FutureTask相关

• 上周因为项目中的线程池参数设置的不合理，引发了一些问题，看了下代码，发现对JUC中的一些概念需要再清晰些。

Runnable

@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

• Runable是一个interface，定义了run()方法，The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread。如果想在其他线程中执行你的task，需要实现这个接口。

Callable

@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;
}

• 有了Runnable，为啥还需要Callable呢，可以看到Runnable和Callable的两个不同，第一，Runnable是没有返回值的，第二，Runnable是不会抛出checked exception的，而有时候我们需要知道任务执行之后的返回，同时也希望利用异常机制完成一些逻辑。所以有了Callable。
• JUC中的Executors这个Factory类，提供了Runnable转Callable的方法。

Future

• future 是一个inteface，提供了一系列方法，来帮助我们获取异步执行的task的执行状况和执行结果。

FutureTask

• FutureTask实现了RunnableFuture接口，即既实现了Runnable接口，又实现了Future接口。所以他有两个功能，第一，作为一个task，提交到别的线程中异步执行，第二，通过future提供的一些接口，获取task的异步执行状态。

/**
     * 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 */
    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;

• 看下FutureTask的几个属性，首先state表示当前task的执行状态，其中，开始状态位NEW表示task还没开始执行。NORMAL，CANCELLED，INTERRUPTED为终态，COMPLETING和INTERRUPTING为临时状态，最终会通过上面的几个状态转移路径，转移到终态。
• callable，表示具体执行的任务。
• outcome， task 执行的返回结果
• runner，执行这个task的线程
• waiters，通过get方法获取此task执行结果被阻塞的线程。
• 看下几个核心的方法，我们知道，futuretask提交到别的线程里后，最终会调用task的run方法执行具体逻辑。

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);
        }
    }

• run方法执行时，首先检查当前的状态是否是NEW，如果不是NEW说明已经被执行过了。开始执行之前，标记执行当前task的线程到runner。
• 调用callable的run方法，执行。抛异常时，设置setException。正常结束时，set结果。看下这两步里都会调到的finishCompletion方法。

/**
     * 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
    }

• 这里主要是在通知所有阻塞在watch这个task结果的线程，通知他们当前task已经执行结束了。
• 在执行结束时，看到finally里还有段逻辑

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);
        }

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();
    }

• 这是在干嘛呢，是因为，即使我们在上一步通过set或者setException设置了当前task的状态，但可能有别的线程在通过调用cancel来设置当前task的状态，如果有的话，这里就自旋空转，直到cancel方法执行结束。
• 那cancel方法是怎么工作的呢

public boolean cancel(boolean mayInterruptIfRunning) {
        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;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
                }
            }
        } finally {
            finishCompletion();
        }
        return true;
    }

• cancel方法其实就是通过找到执行当前task的runner，然后调用thread的interrupt方法，这里需要注意的是，thread.interrupt方法仅仅是设置一个标志位，具体线程有没有响应，要看自己的实现。反正这里就是调一把interrupt然后就走了，然后通知所有watch的线程。
• watch的线程，通过get方法获得执行结果是怎么拿到的呢

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);
        }
    }

• 核心逻辑就是，先把自己这个线程放到watch的waitNodes栈中，然后park 等待，直到task的状态>COMPLETING.

reference

• https://segmentfault.com/a/1190000016572591
• https://segmentfault.com/a/1190000016542779