前言
- 可取消的异步计算。
- 此类提供{@link Future}的基本实现，其中包含启动和取消计算，查询以查看计算是否完成以及检索计算结果的方法。
- 只有在计算完成后才能检索结果；如果计算尚未完成，则{@code get}方法将阻塞。
- 一旦计算完成，就不能重新开始或取消计算（除非使用{@link #runAndReset}调用计算）。
- {@code FutureTask}可以用于包装{@link Callable}或{@link Runnable}对象。
- 由于{@code FutureTask}实现了{@code Runnable}，因此可以将{@code FutureTask}提交给{@link Executor}以便执行。
- 除了用作独立类之外，此类还提供{@code protected}功能，这些功能在创建自定义任务类时可能会很有用。
源码
package java.util.concurrent;
public class FutureTask<V> implements RunnableFuture<V> {
/*
* 修订说明：这与依赖AbstractQueuedSynchronizer的此类的早期版本不同，主要是为了避免用户对取消比赛期间保留中断状态感到惊讶。
当前设计中的同步控制依赖于通过CAS更新的“状态”字段来跟踪完成情况，以及一个简单的Treiber堆栈来保存等待线程。
样式说明：与往常一样，我们绕过使用AtomicXFieldUpdaters的开销，而是直接使用Unsafe内部函数。
*/
/**
* 此任务的运行状态，最初为NEW。
* 运行状态仅在set，setException和cancel方法中转换为终端状态。
* 在完成期间，状态可能会采用COMPLETING（正在设置结果时）或INTERRUPTING（仅在中断跑步者满足cancel（true）时）的瞬态值。
* 从这些中间状态到最终状态的转换使用便宜的有序/惰性写入，因为值是唯一的，无法进一步修改。
* 可能的状态转换：
* 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;
/**
* 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)
return (V)x;
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
/**
* 创建一个{@code FutureTask}，它将在运行时执行给定的{@code Callable}。
*/
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW; // ensure visibility of callable
}
/**
* 创建一个{@code FutureTask}，它将在运行时执行给定的{@code Runnable}，并安排{@code get}在成功完成后返回给定的结果。
*/
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;
}
/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
/**
* @throws CancellationException {@inheritDoc}
*/
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);
}
/**
* 当此任务转换为状态{@code isDone}时（无论是正常还是通过取消）调用受保护的方法。
* 默认实现不执行任何操作。
* 子类可以重写此方法以调用完成回调或执行簿记。
* 请注意，您可以在此方法的实现内部查询状态，以确定此任务是否已取消。
*/
protected void done() { }
/**
* 除非已经设置或取消了该未来，否则将此未来的结果设置为给定值。
* 成功完成计算后，{@link
*
* @param v the value
*/
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* <p>This method is invoked internally by the {@link
* upon failure of the computation.
* 导致此未来报告一个{@link ExecutionException}，将给定throwable作为其原因，除非已经设置或取消了该未来。
* 计算失败时，此方法由{@link
*
*/
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() {
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);
}
}
/**
* 在不设置计算结果的情况下执行计算，然后将此将来状态重置为初始状态，如果计算遇到异常或被取消则无法执行此操作。
* 设计用于本质上执行多次的任务。
*
* @return {@code true} if successfully run and reset
*/
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 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
s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
return ran && s == NEW;
}
/**
* 确保来自可能的cancel（true）的任何中断仅在运行或runAndReset时才传递给任务。
*/
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();
}
/**
* 简单的链表节点可将等待线程记录在Treiber堆栈中。
* 有关更多详细说明，请参见其他类，例如Phaser和SynchronousQueue。
*
*/
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
/**
* 删除并发出所有等待线程的信号，调用done（），并使callable无效。
*/
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
}
/**
* 等待完成，或者在中断或超时时中止。
*/
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);
}
}
/**
* 尝试取消链接超时或中断的等待节点，以避免积累垃圾。
* 内部节点在没有CAS的情况下根本不会被拼接，因为如果释放者无论如何都要遍历它们，这是无害的。
* 为了避免从已删除的节点取消拆分的影响，在出现明显竞争的情况下会重新遍历该列表。
* 当节点很多时，这很慢，但是我们不希望列表足够长以超过开销更高的方案。
*
*/
private void removeWaiter(WaitNode node) {
if (node != null) {
node.thread = null;
retry:
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;
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long stateOffset;
private static final long runnerOffset;
private static final long waitersOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> k = FutureTask.class;
stateOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("state"));
runnerOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("runner"));
waitersOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("waiters"));
} catch (Exception e) {
throw new Error(e);
}
}
}
