Future cancel迷惑性boolean入參解析

/**
 * Attempts to cancel execution of this task.  This attempt will
 * fail if the task has already completed, has already been cancelled,
 * or could not be cancelled for some other reason. If successful,
 * and this task has not started when {@code cancel} is called,
 * this task should never run.  If the task has already started,
 * then the {@code mayInterruptIfRunning} parameter determines
 * whether the thread executing this task should be interrupted in
 * an attempt to stop the task.
 *
 * <p>After this method returns, subsequent calls to {@link #isDone} will
 * always return {@code true}.  Subsequent calls to {@link #isCancelled}
 * will always return {@code true} if this method returned {@code true}.
 *
 * @param mayInterruptIfRunning {@code true} if the thread executing this
 * task should be interrupted; otherwise, in-progress tasks are allowed
 * to complete
 * @return {@code false} if the task could not be cancelled,
 * typically because it has already completed normally;
 * {@code true} otherwise
 */
boolean cancel(boolean mayInterruptIfRunning);

/**
 * 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;

public void run() {
    if (state != NEW ||
        !RUNNER.compareAndSet(this, null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                // 執行任務
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                // 執行異常
                setException(ex);
            }
            if (ran)
                // 正常執行完畢
                set(result);
        }
    } finally {
      //... 省略
    }
}
​
protected void setException(Throwable t) {
  if (STATE.compareAndSet(this, NEW, COMPLETING)) {
    outcome = t;
    STATE.setRelease(this, EXCEPTIONAL); // final state
    finishCompletion();
  }
}
​
protected void set(V v) {
  if (STATE.compareAndSet(this, NEW, COMPLETING)) {
    outcome = v;
    STATE.setRelease(this, NORMAL); // final state
    finishCompletion();
  }
}

public boolean cancel(boolean mayInterruptIfRunning) {
    // NEW為新建或者運行態
    // 1. 此時任務已經不是NEW，說明要麼是完成要麼是異常，取消不瞭，所以返回false
    // 2. 此時任務還是NEW，如果我們傳入true，則CAS標記任務為INTERRUPTING，否則是CANCELLED
    // 防止並發取消任務，CAS隻會有一個線程成功，其餘線程失敗
    if (!(state == NEW && STATE.compareAndSet
          (this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
        return false;
    try {    
        // 傳入true，則打斷該任務的執行線程
        if (mayInterruptIfRunning) {
            try {
                Thread t = runner;
                if (t != null)
                    t.interrupt();
            } finally {
                // 比較任務狀態為INTERRUPTED
                STATE.setRelease(this, INTERRUPTED);
            }
        }
    } finally {
        finishCompletion();
    }
    return true;
}

public void run() {
    if (state != NEW ||
        !RUNNER.compareAndSet(this, 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;
                // 執行異常
                setException(ex);
            }
            if (ran)
                // 正常執行完畢
                set(result);
        }
    } finally {
      //... 省略
    }
}

private void finishCompletion() {
    // assert state > COMPLETING;
    for (WaitNode q; (q = waiters) != null;) {
        // 原子性將WAITERS設置為null
        if (WAITERS.weakCompareAndSet(this, q, null)) {
          
            // 遍歷WAITERS，將阻塞的線程都喚醒
            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;
                q = next;
            }
            break;
        }
    }
​
    // 擴展方法，交給自己實現
    done();
​
    callable = null;
}