FutureTask 原理剖析

ThreadPoolExecutor executor = new ThreadPoolExecutor(5, 10, 60, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
Future future = executor.submit(() -> {   
 System.out.println("hello world");    
return "hello world";});
System.out.println(future.get());

Future接口声明如下：

FutureTask除了实现Future接口外，还实现了Runnable接口。因此，FutureTask可以交给Executor执行，也可以由调用线程直接执行（FutureTask.run()）。根据FutureTask.run()方法被执行的时机，FutureTask可以处于以下3种状态：未启动、运行中、已完成。

当FutureTask处于未启动或已启动状态时，执行FutureTask.get()方法将导致调用线程阻塞；当FutureTask处于已完成状态时，执行FutureTask.get()方法将导致调用线程立即返回结果或抛出异常。

• 当FutureTask处于未启动状态时，执行FutureTask.cancel()方法将导致此任务永远不会被执行；

• 当FutureTask处于已启动状态时，执行FutureTask.cancel（true）方法将以中断执行此任务线程的方式来试图停止任务；

• 当FutureTask处于已启动状态时，执行FutureTask.cancel（false）方法将不会对正在执行此任务的线程产生影响（让正在执行的任务运行完成）；

• 当FutureTask处于已完成状态时，执行FutureTask.cancel（…）方法将返回false。

FutureTask的生命周期如下：

Future.get() 阻塞/唤醒原理

执行future.get()时，如果对应线程还未执行完，则会阻塞当前线程，以FutureTask为例，FutureTask中有一个int型的状态标志，表示future对应线程的运行状态。

/** * 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;

当调用FutureTask.get()时，如果Future对应的任务已完成（正常执行完成或者抛出异常），执行返回；如果Future对应的任务未执行完成，则会将当前线程封装成一个NodeWait，以CAS方式添加到FutureTask.waiters链表上（单向链表，新节点都会作为head node添加上），然后会阻塞当前线程（包括超时阻塞）。

public V get() throws InterruptedException, ExecutionException {   
 int s = state;   
 if (s <= COMPLETING) // 线程未执行完成       
 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) { // 线程已运行完成           
 if (q != null)               
 q.thread = null;           
 return s;        }       
 else if (s == COMPLETING) // cannot time out yet           
 Thread.yield(); // future task已完成，正在赋值outcome，get()返回的值就是outcome，这时不用加入WaitNode即可      
  else if (q == null)            q = new WaitNode(); // 生成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);    }}
private V report(int s) throws ExecutionException {    Object x = outcome;    if (s == NORMAL) // 正常执行结束       
 return (V)x;    if (s >= CANCELLED) // 已取消        throw new CancellationException();    throw new ExecutionException((Throwable)x); // 抛出异常}

在任务执行（run()方法）中，调用result = callable.call方法，正常执行完毕后调用set(result)设置Future结果；出现异常则调用setException(ex)。最后会调用finishCompletion()来唤醒阻塞在Future的所有线程。

设置完数据之后（不管是正常数据还是对应异常），当等待数据的线程来get时，就会返回或者直接给它抛异常；如果当线程已经get过并阻塞在这里时，FutureTask需要将这些线程唤醒起来。

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);    }}protected void set(V v) {    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {        outcome = v;        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state        finishCompletion();    }}protected void setException(Throwable t) {    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {        outcome = t;        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state        finishCompletion();    }}// 唤醒所有等待线程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}

小结

FutureTask中的waiters是一个单向链表，如果多个线程阻塞在该Future上，最新阻塞的线程排列在链表前面，唤醒线程时依次从前到后遍历链表唤醒线程，这样处理貌似对最开始阻塞在Future上的线程不太公平哈，因为最开始阻塞的线程是到最后才被唤醒的。