jdk1.8 J.U.C之FutureTask实现机制分析

我画了一张关于FutureTask的类图，主要包括FutureTask的几个重要的函数和字段，还有它和父类的关系。

根据上面图我们可以清晰的看出FutureTask的继承关系。FutureTask继承一个最重要的类是Future，有几个比较重要的方法get,cancel,isCancelled等。FutureTask是为了弥补Thread的不足而设计的，它可以让程序员准确地知道线程什么时候执行完成并获得到线程执行完成后返回的结果（如果有需要）。FutureTask是一种可以取消的异步的计算任务。它的计算是通过Callable实现的，它等价于可以携带结果的Runnable，并且有三个状态：等待、运行和完成。完成包括所有计算以任意的方式结束，包括正常结束、取消和异常。
```java
public class FutureTaskTest {
public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool();
FutureTask futureTask = new FutureTask(new Callable() {
public Integer call() throws Exception {
System.out.println("执行一个比较耗时的任务");
Thread.sleep(3000);
int sum = 0;
for(int i=0;i System.out.println("主线程....");

try {
futureTask.get();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}

System.out.println("所有的任务执行完毕....");
}


}

从上面的例子中我们可以看到FutureTask在执行异步的时候是需要依托线程池的。也就是调用 Executors.newCachedThreadPool()，submit是把futureTask这个Task交给线程池，我们可以看下AbstractExecutorService的submit:
```java
   public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

然后执行execute，这个方法是Executor接口中的唯一方法

public interface Executor {
    void execute(Runnable command);
}

方法是实现是在ThreadPollExecutor，这个方法主要是从线程池中获取一个线程来执行call任务，具体的逻辑我暂时还不是很清楚，因为这块源码我还没有研究过，有空的时候再研究下到时候更新下这篇文章。

 public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();

        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

下面是FutureTask的源码分析，futureTask初始化

public FutureTask(Callable<V> callable) {
    if (callable == null)
        throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;       // ensure visibility of callable
}

state是一个volatile的变量,用线程的状态

/**
 * 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;

注释已经写得很清楚了，初始化的时候是NEW，Callable的call执行完就变为COMPLETING没有异常就再改为 NORMAL，否则就是EXCEPTIONAL

当线程开始执行的时候会先执行run函数。那么一般的我们的task会在初始化的时候new一个Callable对象，并重写call函数。所以在run的时候是会执行call这个函数。看下call代码就一目了然了。

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);
    }
}

所以当call还没执行完我们的主线程会继续执行直到遇到get方法。get会执行阻塞，那么call什么时候才会被唤醒呢，这个就需要操作系统的支持了，这里的处理方式跟AQS一样是托管给操作系统，先把线程挂起，等待操作系统唤醒，那么get这个方法到底做了什么处理呢？

public V get() throws InterruptedException, ExecutionException {
    int s = state;
    if (s <= COMPLETING)
        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;
        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);
    }
}

这个waitDone最重要的职能有两点一个是把当前线程的WaitNode赋值给waiters，通过CAS。二是把这个线程阻塞，通过LockSupport.park，直到call执行完毕，那么run就会执行set方法，set主要是修改state的状态：

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;
                    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
}

有意思的是FutureTask预留了一个done函数给它的子类用，有需要的开发者可以对这个函数进行重写。

protected void done() { 

参考：

http://ifeve.com/abstractqueuedsynchronizer-use/#more-18899