J.U.C并发框架源码阅读（十六）FutureTask

/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/ */

package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;

/**
* A cancellable asynchronous computation.  This class provides a base
* implementation of {@link Future}, with methods to start and cancel
* a computation, query to see if the computation is complete, and
* retrieve the result of the computation.  The result can only be
* retrieved when the computation has completed; the {@code get}
* methods will block if the computation has not yet completed.  Once
* the computation has completed, the computation cannot be restarted
* or cancelled (unless the computation is invoked using
* {@link #runAndReset}).
*
* <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
* {@link Runnable} object.  Because {@code FutureTask} implements
* {@code Runnable}, a {@code FutureTask} can be submitted to an
* {@link Executor} for execution.
*
* <p>In addition to serving as a standalone class, this class provides
* {@code protected} functionality that may be useful when creating
* customized task classes.
*
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this FutureTask's {@code get} methods
*/
public class FutureTask<V> implements RunnableFuture<V> {
/*
* Revision notes: This differs from previous versions of this
* class that relied on AbstractQueuedSynchronizer, mainly to
* avoid surprising users about retaining interrupt status during
* cancellation races. Sync control in the current design relies
* on a "state" field updated via CAS to track completion, along
* with a simple Treiber stack to hold waiting threads.
*
* Style note: As usual, we bypass overhead of using
* AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
*/

/**
* 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;

/**
* 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);
}

/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Callable}.
*
* @param  callable the callable task
* @throws NullPointerException if the callable is null
*/
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;       // ensure visibility of callable
}

/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Runnable}, and arrange that {@code get} will return the
* given result on successful completion.
*
* @param runnable the runnable task
* @param result the result to return on successful completion. If
* you don't need a particular result, consider using
* constructions of the form:
* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
* @throws NullPointerException if the runnable is null
*/
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)
return false;
if (mayInterruptIfRunning) {
if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))
return false;
Thread t = runner;
if (t != null)
t.interrupt();
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state
}
else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))
return false;
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);
}

/**
* Protected method invoked when this task transitions to state
* {@code isDone} (whether normally or via cancellation). The
* default implementation does nothing.  Subclasses may override
* this method to invoke completion callbacks or perform
* bookkeeping. Note that you can query status inside the
* implementation of this method to determine whether this task
* has been cancelled.
*/
protected void done() { }

/**
* Sets the result of this future to the given value unless
* this future has already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon successful completion of the computation.
*
* @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();
}
}

/**
* Causes this future to report an {@link ExecutionException}
* with the given throwable as its cause, unless this future has
* already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon failure of the computation.
*
* @param t the cause of failure
*/
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);
}
}

/**
* Executes the computation without setting its result, and then
* resets this future to initial state, failing to do so if the
* computation encounters an exception or is cancelled.  This is
* designed for use with tasks that intrinsically execute more
* than once.
*
* @return 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;
}

/**
* 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) {
// 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();
}

/**
* 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(); }
}

/**
* 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
}

/**
* Awaits completion or aborts on interrupt or timeout.
*
* @param timed true if use timed waits
* @param nanos time to wait, if timed
* @return state upon completion
*/
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);
}
}

/**
* Tries to unlink a timed-out or interrupted wait node to avoid
* accumulating garbage.  Internal nodes are simply unspliced
* without CAS since it is harmless if they are traversed anyway
* by releasers.  To avoid effects of unsplicing from already
* removed nodes, the list is retraversed in case of an apparent
* race.  This is slow when there are a lot of nodes, but we don't
* expect lists to be long enough to outweigh higher-overhead
* schemes.
*/
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);
}
}

}

private static final int NEW          = 0;//新建，实际上计算任务可能正在执行
private static final int COMPLETING   = 1;//执行中，实际上计算任务已经执行完毕（可能正常，也可能是发生异常）
private static final int NORMAL       = 2;//正常结束
private static final int EXCEPTIONAL  = 3;//异常结束
private static final int CANCELLED    = 4;//已取消
private static final int INTERRUPTING = 5;//中断中
private static final int INTERRUPTED  = 6;//已中断

* Possible state transitions:
* NEW -> COMPLETING -> NORMAL
* NEW -> COMPLETING -> EXCEPTIONAL
* NEW -> CANCELLED
* NEW -> INTERRUPTING -> INTERRUPTED

