FutureTask源码阅读

public class FutureTask<V> implements RunnableFuture<V> { /** * 内部状态可能得迁转过程: * NEW -> COMPLETING -> NORMAL //正常完成 * NEW -> COMPLETING -> EXCEPTIONAL //发生异常 * NEW -> CANCELLED //取消 * NEW -> INTERRUPTING -> INTERRUPTED //中断 */ 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; /** 内部的callable，运行完成后设置为null */ private Callable<V> callable; /** The result to return or exception to throw from get() */ private Object outcome; // 没有volatile修饰, 依赖state的读写来保证可见性 /** 执行内部callable的线程*/ private volatile Thread runner; /** Treiber stack of waiting threads */ private volatile WaitNode waiters; /** * Returns result or throws exception for completed task. * * @param s completed state value */ @SuppressWarnings("unchecked") private V report(int s) throws ExecutionException { Object x = outcome; if (s == NORMAL)//任务完成 return (V)x; if (s >= CANCELLED) throw new CancellationException(); throw new ExecutionException((Throwable)x); } 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) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable } public boolean isCancelled() { return state >= CANCELLED; } public boolean isDone() { return state != NEW; } public boolean cancel(boolean mayInterruptIfRunning) { if (!(state == NEW && UNSAFE.compareAndSwapInt(this, stateOffset, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) return false; try { // in case call to interrupt throws exception if (mayInterruptIfRunning) { try { Thread t = runner; if (t != null) t.interrupt(); } finally { // final state UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); } } } finally { finishCompletion(); } return true; } public V get() throws InterruptedException, ExecutionException { int s = state; if (s <= COMPLETING) s = awaitDone(false, 0L);//如果任务还没执行完毕，等待任务执行完毕 return report(s);//如果任务执行完毕，获取执行结果 } 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); } protected void done() { } protected void set(V v) { //当前任务状态state从NEW改为COMPLETING if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = v; UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state finishCompletion();//唤醒Treiber Stack中所有等待线程 } } protected void setException(Throwable t) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = t; UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state finishCompletion(); } } public void run() { //如果state不为NE或者尝试设置runner为当前线程，失败就退出 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();//执行任务 ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex);//如果发生异常，设置异常 } if (ran)//如果正常执行完成，设置执行结果 set(result); } } finally { // runner必须在设置了state之后再置空，避免run方法出现并发问题 runner = null; int s = state; if (s >= INTERRUPTING)//处理可能发生的取消中断 handlePossibleCancellationInterrupt(s); } } 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; } /** * Ensures that any interrupt from a possible cancel(true) is only * delivered to a task while in run or runAndReset. */ private void handlePossibleCancellationInterrupt(int s) { // 如果当前正在中断过程中，自旋等中断完成 if (s == INTERRUPTING) while (state == INTERRUPTING) Thread.yield(); // wait out pending interrupt } /** * Simple linked list nodes to record waiting threads in a Treiber * stack. See other classes such as Phaser and SynchronousQueue * for more detailed explanation. */ static final class WaitNode { volatile Thread thread; volatile WaitNode next; WaitNode() { thread = Thread.currentThread(); } } //移除Treiber Stack，并将Treiber Stack中所有等待获取任务结果的线程唤醒，然后回调下done钩子方法 private void finishCompletion() { // assert state > COMPLETING; for (WaitNode q; (q = waiters) != null;) { //尝试将waiters设置为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 } 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()) { //如果当前线程被中断，移除等待节点q，然后抛出中断异常 removeWaiter(q); throw new InterruptedException(); } int s = state; if (s > COMPLETING) {//如果任务已经执行完毕 if (q != null) q.thread = null;//q不为null，将q中的thread置null return s;//返回状态 } else if (s == COMPLETING) // cannot time out yet Thread.yield();//如果当前正在完成过程中，出让CPU else if (q == null) q = new WaitNode();//创建一个等待节点 else if (!queued)//将q(包含当前线程的等待节点)入队 queued = UNSAFE.compareAndSwapObject(this, waitersOffset, q.next = waiters, q); else if (timed) { nanos = deadline - System.nanoTime(); if (nanos <= 0L) { removeWaiter(q);//如果超时，移除等待节点q return state; } LockSupport.parkNanos(this, nanos);//超时的话，就阻塞给定时间 } else LockSupport.park(this);//没设置超时的话，就阻塞当前线程 } } private void removeWaiter(WaitNode node) { if (node != null) { node.thread = null;//将node的thread域置空 retry://下面过程中会将node从等待队列中移除，以thread域为null为依据 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; else if (pred != null) { pred.next = s; if (pred.thread == null) // check for race continue retry; } else if (!UNSAFE.compareAndSwapObject(this, waitersOffset, q, s)) continue retry; } break; } } } }