【JUC】可回调任务FutureTask原理

上一篇观察ThreadPoolExecutor的submit方法的时候，发现了它是靠FutureTask实现结果回调的：

public <T> Future<T> submit(Callable<T> task) {
    if (task == null) throw new NullPointerException();
    // ## 声明一个可回调任务，本质是一个FutureTask
    RunnableFuture<T> ftask = newTaskFor(task);
    // 线程池篇分析过
    execute(ftask);
    return ftask;
}

protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
    return new FutureTask<T>(callable);
}

一、FutureTask使用样例

// 1.声明一个可回调任务
FutureTask<String> task = new FutureTask(()->"hello world");
Thread threadA = new Thread(task);
threadA.start();

// 2.阻塞方式获取任务执行结果：threadA未执行完，当前线程TreadB会阻塞于此
System.out.println(task.get());

FutureTask实现了RunnableFuture接口，而RunnableFuture=Runnable接口+Future接口

1. 线程A执行start方法时，会调用FutureTask的run()方法（Runnable接口）
   run()方法会触发FutureTask内部的state状态变更，并调用Callable的call()方法
2. 此时线程B以及其它线程调用FutureTask的get()方法（Future接口），这些线程会阻塞等待run()方法完成
   源码层面会构建一个名为waiters的单项链表，以LockSupport.part的形式将线程阻塞在节点上
3. call()方法执行完成，state状态最终变更为NORMAL，同时释放阻塞线程

// ## 状态
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;

// 结果返回接口
private Callable<V> callable;
// 线程执行方法的返回结果
private Object outcome; // non-volatile, protected by state reads/writes
// 正在执行callable接口发的线程
private volatile Thread runner;
// 等待节点
private volatile WaitNode waiters;

二、run()

public void run() {
    if (state != NEW 
        // runner变量赋值
        || !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
        
    try {
        Callable<V> c = callable;
        // NEW状态下执行
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                // 调用Callable的call方法，获取返回值
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                setException(ex);
            }
            if (ran)
                // == call方法执行成功，设置结果
                set(result);
        }
    } 
}

protected void set(V v) {
    // 状态变更NEW->COMPLETING
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        // 执行结果赋值给outcome
        outcome = v;
        // 状态变更COMPLETING->NORMAL，表示执行完成
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
        // == 释放等待队列中的阻塞线程
        finishCompletion();
    }
}

private void finishCompletion() {
    // assert state > COMPLETING;
    for (WaitNode q; (q = waiters) != null;) {
        // cas方式将waiters变量设置为null
        if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
            // ## 遍历队列（单向链表）中的WaitNode节点，释放全部的等待线程
            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;
        }
    }

    // 提供的监听方法，需用户自定义实现
    done();

    callable = null;        // to reduce footprint
}

三、get()

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    // NEW和COMPLETING状态触发等待
    if (s <= COMPLETING)
        // == 等待完成
        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;
        // -- 检查状态，如果此时已变成NORMAL则无需等待
        if (s > COMPLETING) {
            if (q != null)
                q.thread = null;
            return s;
        }
        // -- 检查状态，如果此时是COMPLETING，切换其它线程执行
        else if (s == COMPLETING) // cannot time out yet
            Thread.yield();
        // -- 新建等待节点
        else if (q == null)
            q = new WaitNode();
        // -- waiters变量赋值
        else if (!queued)
            // ## 头插
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                 q.next = waiters, q);
        // -- 有超时设置情况
        else if (timed) {
            nanos = deadline - System.nanoTime();
            // ## 如果已超时，移除节点
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
            // ## 如果未超时，阻塞指定时间
            LockSupport.parkNanos(this, nanos);
        }
        // -- 线程阻塞
        else
            LockSupport.park(this);
    }
}