FutureTask類圖
從類圖可以看出FutureTask簡介實現(xiàn)了Runnable赔癌、Future接口气嫁,我們知道Future用于表示異步計算的結(jié)果类茂,所以通過它我們可以獲得線程的執(zhí)行結(jié)果母市。如何獲得線程的執(zhí)行結(jié)果呢矾兜?
FutureTask獲取執(zhí)行結(jié)果
一般我們按照下面的方式使用FutureTask
//1. 創(chuàng)建FutureTask
FutureTask futureTask = new FutureTask(new CallableTest());
//2. 啟動線程
new Thread(futureTask).start();
//3 獲取線程執(zhí)行結(jié)果
String result = (String) futureTask.get();
源碼走讀
//創(chuàng)建FutureTask對象
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
//調(diào)用run方法,執(zhí)行我們的業(yè)務(wù)邏輯
public void run() {
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 {
//我們重寫的call的方法的執(zhí)行
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)//call方法執(zhí)行完成執(zhí)行喚醒操作
set(result);
}
} finally {
runner = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
//執(zhí)行call方法的時候窒篱,我們主線程接著執(zhí)行g(shù)et()方法
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
//阻塞主方法的邏輯
s = awaitDone(false, 0L);
return report(s);
}
//未被喚醒會一直等待直到超時或知道被中斷
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
//等待隊列
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;
}
//阻塞操作(有超時時間)
LockSupport.parkNanos(this, nanos);
}
else
//阻塞操作
LockSupport.park(this);
}
}
//run()中的喚醒操作
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
//執(zhí)行后結(jié)果賦值給outcome
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
//依次喚醒阻塞的線程
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;
q = next;
}
break;
}
}
done();
callable = null;
}
從上面的分析可以看出:異步計算線程啟動后焕刮,主線程(或者其他調(diào)用get()方法的線程)將被放在一個等待對列中,同時被阻塞(通過LockSupport類的park方法)墙杯,知道異步計算線程執(zhí)行完成后配并,等待隊列中的線程將被依次喚醒,并且或得計算結(jié)果高镐。