public class FutureTask<V> implements RunnableFuture<V> {
/**
* 内部状态可能得迁转过程:
* NEW -> COMPLETING -> NORMAL //正常完成
* NEW -> COMPLETING -> EXCEPTIONAL //发生异常
* NEW -> CANCELLED //取消
* NEW -> INTERRUPTING -> INTERRUPTED //中断
*/
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;
/** 内部的callable,运行完成后设置为null */
private Callable<V> callable;
/** The result to return or exception to throw from get() */
private Object outcome; // 没有volatile修饰, 依赖state的读写来保证可见性
/** 执行内部callable的线程*/
private volatile Thread runner;
/** Treiber stack of waiting threads */
private volatile WaitNode waiters;
/**
* Returns result or throws exception for completed task.
*
* @param s completed state value
*/
@SuppressWarnings("unchecked")
private V report(int s) throws ExecutionException {
Object x = outcome;
if (s == NORMAL)//任务完成
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}
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;
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);//如果任务还没执行完毕,等待任务执行完毕
return report(s);//如果任务执行完毕,获取执行结果
}
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);
}
protected void done() { }
protected void set(V v) {
//当前任务状态state从NEW改为COMPLETING
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();//唤醒Treiber Stack中所有等待线程
}
}
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
public void run() {
//如果state不为NE或者尝试设置runner为当前线程,失败就退出
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();//执行任务
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);//如果发生异常,设置异常
}
if (ran)//如果正常执行完成,设置执行结果
set(result);
}
} finally {
// runner必须在设置了state之后再置空,避免run方法出现并发问题
runner = null;
int s = state;
if (s >= INTERRUPTING)//处理可能发生的取消中断
handlePossibleCancellationInterrupt(s);
}
}
protected boolean runAndReset() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return false;
boolean ran = false;
int s = state;
try {
Callable<V> c = callable;
if (c != null && s == NEW) {
try {
c.call(); // don't set result
ran = true;
} catch (Throwable ex) {
setException(ex);
}
}
} finally {
runner = null;
s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW;
}
/**
* Ensures that any interrupt from a possible cancel(true) is only
* delivered to a task while in run or runAndReset.
*/
private void handlePossibleCancellationInterrupt(int s) {
// 如果当前正在中断过程中,自旋等中断完成
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield(); // wait out pending interrupt
}
/**
* Simple linked list nodes to record waiting threads in a Treiber
* stack. See other classes such as Phaser and SynchronousQueue
* for more detailed explanation.
*/
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
//移除Treiber Stack,并将Treiber Stack中所有等待获取任务结果的线程唤醒,然后回调下done钩子方法
private void finishCompletion() {
// assert state > COMPLETING;
for (WaitNode q; (q = waiters) != null;) {
//尝试将waiters设置为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
}
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()) {
//如果当前线程被中断,移除等待节点q,然后抛出中断异常
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) {//如果任务已经执行完毕
if (q != null)
q.thread = null;//q不为null,将q中的thread置null
return s;//返回状态
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();//如果当前正在完成过程中,出让CPU
else if (q == null)
q = new WaitNode();//创建一个等待节点
else if (!queued)//将q(包含当前线程的等待节点)入队
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);//如果超时,移除等待节点q
return state;
}
LockSupport.parkNanos(this, nanos);//超时的话,就阻塞给定时间
}
else
LockSupport.park(this);//没设置超时的话,就阻塞当前线程
}
}
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;//将node的thread域置空
retry://下面过程中会将node从等待队列中移除,以thread域为null为依据
for (;;) { // restart on removeWaiter race
for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
s = q.next;
if (q.thread != null)
pred = q;
else if (pred != null) {
pred.next = s;
if (pred.thread == null) // check for race
continue retry;
}
else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
q, s))
continue retry;
}
break;
}
}
}
}
FutureTask源码阅读
最新推荐文章于 2024-09-18 23:26:24 发布