首先我们看下FutureTask与其他接口的关系图
image.png
可以看到FutureTask实现了RunnableFuture，RunnableFuture继承了Runnable和Future接口。
在我们的demo例子中 FutureTask如何得知线程执行完毕的。
//FutureTask类
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;
//当我们new出来FutureTask时候，这个state为0
public FutureTask(Runnable runnable, V result) {
//调用了Executors中的方法
this.callable = Executors.callable(runnable, result);
this.state = NEW;
}
// Executors类
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
//在重写call方法中会调用task的run方法。
public T call() {
task.run();
return result;
}
}
我们执行线程start之后，因为FutureTask实现了RunnableFuture，RunnableFuture继承了Runnable和Future接口，所以我们会运行到FutureTask的run方法。
// FutureTask类
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 {
/*call会调用call中Runnable的run方法,所以 call会调用我们demo中
重写的run方法 */
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
//如果正确执行完run的内容，则去置state的状态
if (ran)
set(result);
}
} finally {
runner = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
//修改state装填并且唤醒挂起的线程
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL);
finishCompletion();
}
}
//当任务执行完毕会唤醒所有挂起的线程，这里的waiters是一个链表，后面会有说明
private void finishCompletion() {
for (WaitNode q; (q = waiters) != null;) {
// CAS直接将挂起线程链表waiters指向为空
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
//唤醒所有挂起的线程
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;
}
在demo中，当我们执行FutureTask类的get方法时
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
// 获取state的状态
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//如果处于已经完成或者异常状态，直接返回
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
//如果处于COMPLETING，则让线程再次参与竞争cpu
else if (s == COMPLETING)
Thread.yield();
else if (q == null)
q = new WaitNode();
else if (!queued)
//形成一个后进先出的链表
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
//线程挂起
LockSupport.parkNanos(this, nanos);
}
else
//线程挂起
LockSupport.park(this);
}
}
在上面的源码中，我们可以看出来，awaitDone方法中我们会对比当前state，如果state==new，我们会将线程挂起，等待finishCompletion方法中唤醒挂起的线程。
private V report(int s) throws ExecutionException {
Object x = outcome;
//如果线程正确执行完毕，则返回结果，否则抛出异常
if (s == NORMAL)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
