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Future详解( 二 )

生活百科Future详解

2）构造方法
public FutureTask(Callable<V> callable) {if (callable == null)throw new NullPointerException();this.callable = callable;this.state = NEW;// ensure visibility of callable}public FutureTask(Runnable runnable, V result) {//封装成callable，但返回值为传入的值this.callable = Executors.callable(runnable, result);this.state = NEW;}3）核心方法
1.run()方法
public void run() {if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread()))return;// 只有当任务状态为new并且runner旧值为null才会执行到这里try {Callable<V> c = callable;if (c != null && state == NEW) {V result;boolean ran;try {// 调用callable.run()并返回结果result = c.call();ran = true;} catch (Throwable ex) {result = null;ran = false;setException(ex);}if (ran)// 内部设置outcome为callable执行的结果，并且更新任务的状态为NORMAL（任务正常执行）并且唤醒阻塞的线程set(result);}} finally {runner = null;int s = state;if (s >= INTERRUPTING)// 如果当前任务处于中断中，则执行这个方法线程会不断让出cpu直到任务处于已中断状态handlePossibleCancellationInterrupt(s);}}2.set(V v)方法
protected void set(V v) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {// 设置outcome（结果）为callable.run()返回的结果outcome = v;//修改状态UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state// 唤醒调用get()的所有等待的线程并清空栈finishCompletion();}}protected void setException(Throwable t) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {// 设置outcome（结果）为callable.run()抛出的异常outcome = t;UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final statefinishCompletion();}}3.get()方法
【Future详解】public V get() throws InterruptedException, ExecutionException {int s = state;// 条件成立会调用awaitDone方法自旋等待直到任务完成if (s <= COMPLETING) s = awaitDone(false, 0L);return report(s);}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);}//这个方法是真正用来获取任务的返回结果的，这个方法在get()方法里面会被调用，如果该方法被调用，说明任务已经执行完了 。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);}4.awaitDone(boolean timed, long nanos)方法
// 这个方法的作用是等待任务被完成（正常完成或出现异常完成都算完成），被中断，或是被超时private int awaitDone(boolean timed, long nanos)throws InterruptedException {final long deadline = timed ? System.nanoTime() + nanos : 0L;WaitNode q = null;boolean queued = false;for (;;) {// 如果当前线程出现中断异常，则将该线程代表的WaitNode结点移出栈并抛出中断异常if (Thread.interrupted()) {removeWaiter(q);throw new InterruptedException();}int s = state;// 如果当前任务状态大于COMPLETING，说明当前任务已经有结果了（任务完成、中断、取消），直接返回任务状态if (s > COMPLETING) {if (q != null)q.thread = null;return s;}// 当前任务处于临界状态，即将完成，则当前线程释放cpuelse if (s == COMPLETING) // cannot time out yetThread.yield();// 第一次自旋，如果当前WitNode为null，new一个WaitNode结点else if (q == null)q = new WaitNode();// 第二次自旋，如果当前WaitNode节点没有入队，则尝试入队else if (!queued)queued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);// 第三次自旋，到这里表示是否定义了超时时间else if (timed) {nanos = deadline - System.nanoTime();if (nanos <= 0L) {removeWaiter(q);return state;}// 未超出时间，挂起当前线程一定时间LockSupport.parkNanos(this, nanos);}else// 挂起当前线程，该线程会休眠（什么时候该线程会继续执行呢？除非有其他线程调用unpark()或者中断该线程）LockSupport.park(this);}}

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