類結構圖
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狀態圖
數據結構
等待線程那一塊用了鏈表。
outcome 是結果存儲的變量。
核心代碼
線程任務執行
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;
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);
}
}
- 該方法會把執行該任務的線程記錄下來,放入runner變量中。
- 線程執行Callable方法,如果調用成功則調用set方法存入outcome變量中。
- 最後清空runner變量。
protected void set(V v) {
if (STATE.compareAndSet(this, NEW, COMPLETING)) {
outcome = v;
STATE.setRelease(this, NORMAL); // final state
finishCompletion();
}
}
- 把當前任務狀態由NEW變更爲COMPLETING,則把參數存入outcome變量中。
- 把當前變量COMPLETING變更爲NORMAL。
- 調用finishCompletion方法,喚醒等待線程,即waiters。
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
if (WAITERS.weakCompareAndSet(this, 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
}
- 把等待線程waiters依次喚醒。
其他線程獲取調用結果
/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
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);
}
- 如果任務狀態爲NORMAL,直接調用report方法拿到outcome的值。
- 否則,awaitDone阻塞線程。
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
// The code below is very delicate, to achieve these goals:
// - call nanoTime exactly once for each call to park
// - if nanos <= 0L, return promptly without allocation or nanoTime
// - if nanos == Long.MIN_VALUE, don't underflow
// - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
// and we suffer a spurious wakeup, we will do no worse than
// to park-spin for a while
long startTime = 0L; // Special value 0L means not yet parked
WaitNode q = null;
boolean queued = false;
for (;;) {
int s = state;
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING)
// We may have already promised (via isDone) that we are done
// so never return empty-handed or throw InterruptedException
Thread.yield();
else if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
else if (q == null) {
if (timed && nanos <= 0L)
return s;
q = new WaitNode();
}
else if (!queued)
queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
else if (timed) {
final long parkNanos;
if (startTime == 0L) { // first time
startTime = System.nanoTime();
if (startTime == 0L)
startTime = 1L;
parkNanos = nanos;
} else {
long elapsed = System.nanoTime() - startTime;
if (elapsed >= nanos) {
removeWaiter(q);
return state;
}
parkNanos = nanos - elapsed;
}
// nanoTime may be slow; recheck before parking
if (state < COMPLETING)
LockSupport.parkNanos(this, parkNanos);
}
else
LockSupport.park(this);
}
}
- 如果任務狀態爲NORMAL,直接返回。
- 如果未超時且不爲NORMAL,使用頭插法塞入到waiters鏈表中並阻塞當前線程。