/**
* Creates a {@code FutureTask} that will, upon running, execute the
* given {@code Runnable}, and arrange that {@code get} will return the
* given result on successful completion.
*
* @param runnable the runnable task
* @param result the result to return on successful completion. If
* you don't need a particular result, consider using
* constructions of the form:
* {@code Future<?> f = new FutureTask<Void>(runnable, null)}
* @throws NullPointerException if the runnable is null
*/
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW;       // ensure visibility of callable
}

public void run() {
if (state != NEW ||//当前状态必须为NEW，工作线程必须为null，然后将工作线程用cas操作设置为当前线程
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;
if (c != null && state == NEW) {//这个时候已经可以保证FutureTask是由当前线程独占了，只要判断当前线程没有已经执行过这个FutureTask即可
V result;
boolean ran;
try {
result = c.call();//当前线程执行计算任务
ran = true;//成功跑完，标记一下
} catch (Throwable ex) {//计算过程中抛出异常
result = null;
ran = false;//标记任务未完成
setException(ex);//标记任务异常
}
if (ran)
set(result);//标记任务正常完成
}
} finally {
// runner must be non-null until state is settled to
// prevent concurrent calls to run()
runner = null;//先设置state，再设置runner为null，防止并发调用call出问题
// state must be re-read after nulling runner to prevent
// leaked interrupts
int s = state;
if (s >= INTERRUPTING)//处理可能的中断
handlePossibleCancellationInterrupt(s);
}
}

/**
* Sets the result of this future to the given value unless
* this future has already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon successful completion of the computation.
*
* @param v the value
*/
protected void set(V v) {//设置FutureTask为正常结束
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}

/**
* Causes this future to report an {@link ExecutionException}
* with the given throwable as its cause, unless this future has
* already been set or has been cancelled.
*
* <p>This method is invoked internally by the {@link #run} method
* upon failure of the computation.
*
* @param t the cause of failure
*/
protected void setException(Throwable t) {//设置FutureTask为异常结束
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}

/**
* Executes the computation without setting its result, and then
* resets this future to initial state, failing to do so if the
* computation encounters an exception or is cancelled.  This is
* designed for use with tasks that intrinsically execute more
* than once.
*
* @return 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//不设置结果
ran = true;//如果任务计算过程中抛出异常或者被中断，ran变量为false
} 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;//如果任务是正常结束，重置FutureTask状态
}

public boolean cancel(boolean mayInterruptIfRunning) {
if (state != NEW)
return false;
if (mayInterruptIfRunning) {
if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING))//设置FutureTask的状态为INTERRUPTING
return false;
Thread t = runner;
if (t != null)
t.interrupt();//发送中断信号
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state//设置FutureTask的状态为INTERRUPTED
}
else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED))//设置FutureTask的状态为CANCELLED
return false;
finishCompletion();
return true;
}

/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)//任务未完成，排队等待结果
s = awaitDone(false, 0L);
return report(s);
}

/**
* Awaits completion or aborts on interrupt or timeout.
*
* @param timed true if use timed waits
* @param nanos time to wait, if timed
* @return state upon completion
*/
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;//计算等待结果线程的唤醒时间
WaitNode q = null;
boolean queued = false;
for (;;) {//死循环中等待结果
if (Thread.interrupted()) {//等待线程被中断
removeWaiter(q);//从等待队列中移除
throw new InterruptedException();
}

int s = state;
if (s > COMPLETING) {//计算任务结束
if (q != null)//函数返回
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
Thread.yield();//计算任务尚未结束，退让，减少开销
else if (q == null)
q = new WaitNode();//创建新的等待节点
else if (!queued)//如果当前等待节点还未入栈
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);//用CAS操作将当前等待节点入栈
else if (timed) {
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);//入栈成功，等待
}
else
LockSupport.park(this);//入栈成功，等待
}
}

/**
* 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);//唤醒等待线程
}
WaitNode next = q.next;//查找下一个等待线程
if (next == null)
break;
q.next = null; // unlink to help gc
q = next;
}
break;
}
}

done();

callable = null;        // to reduce footprint
}