类关系图

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FutureTask实现了RunableFuture接口。间接实现了Runable接口和Funture。所以他也是可以丢给线程去执行

构造器

  1.     public FutureTask(Callable<V> callable) {
  2.         if (callable == null)
  3.             throw new NullPointerException();
  4.         this.callable = callable;//真正执行的方法
  5.         this.state = NEW;       // ensure visibility of callable 确保callable的可见性
  6.     }

构造器初始化的时候要传一个Callable，这个是我们程序真正要执行的方法

Callable接口

  1. @FunctionalInterface
  2. public interface Callable<V> {
  4.     V call() throws Exception;
  5. }

需要执行的方法类去实现callable这个接口，并返回结果

run()线程调度的方法

  1.     public void run() {
  2.         if (state != NEW ||
  3.             !UNSAFE.compareAndSwapObject(this, runnerOffset,
  4.                                          null, Thread.currentThread()))
  5.             return;
  6.         try {
  7.             Callable<V> c = callable;//构造器传过来的需要完成的任务
  8.             if (c != null && state == NEW) {
  9.                 V result;
  10.                 boolean ran;
  11.                 try {
  12.                     result = c.call();//执行任务
  13.                     ran = true;
  14.                 } catch (Throwable ex) {
  15.                     result = null;
  16.                     ran = false;
  17.                     setException(ex);//设置异常
  18.                 }
  19.                 if (ran)
  20.                     set(result);//设置结果
  21.             }
  22.         } finally {
  23.             // runner must be non-null until state is settled to
  24.             // prevent concurrent calls to run()
  25.             runner = null;
  26.             // state must be re-read after nulling runner to prevent
  27.             // leaked interrupts
  28.             int s = state;
  29.             if (s >= INTERRUPTING)
  30.                 handlePossibleCancellationInterrupt(s);
  31.         }
  32.     }

调用.state启动线程后，cpu会去调用run方法。run方法调用call去执行真正的任务

set(V v)保存结果

  1.     protected void set(V v) {
  2.         if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {//cas保证线程安全
  3.             outcome = v;//保存结果
  4.             UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state 设置最终状态
  5.             finishCompletion();//唤醒get的时候阻塞的线程
  6.         }
  7.     }

finishCompletion() 唤醒等待结果的线程

  1.     private void finishCompletion() {
  2.         // assert state > COMPLETING;
  3.         for (WaitNode q; (q = waiters) != null;) {
  4.             if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {//cas
  5.                 for (;;) {
  6.                     Thread t = q.thread;
  7.                     if (t != null) {
  8.                         q.thread = null;
  9.                         LockSupport.unpark(t);//唤醒线程
  10.                     }
  11.                     WaitNode next = q.next;
  12.                     if (next == null)
  13.                         break;
  14.                     q.next = null; // unlink to help gc
  15.                     q = next;
  16.                 }
  17.                 break;
  18.             }
  19.         }
  20.         done();
  21.         callable = null;        // to reduce footprint
  22.     }

V get() 获取结果

  1.     public V get() throws InterruptedException, ExecutionException {
  2.         int s = state;
  3.         if (s <= COMPLETING)//结果还未出来
  4.             s = awaitDone(false, 0L);//进入等待
  5.         return report(s);//获取结果
  6.     }

awaitDone(boolean timed, long nanos)等待执行任务完成

  1.     private int awaitDone(boolean timed, long nanos)
  2.         throws InterruptedException {
  3.         final long deadline = timed ? System.nanoTime() + nanos : 0L;//超时等待
  4.         WaitNode q = null;
  5.         boolean queued = false;
  6.         for (;;) {
  7.             if (Thread.interrupted()) {//中断标记
  8.                 removeWaiter(q);
  9.                 throw new InterruptedException();
  10.             }
  12.             int s = state;
  13.             if (s > COMPLETING) {//线程结束
  14.                 if (q != null)
  15.                     q.thread = null;
  16.                 return s;
  17.             }
  18.             else if (s == COMPLETING) // cannot time out yet 马上结束了
  19.                 Thread.yield();
  20.             else if (q == null)
  21.                 q = new WaitNode();//创建一个等待节点
  22.             else if (!queued)
  23.                 queued = UNSAFE.compareAndSwapObject(this, waitersOffset,//将等待节点加入等待队列
  24.                                                      q.next = waiters, q);
  25.             else if (timed) {
  26.                 nanos = deadline - System.nanoTime();
  27.                 if (nanos <= 0L) {
  28.                     removeWaiter(q);
  29.                     return state;
  30.                 }
  31.                 LockSupport.parkNanos(this, nanos);//超时等待的方法，到时间就自动唤醒
  32.             }
  33.             else
  34.                 LockSupport.park(this);//等待被唤醒
  35.         }
  36.     }

V report(int s)真正获取结果

  1.     private V report(int s) throws ExecutionException {
  2.         Object x = outcome;
  3.         if (s == NORMAL)
  4.             return (V)x;//获取结果
  5.         if (s >= CANCELLED)
  6.             throw new CancellationException();//取消
  7.         throw new ExecutionException((Throwable)x);//执行异常
  8.     }

如果执行异常也会获取并抛出