Java线程池中的Future实现详解

  Future<Integer> result = executor.submit(task); //有返回值
  executor.executor(task)  //没有返回值

public class Test {
    public static void main(String[] args) {
        ExecutorService executor = Executors.newCachedThreadPool();
        Task task = new Task();
        Future<Integer> result = executor.submit(task);
        executor.shutdown();
         
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }
         
        System.out.println("主线程在执行任务");
         
        try {
            System.out.println("task运行结果"+result.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        } 
        System.out.println("所有任务执行完毕");
    }
}
class Task implements Callable<Integer>{
    @Override
    public Integer call() throws Exception {
        System.out.println("子线程在进行计算");
        Thread.sleep(3000);
        int sum = 0;
        for(int i=0;i<100;i++)
            sum += i;
        return sum;
    }
}

public void run() {
   // 如果状态不是 NEW，说明任务已经执行过或者已经被取消，直接返回
   // 如果状态是 NEW，则尝试把执行线程保存在 runnerOffset（runner字段），如果赋值失败，则直接返回
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
       // 获取构造函数传入的 Callable 值
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
               // 正常调用 Callable 的 call 方法就可以获取到返回值
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
               // 保存 call 方法抛出的异常
                setException(ex);
            }
            if (ran)
               // 保存 call 方法的执行结果
                set(result);
        }
    } finally {        
        runner = null;       
        int s = state;
       // 如果任务被中断，则执行中断处理
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes

// 保存异常结果
protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}

// 保存正常结果
protected void set(V v) {
  if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
    outcome = v;
    UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
    finishCompletion();
  }
}

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;
                   // 唤醒等待队列中的线程
                    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
}  

public V get() throws InterruptedException, ExecutionException {
    int s = state;
   // 如果 state 还没到 set outcome 结果的时候，则调用 awaitDone() 方法阻塞自己
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
   // 返回结果
    return report(s);
}

// get 方法支持超时限制，如果没有传入超时时间，则接受的参数是 false 和 0L
// 有等待就会有队列排队或者可响应中断，从方法签名上看有 InterruptedException，说明该方法这是可以被中断的
private int awaitDone(boolean timed, long nanos)
    throws InterruptedException {
   // 计算等待截止时间
    final long deadline = timed ? System.nanoTime() + nanos : 0L;
    WaitNode q = null;
    boolean queued = false;
    for (;;) {
       // 如果当前线程被中断，如果是，则在等待对立中删除该节点，并抛出 InterruptedException
        if (Thread.interrupted()) {
            removeWaiter(q);
            throw new InterruptedException();
        }

        int s = state;
       // 状态大于 COMPLETING 说明已经达到某个最终状态（正常结束/异常结束/取消）
       // 把 thread 只为空，并返回结果
        if (s > COMPLETING) {
            if (q != null)
                q.thread = null;
            return s;
        }
       // 如果是COMPLETING 状态（中间状态），表示任务已结束，但 outcome 赋值还没结束，这时主动让出执行权，让其他线程优先执行（只是发出这个信号，至于是否别的线程执行一定会执行可是不一定的）
        else if (s == COMPLETING) // cannot time out yet
            Thread.yield();
       // 等待节点为空
        else if (q == null)
           // 将当前线程构造节点
            q = new WaitNode();
       // 如果还没有入队列，则把当前节点加入waiters首节点并替换原来waiters
        else if (!queued)
            queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                 q.next = waiters, q);
       // 如果设置超时时间
        else if (timed) {
            nanos = deadline - System.nanoTime();
           // 时间到，则不再等待结果
            if (nanos <= 0L) {
                removeWaiter(q);
                return state;
            }
           // 阻塞等待特定时间
            LockSupport.parkNanos(this, nanos);
        }
        else
           // 挂起当前线程，知道被其他线程唤醒
            LockSupport.park(this);
    }
}

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
       // 需要中断任务执行线程
        if (mayInterruptIfRunning) {
            try {
                Thread t = runner;
               // 中断线程
                if (t != null)
                    t.interrupt();
            } finally { // final state
               // 修改为最终状态 INTERRUPTED
                UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
            }
        }
    } finally {
       // 唤醒等待中的线程
        finishCompletion();
    }
    return true;
}