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在 Java 中,Runnable 接口示意一个没有返回后果的工作,而 Callable 接口示意具备返回后果的工作。
在并发编程中,异步执行提交工作和获取工作后果这两个操作,能够进步零碎的吞吐量。Future 接口应运而生,它示意异步工作的执行后果,并提供了查看工作是否执行完、勾销工作、获取工作执行后果等性能。FutureTask 是 Future 接口的根本实现,常与线程池实现类 ThreadPoolExecutor 配合应用。
本文基于 jdk1.8.0_91
1. 继承体系
RunnableFuture 接口同时实现了 Runnable 接口和 Future 接口,是一种冗余设计。
java.util.concurrent.RunnableFuture
/**
* A {@link Future} that is {@link Runnable}. Successful execution of
* the {@code run} method causes completion of the {@code Future}
* and allows access to its results.
*
* @see FutureTask
* @see Executor
* @since 1.6
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();}
FutureTask 是一个可勾销的异步工作,是对 Future 接口的根本实现,具备以下性能:
- 启动或中断的工作的执行;
- 判断工作是否执行实现;
- 获取工作执行实现后的后果。
同时,FutureTask 能够用于包装 Callable 或 Runnable 对象。
因为它实现了 Runnable 接口,能够提交给 Executor 执行。
/**
* A cancellable asynchronous computation.
*
* @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>
java.util.concurrent.Executor
/**
* An object that executes submitted {@link Runnable} tasks.
*
* @since 1.5
* @author Doug Lea
*/
public interface Executor {void execute(Runnable command);
}
2. 属性
java.util.concurrent.FutureTask
// The run state of this task, initially NEW.
// 工作的执行状态,初始为 NEW。private volatile int state;
/** The underlying callable; nulled out after running */
// 须要执行的工作,工作执行完后为空
private Callable<V> callable;
/** The result to return or exception to throw from get() */
// 工作的执行后果,或者工作抛出的异样
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
// 执行工作的线程
private volatile Thread runner;
/** Treiber stack of waiting threads */
// 指向栈顶的指针,栈构造用于存储期待工作执行后果的线程
private volatile WaitNode waiters;
其中 state、runner、waiters 三个属性在并发时存在争用,采纳 CAS 保护其准确性。
// 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);
}
}
2.1 状态定义
/**
* 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;
FutureTask 中应用 state 代表工作在运行过程中的状态。随着工作的执行,状态将一直地进行转变。
状态的阐明:
- NEW:新建状态,工作都从该状态开始。
- COMPLETING:工作正在执行中。
- NORMAL:工作失常执行实现。
- EXCEPTIONAL:工作执行过程中抛出了异样。
- CANCELLED:工作被勾销(不响应中断)。
- INTERRUPTING:工作正在被中断。
- INTERRUPTED:工作曾经中断。
状态转移过程:
NEW -> COMPLETING -> NORMAL
NEW -> COMPLETING -> EXCEPTIONAL
NEW -> CANCELLED
NEW -> INTERRUPTING -> INTERRUPTED
状态的分类:
- 工作的初始状态:NEW
- 工作的中间状态:COMPLETING、INTERRUPTING
- 工作的终止状态:NORMAL、EXCEPTIONAL、CANCELLED、INTERRUPTED
2.2 状态应用
FutureTask 中判断工作是否已勾销、是否已实现,是依据 state 来判断的。
public boolean isCancelled() {return state >= CANCELLED; // CANCELLED、INTERRUPTING、INTERRUPTED}
public boolean isDone() {return state != NEW;}
能够看到:
- 被勾销或被中断的工作(CANCELLED、INTERRUPTING、INTERRUPTED),都视为已勾销。
- 当工作来到了初始状态 NEW,就视为工作已完结。工作的两头态很短暂,并不代表工作正在执行,而是工作曾经执行完了,正在设置最终的返回后果。
依据状态值,FutureTask 能够保障曾经实现的工作不会被再次运行或者被勾销。
中间状态尽管是一个刹时状态,在 FutureTask 中用于线程间的通信。例如:
- 在 FutureTask#run 中检测到状态 >= INTERRUPTING,阐明其余线程发动了勾销操作,以后线程需期待对方实现中断。
- 在 FutureTask#get 中检测到状态 <= COMPLETING,阐明执行工作的线程尚未解决完,以后线程需期待对方实现工作。
