FutureTask相关
2024-09-08 09:25:03
- 上周因为项目中的线程池参数设置的不合理，引发了一些问题，看了下代码，发现对JUC中的一些概念需要再清晰些。
Runnable
@FunctionalInterface
public interface Runnable {
/**
* When an object implementing interface <code>Runnable</code> is used
* to create a thread, starting the thread causes the object's
* <code>run</code> method to be called in that separately executing
* thread.
* <p>
* The general contract of the method <code>run</code> is that it may
* take any action whatsoever.
*
* @see java.lang.Thread#run()
*/
public abstract void run();
}
- Runable是一个interface，定义了run()方法，The Runnable interface should be implemented by any class whose instances are intended to be executed by a thread。如果想在其他线程中执行你的task，需要实现这个接口。
Callable
@FunctionalInterface
public interface Callable<V> {
/**
* Computes a result, or throws an exception if unable to do so.
*
* @return computed result
* @throws Exception if unable to compute a result
*/
V call() throws Exception;
}
- 有了Runnable，为啥还需要Callable呢，可以看到Runnable和Callable的两个不同，第一，Runnable是没有返回值的，第二，Runnable是不会抛出checked exception的，而有时候我们需要知道任务执行之后的返回，同时也希望利用异常机制完成一些逻辑。所以有了Callable。
- JUC中的Executors这个Factory类，提供了Runnable转Callable的方法。
Future
- future 是一个inteface，提供了一系列方法，来帮助我们获取异步执行的task的执行状况和执行结果。
FutureTask
- FutureTask实现了RunnableFuture接口，即既实现了Runnable接口，又实现了Future接口。所以他有两个功能，第一，作为一个task，提交到别的线程中异步执行，第二，通过future提供的一些接口，获取task的异步执行状态。
/**
* The run state of this task, initially NEW. The run state
* transitions to a terminal state only in methods set,
* setException, and cancel. During completion, state may take on
* transient values of COMPLETING (while outcome is being set) or
* INTERRUPTING (only while interrupting the runner to satisfy a
* cancel(true)). Transitions from these intermediate to final
* states use cheaper ordered/lazy writes because values are unique
* and cannot be further modified.
*
* 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;
/** The underlying callable; nulled out after running */
private Callable<V> callable;
/** The result to return or exception to throw from get() */
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
- 看下FutureTask的几个属性，首先state表示当前task的执行状态，其中，开始状态位NEW表示task还没开始执行。NORMAL，CANCELLED，INTERRUPTED为终态，COMPLETING和INTERRUPTING为临时状态，最终会通过上面的几个状态转移路径，转移到终态。
- callable，表示具体执行的任务。
- outcome， task 执行的返回结果
- runner，执行这个task的线程
- waiters，通过get方法获取此task执行结果被阻塞的线程。
- 看下几个核心的方法，我们知道，futuretask提交到别的线程里后，最终会调用task的run方法执行具体逻辑。
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 {
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);
}
}
- run方法执行时，首先检查当前的状态是否是NEW，如果不是NEW说明已经被执行过了。开始执行之前，标记执行当前task的线程到runner。
- 调用callable的run方法，执行。抛异常时，设置setException。正常结束时，set结果。看下这两步里都会调到的finishCompletion方法。
/**
* Removes and signals all waiting threads, invokes done(), and
* nulls out callable.
*/
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
}
- 这里主要是在通知所有阻塞在watch这个task结果的线程，通知他们当前task已经执行结束了。
- 在执行结束时，看到finally里还有段逻辑
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);
}
private void handlePossibleCancellationInterrupt(int s) {
// It is possible for our interrupter to stall before getting a
// chance to interrupt us. Let's spin-wait patiently.
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield(); // wait out pending interrupt
// assert state == INTERRUPTED;
// We want to clear any interrupt we may have received from
// cancel(true). However, it is permissible to use interrupts
// as an independent mechanism for a task to communicate with
// its caller, and there is no way to clear only the
// cancellation interrupt.
//
// Thread.interrupted();
}
- 这是在干嘛呢，是因为，即使我们在上一步通过set或者setException设置了当前task的状态，但可能有别的线程在通过调用cancel来设置当前task的状态，如果有的话，这里就自旋空转，直到cancel方法执行结束。
- 那cancel方法是怎么工作的呢
public boolean cancel(boolean mayInterruptIfRunning) {
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
try { // in case call to interrupt throws exception
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
t.interrupt();
} finally { // final state
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}
- cancel方法其实就是通过找到执行当前task的runner，然后调用thread的interrupt方法，这里需要注意的是，thread.interrupt方法仅仅是设置一个标志位，具体线程有没有响应，要看自己的实现。反正这里就是调一把interrupt然后就走了，然后通知所有watch的线程。
- watch的线程，通过get方法获得执行结果是怎么拿到的呢
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);
}
}
- 核心逻辑就是，先把自己这个线程放到watch的waitNodes栈中，然后park 等待，直到task的状态>COMPLETING.
reference
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