讀FutureTask源碼

//一個可以異步返回計算的結果
//它同時實現了Future和Runnable
//先看構造函數
 public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

//運行runnable並返回給定的result
public FutureTask(Runnable runnable, V result) {
        //適配器模式轉化runnable接口
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

//適配器模式轉化runnable接口
 static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }


 public void run() {
        //如果state不等於0或者設置當前已經被其他線程佔用了直接返回。
        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);
                }
		//如果已經執行了設置值
                if (ran)
                    set(result);
            }
        } finally {

            runner = null;
            int s = state;
	    //如果被取消了
            if (s >= INTERRUPTING)
	    //讓出執行權
                handlePossibleCancellationInterrupt(s);
        }
    }


//設定指定值
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() {
        //釋放所有等待線程
        for (WaitNode q; (q = waiters) != null;) {
	    //清空當前線程成功
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
			//釋放線程
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        //鉤子方法
        done();

        callable = null;        // to reduce footprint
    }


private void handlePossibleCancellationInterrupt(int s) {

        if (s == INTERRUPTING)
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending interrupt

    }

protected void done() { }

//獲取結果
public V get() throws InterruptedException, ExecutionException {
        int s = state;
	//如果還爲完成
        if (s <= COMPLETING)
	    //在這上面阻塞
            s = awaitDone(false, 0L);
        return report(s);
    }

//加入隊列阻塞當前線程
 private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
	    //如果當前線程已經被中斷了
            if (Thread.interrupted()) {
	        //從等待隊列中清空他
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
	    //已經執行過了
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
	        //讓出執行權
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
	        //加入隊列
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
		//超時刪除q
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
		//掛起當前線程
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
	            //到這裏說明q.thread==null,q是需要刪除的節點。
                    else if (pred != null) {
		        //修改上一個節點的next
                        pred.next = s;
			//上一個節點被刪了,重新循環。
                        if (pred.thread == null) 
                            continue retry;
                    }
		    //走到這裏說明第一個節點就是被刪除的節點
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
			//設置失敗重新循環
                        continue retry;
                }
                break;
            }
        }
    }

//獲取值
private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }

//在一定時間內等待獲取結果超時拋出異常
public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }


//任務是否取消
public boolean isCancelled() {
        return state >= CANCELLED;
    }

//任務是否完成
 public boolean isDone() {
        return state != NEW;
    }


//試圖取消任務的執行
public boolean cancel(boolean mayInterruptIfRunning) {
        //任務已經開始直接返回false
        if (state != NEW)
            return false;
	 //試圖中斷
        if (mayInterruptIfRunning) {
            if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
                return false;
            Thread t = runner;
            if (t != null)
                t.interrupt();
            UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
        }
	//嘗試取消
        else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
            return false;
        finishCompletion();
        return true;
    }

//將結果設置爲異常
 protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }


//執行計算但不設置結果執行完後重置
protected boolean runAndReset() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don't set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } finally {
            runner = null;
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }