futuretask源码分析（推荐）

public class FutureTask<V> implements RunnableFuture<V> {      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;    private Callable<V> callable;    private Object outcome; // non-volatile, protected by state reads/writes    private volatile Thread runner;    private volatile WaitNode waiters;    @SuppressWarnings("unchecked")  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);  }    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  }  //判断任务是否已取消(异常中断、取消等)  public boolean isCancelled() {    return state >= CANCELLED;  }    public boolean cancel(boolean mayInterruptIfRunning) {    if (state != NEW)      return false;    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    }    else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))      return false;    finishCompletion();    return true;  }    public V get() throws InterruptedException, ExecutionException {    int s = state;    //如果任务未彻底完成，那么则阻塞直至任务完成后唤醒该线程    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);  }    protected void done() { }    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();    }  }    public void run() {    //只有当任务状态=new时才被运行继续执行    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 {          //调用Callable的Call方法          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 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 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      s = state;      if (s >= INTERRUPTING)        handlePossibleCancellationInterrupt(s);    }    return ran && s == NEW;  }    private void handlePossibleCancellationInterrupt(int s) {    // It is possible for our interrupter to stall before getting a    // chance to interrupt us. Let's spin-wait patiently.    if (s == INTERRUPTING)      while (state == INTERRUPTING)        Thread.yield(); // wait out pending interrupt    // assert state == INTERRUPTED;    // We want to clear any interrupt we may have received from    // cancel(true). However, it is permissible to use interrupts    // as an independent mechanism for a task to communicate with    // its caller, and there is no way to clear only the    // cancellation interrupt.    //    // Thread.interrupted();  }    static final class WaitNode {    volatile Thread thread;    volatile WaitNode next;    WaitNode() { thread = Thread.currentThread(); }  }    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  }    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);    }  }    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;          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;      }    }  }  // Unsafe mechanics  private static final sun.misc.Unsafe UNSAFE;  private static final long stateOffset;  private static final long runnerOffset;  private static final long waitersOffset;  static {    try {      UNSAFE = sun.misc.Unsafe.getUnsafe();      Class<?> k = FutureTask.class;      stateOffset = UNSAFE.objectFieldOffset        (k.getDeclaredField("state"));      runnerOffset = UNSAFE.objectFieldOffset        (k.getDeclaredField("runner"));      waitersOffset = UNSAFE.objectFieldOffset        (k.getDeclaredField("waiters"));    } catch (Exception e) {      throw new Error(e);    }  }}