
                    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
}   

用偏移地址来指代那些变量

// 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);
    }
}