介绍
JDK future框架，提供了一种异步编程模式，基于线程池的。将任务runnable/callable提交到线程池executor，返回一个Future对象。通过future.get()获取执行结果，这里提交到线程池，后面的操作不会阻塞。future.get()获取结果会阻塞，其实也是用多线线程执行任务。
使用
- future + callable
public static void main(String[] args) throws ExecutionException, InterruptedException {
ExecutorService executorService = Executors.newSingleThreadExecutor();
Future<Integer> result = executorService.submit(new Callable<Integer>() {
@Override
public Integer call() throws Exception {
TimeUnit.SECONDS.sleep(10);
return 1;
}
});
System.out.println(result.get());
}
- futureTask( extends Runnable, Future)
public static void main(String[] args) throws ExecutionException, InterruptedException {
ExecutorService executorService = Executors.newSingleThreadExecutor();
FutureTask task = new FutureTask(() -> {
TimeUnit.SECONDS.sleep(10);
return 1;
});
executorService.execute(task);
System.out.println(task.get());
}
原理
主线程等待
在Future.get()方法内，实现了等待，哪个线程调用了某个FutureTask的get方法，就会进入到FutureTask的等待单链表里面去：WaitNode
public V get() throws InterruptedException, ExecutionException {
int s = state;
// 1.
if (s <= COMPLETING)
s = awaitDone(false, 0L);
// 2
return report(s);
}
任务状态
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;
直接看命名就应该知道什么意思，就不挨个说了。
-
某个线程调用futureTask的get方法，首先需要判断这个task是否在创建状态(NEW)或者进行中(COMPLETING)，只有在创建或者进行中，才会把这个线程放入等待队列
-
如果不需要等待，就返回结果
工作线程会把结果放在outcome这个变量上面，report就是去这个变量上取结果并且给当前线程的，当然还需要校验任务状态
主要的逻辑就在awaitDone 这个方法里面
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;
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);
}
}
有些代码是关于等待时间的，也就是说调用get方法的线程可以设置一定的等待时间，这里不关心
这个代码逻辑也比较简单，核心就是为了让当前线程(调用futureTask的get方法的线程)执行LockSupport.park(this);
工作线程计算结果并唤醒
线程池最终回去调用任务的run方法，所以直接看run方法的实现
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
// 1
if (c != null && state == NEW) {
V result;
boolean ran;
try {
// 2
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
// 3
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);
}
}
- 判断当前任务是不是在新建状态
- 执行call里面的业务逻辑，并拿到结果
- 把第二步执行的结果放到成员变量outcome里面去，并且修改任务状态为正常结束(NORMAL)
- 通知所有的waiters(所有调用了当前futureTask的get方法的线程)
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
上面第四部的逻辑就在finishCompletion里面
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
}
循环的从waiters里面拿线程，然后调用LockSupport.unpark(t)，这个时候所有想要通过futureTask的get方法拿到任务的结果的线程，都被唤醒了
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