2.2 栈(Treiber stack)
/** Treiber stack of waiting threads */
private volatile WaitNode waiters; // 栈顶指针
/**
* 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; // 期待工作执行后果的线程
volatile WaitNode next; // 栈的下一个节点
WaitNode() { thread = Thread.currentThread(); }
}
FutureTask 应用链表来结构栈(Treiber stack,应用 CAS 保障栈操作的线程平安,参考 java.util.concurrent.SynchronousQueue.TransferStack)。
其中 waiters 是链表的头节点,代表栈顶的指针。
栈的作用:
FutureTask 实现了 Future 接口,如果获取后果时,工作还没有执行结束,那么获取后果的线程就在栈中挂起,直到工作执行结束被唤醒。
3. 构造函数
赋值工作,设置工作的初始状态。
/**
* 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
}
值得注意的两个中央:
- FutureTask 创立的时候,状态为 NEW。
- 因为 FutureTask 应用 Callable 示意工作,需用 Executors#callable 办法将 Runnable 转换为 Callable。
测试:
@Test
public void executors() throws Exception {Callable<String> callable = Executors.callable(new Runnable() {
@Override
public void run() {System.out.println("run!");
}
}, "haha");
String call = callable.call();
System.out.println("call =" + call);
}
执行后果:
run!
call = haha
4. Runnable 实现
4.1 FutureTask#run
代码流程:
- 校验工作是否可执行:工作已执行或其余线程已获取执行权,则无奈执行。
- 调用 Callable#call 执行工作。
- 若工作执行失败,应用 setException 办法设置异样。
- 若工作执行胜利,应用 set 办法设置返回后果。
- 最初,革除对以后线程的记录,判断是否期待中断。
留神,在工作执行完结后,属性 runner、callable 都会被清空。
java.util.concurrent.FutureTask#run
public void run() {
// state != NEW 阐明工作曾经执行结束,不再反复执行
// 将 runner 属性设置为以后线程,若设置失败阐明其余线程已获取执行权
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 Callable#call
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex); // 执行失败,设置异样
}
if (ran)
set(result); // 执行胜利,设置后果
}
} 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) // INTERRUPTING、INTERRUPTED
handlePossibleCancellationInterrupt(s);
}
}
4.1.1 FutureTask#set
工作执行胜利之后,调用该办法。
用于设置工作状态、设置工作执行后果、唤醒栈中期待工作执行后果的线程。
java.util.concurrent.FutureTask#set
/**
* 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)) { // state: NEW -> COMPLETING
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state: COMPLETING -> NORMAL
finishCompletion();}
}
状态变动:NEW -> COMPLETING -> NORMAL
因为 state 属性是 volatile,这里 putOrderedInt 和 putIntVolatile 是等价的,保障可见性。
为什么 这里应用 lazySet 而没有用 CAS:
- 在并发状况下,只有一个线程执行 CAS 将 state 从 NEW 批改为 COMPLETING 会胜利,其余线程均失败。
- 因而随后只有一个线程持续批改 state 为 NORMAL,不存在争用,无需应用 CAS。
4.1.2 FutureTask#setException
工作执行产生异样,调用该办法。
除了设置工作状态不同,其余与 FutureTask#set 雷同。
状态变动:NEW -> COMPLETING -> EXCEPTIONAL
java.util.concurrent.FutureTask#setException
/**
* 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)) { // state: NEW -> COMPLETING
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state: COMPLETING -> EXCEPTIONAL
finishCompletion();}
}
4.1.3 FutureTask#finishCompletion
执行结束,唤醒期待线程。
java.util.concurrent.FutureTask#finishCompletion
/**
* 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)) {
// CAS 将 waiters 属性置空:1. CAS 胜利,遍历链表唤醒所有节点;2. CAS 失败,从新读取 waiters
for (;;) {
Thread t = q.thread;
if (t != null) {
q.thread = null;
LockSupport.unpark(t); // 唤醒节点上的线程
}
