futuretask源码分析（推荐）

public class FutureTask<V> implements RunnableFuture<V> {  /*   * Revision notes: This differs from previous versions of this   * class that relied on AbstractQueuedSynchronizer, mainly to   * avoid surprising users about retaining interrupt status during   * cancellation races. Sync control in the current design relies   * on a "state" field updated via CAS to track completion, along   * with a simple Treiber stack to hold waiting threads.   *   * Style note: As usual, we bypass overhead of using   * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.   */  /**   * 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;  /** The underlying callable; nulled out after running */  private Callable<V> callable;  /** 用来存储任务执行结果或者异常对象,根据任务state在get时候选择返回执行结果还是抛出异常 */  private Object outcome; // non-volatile, protected by state reads/writes  /** 当前运行Run方法的线程 */  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)      return (V)x;    if (s >= CANCELLED)      throw new CancellationException();    throw new ExecutionException((Throwable)x);  }  /**   * Creates a {@code FutureTask} that will, upon running, execute the   * given {@code Callable}.   *   * @param callable the callable task   * @throws NullPointerException if the callable is null   */  public FutureTask(Callable<V> callable) {    if (callable == null)      throw new NullPointerException();    this.callable = callable;    this.state = NEW;    // ensure visibility of callable  }  /**   * Creates a {@code FutureTask} that will, upon running, execute the   * given {@code Runnable}, and arrange that {@code get} will return the   * given result on successful completion.   *   * @param runnable the runnable task   * @param result the result to return on successful completion. If   * you don't need a particular result, consider using   * constructions of the form:   * {@code Future<?> f = new FutureTask<Void>(runnable, null)}   * @throws NullPointerException if the runnable is null   */  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;  }  /**  判断任务是否已结束（取消、异常、完成、NORMAL都等于结束）  **  public boolean isDone() {    return state != NEW;  }  /**  mayInterruptIfRunning用来决定任务的状态。          true : 任务状态= INTERRUPTING = 5。如果任务已经运行，则强行中断。如果任务未运行，那么则不会再运行          false：CANCELLED  = 4。如果任务已经运行，则允许运行完成（但不能通过get获取结果）。如果任务未运行，那么则不会再运行  **/  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;  }  /**   * @throws CancellationException {@inheritDoc}   */  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);  }  /**   * @throws CancellationException {@inheritDoc}   */  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 method invoked when this task transitions to state   * {@code isDone} (whether normally or via cancellation). The   * default implementation does nothing. Subclasses may override   * this method to invoke completion callbacks or perform   * bookkeeping. Note that you can query status inside the   * implementation of this method to determine whether this task   * has been cancelled.   */  protected void done() { }  /**  该方法在FutureTask里只有run方法在任务完成后调用。  主要保存任务执行结果到成员变量outcome 中，和切换任务执行状态。  由该方法可以得知：  COMPLETING ： 任务已执行完成（也可能是异常完成），但还未设置结果到成员变量outcome中，也意味着还不能get  NORMAL  ： 任务彻底执行完成  **/  protected void set(V v) {    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {      outcome = v;      UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state      finishCompletion();    }  }  /**   * Causes this future to report an {@link ExecutionException}   * with the given throwable as its cause, unless this future has   * already been set or has been cancelled.   *   * <p>This method is invoked internally by the {@link #run} method   * upon failure of the computation.   *   * @param t the cause of failure   */  protected void setException(Throwable t) {    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {      outcome = t;      UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state      finishCompletion();    }  }  /**  由于实现了Runnable接口的缘故，该方法可由执行线程所调用。  **/  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);    }  }  /**  如果该任务在执行过程中不被取消或者异常结束，那么该方法不记录任务的执行结果，且不修改任务执行状态。  所以该方法可以重复执行N次。不过不能直接调用，因为是protected权限。  **/  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;  }  /**   * 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) {    // 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();  }  /**   * 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(); }  }  /**  该方法在任务完成（包括异常完成、取消）后调用。删除所有正在get获取等待的节点且唤醒节点的线程。和调用done方法和置空callable.  **/  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  }  /**  阻塞等待任务执行完成（中断、正常完成、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 (;;) {      /**      这里的if else的顺序也是有讲究的。      1.先判断线程是否中断，中断则从队列中移除(也可能该线程不存在于队列中)      2.判断当前任务是否执行完成，执行完成则不再阻塞，直接返回。      3.如果任务状态=COMPLETING，证明该任务处于已执行完成，正在切换任务执行状态，CPU让出片刻即可      4.q==null，则证明还未创建节点，则创建节点      5.q节点入队      6和7.阻塞      **/      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);    }  }  /**   * Tries to unlink a timed-out or interrupted wait node to avoid   * accumulating garbage. Internal nodes are simply unspliced   * without CAS since it is harmless if they are traversed anyway   * by releasers. To avoid effects of unsplicing from already   * removed nodes, the list is retraversed in case of an apparent   * race. This is slow when there are a lot of nodes, but we don't   * expect lists to be long enough to outweigh higher-overhead   * schemes.   */  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);    }  }}