futuretask源码分析（推荐）

futuretask只实现runnablefuture接口：

该接口继承了java.lang.runnable和future接口，也就是继承了这两个接口的特性。

1.可以不必直接继承thread来生成子类，只要实现run方法，且把实例传入到thread构造函数，thread就可以执行该实例的run方法了（ thread(runnable) ）。

2.可以让任务独立执行，get获取任务执行结果时，可以阻塞直至执行结果完成。也可以中断执行，判断执行状态等。

futuretask是一个支持取消行为的异步任务执行器。该类实现了future接口的方法。

如： 1. 取消任务执行

2. 查询任务是否执行完成

3. 获取任务执行结果（”get“任务必须得执行完成才能获取结果，否则会阻塞直至任务完成）。

注意：一旦任务执行完成，则不能执行取消任务或者重新启动任务。(除非一开始就使用runandreset模式运行任务)
futuretask支持执行两种任务， callable 或者 runnable的实现类。且可把futuretask实例交由executor执行。

源码部分(很简单)：

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);
    }
  }
}

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