WaitNode next = q.next;
if (next == null)
break;
q.next = null; // unlink to help gc // 出栈
q = next;
}
break;
}
}
done(); // 预留办法
callable = null; // to reduce footprint
}
4.1.4 FutureTask#handlePossibleCancellationInterrupt
在 FutureTask#cancel 办法中,会先将 state 设为 INTERRUPTING,再中断 runner 线程,最初将 state 设为 INTERRUPTED。
所以在 FutureTask#run 的 finally 块中如果查看到 state == INTERRUPTING
,阐明其余线程发动了 cancel(true) 操作,这里须要期待其余线程中断以后线程。直到检测到 state != INTERRUPTING
,阐明其余线程已实现中断以后线程操作。
java.util.concurrent.FutureTask#handlePossibleCancellationInterrupt
/**
* 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) // 其余线程中断以后线程之后,会设置 state 为 INTERRUPTED,使这里完结循环
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();}
4.2 FutureTask#runAndReset
反对周期性执行工作:
- 执行工作胜利,不必返回工作后果,也不必扭转工作状态(放弃为 NEW),下次能够再次执行工作。
- 执行工作失败,则设置异样后果,并批改工作状态(不为 NEW),下次无奈再次执行工作。
- 勾销执行工作,则期待其余线程中断以后线程,并批改工作状态(不为 NEW),下次无奈再次执行工作。
/**
* designed for use with tasks that intrinsically execute more // 设计用来反对定时工作
* than once.
*
* @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); // 批改 state: NEW -> COMPLETING -> EXCEPTIONAL
}
}
} 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; // 返回 true 则容许下次再执行 runAndReset
}
5. Future 实现
5.1 Future#get
获取工作执行的后果:
- 如果工作未实现(NEW、COMPLETING),取后果的线程会阻塞(或自旋)。
- 如果工作执行出错(EXCEPTIONAL),抛出 ExecutionException
- 如果工作被勾销了(CANCELLED、INTERRUPTING、INTERRUPTED),抛出 CancellationException
- 如果线程期待被中断,抛出 InterruptedException
java.util.concurrent.FutureTask#get()
/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L); // 自旋或阻塞期待工作实现
return report(s); // 获取工作执行后果或抛出异样
}
5.1.1 FutureTask#awaitDone
期待工作实现(工作执行实现、工作执行出现异常、工作勾销执行),若以后线程产生中断、超时则进行期待。
在自旋中进行判断:
- 若以后线程已中断,则将节点出栈,抛出 InterruptedException。
- 若 state > COMPLETING,阐明工作曾经实现,返回以后 state。
- 若 state == COMPLETING,阐明工作行将实现,以后线程持续自旋。
-
若 state < COMPLETING,须要将以后线程入栈期待:
- 无超时工夫,始终期待直到被其余线程唤醒(FutureTask#run 或 FutureTask#cancel)或产生中断(Thread#interrupt);
- 有超时工夫,阻塞直到超时、被唤醒、产生中断。若已超时,将节点出栈,返回 state。
java.util.concurrent.FutureTask#awaitDone
/**
* 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()) { // 查看并革除中断状态
removeWaiter(q); // 已中断,将节点出栈
throw new InterruptedException();}
int s = state;
if (s > COMPLETING) { // 其余线程已实现工作,完结期待
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(); // 创立节点,设置 q.thread
else if (!queued)
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q); // 节点 q 入栈,作为新的头节点 waiters
else if (timed) {nanos = deadline - System.nanoTime();
if (nanos <= 0L) {removeWaiter(q); // 已超时,将节点出栈
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
LockSupport.park(this); // 进入阻塞,由 FutureTask#run 或 FutureTask#cancel 来唤醒(外部均调用 FutureTask#finishCompletion)}
}
5.1.2 FutureTask#report
以后线程期待结束,获取工作的执行后果,或者抛出异样。
java.util.concurrent.FutureTask#report
/**
* 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) // CANCELLED、INTERRUPTING、INTERRUPTED
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}
5.2 Future#get(timeout, unit)
在肯定的工夫之内,期待获取工作执行的后果。
/**
* @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(); // 期待超时了,工作还没有执行完,则抛出 TimeoutException
return report(s);
}
5.3 Future#cancel
尝试勾销工作的执行:
- 如果工作已实现或已勾销,则勾销操作会失败,返回 false。
- 如果工作还未执行,则勾销操作会胜利,返回 true。
-
如果工作正在执行,办法的参数就会批示线程是否须要中断:
- mayInterruptIfRunning 为 true,则以后正在执行的工作会被中断;
- mayInterruptIfRunning 为 false,则容许正在执行的工作持续运行,直到它执行完。
状态变动:
NEW -> CANCELLED
NEW -> INTERRUPTING -> INTERRUPTED
public boolean cancel(boolean mayInterruptIfRunning) {
// 如果工作还没有启动(NEW),则批改工作状态(INTERRUPTING or CANCELLED),批改胜利则进入下一步
// 如果工作状态不是 NEW,则间接返回。阐明工作已完结(已实现、已勾销、出现异常),无奈勾销,返回 false
if (!(state == NEW &&
UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
return false;
try { // in case call to interrupt throws exception
// 进入这里,阐明工作状态为 INTERRUPTING or CANCELLED
// mayInterruptIfRunning 为 true 阐明须要中断执行工作的线程,为 false 容许工作继续执行完
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
t.interrupt();} finally { // final state
// 只有一个线程会执行到这里,无需应用 CAS
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // INTERRUPTING -> INTERRUPTED
}
}
} finally {finishCompletion(); // 唤醒期待线程
}
return true;
}
6. 示例
应用三个线程顺次执行:提交工作、期待工作、勾销工作。
察看执行后果,了解并发状况下多个线程之间如何应用 Future 进行交互。
/**
* 三个线程顺次执行:提交工作、期待工作、勾销工作
* 在工作未执行完的时候,勾销工作。*
* @author Sumkor
* @since 2021/4/28
*/
@Test
public void cancel() throws InterruptedException {
// 定义工作
FutureTask<String> futureTask = new FutureTask<>(new Callable<String>() {
@Override
public String call() throws Exception {Thread.sleep(10000);
return "哦豁";
}
});
CountDownLatch submitGate = new CountDownLatch(1); // 期待工作提交
CountDownLatch endGate = new CountDownLatch(3); // 期待线程执行完
// 提交工作
new Thread(new Runnable() {
@Override
public void run() {
try {submitGate.countDown();
System.out.println(Thread.currentThread().getName() + "执行工作开始");
futureTask.run();
System.out.println(Thread.currentThread().getName() + "执行工作完结");
} finally {endGate.countDown();
}
}
}).start();
// 期待工作
new Thread(new Runnable() {
@Override
public void run() {
try {submitGate.await();
Thread.sleep(1000);// 期待 futureTask.run() 执行一段时间后再获取后果
System.out.println(Thread.currentThread().getName() + "获取工作后果开始");
String result = futureTask.get();
System.out.println(Thread.currentThread().getName() + "获取工作后果完结" + result);
} catch (Exception e) {System.out.println(Thread.currentThread().getName() + "获取工作后果失败" + e.getMessage());
e.printStackTrace();} finally {endGate.countDown();
}
}
}).start();
// 勾销工作
new Thread(new Runnable() {
@Override
public void run() {
try {submitGate.await();
Thread.sleep(2000);// 期待 futureTask.get() 执行一段时间后再勾销工作
System.out.println(Thread.currentThread().getName() + "勾销工作开始");
boolean cancel = futureTask.cancel(true);
System.out.println(Thread.currentThread().getName() + "勾销工作完结" + cancel);
} catch (Exception e) {System.out.println(Thread.currentThread().getName() + "勾销工作失败" + e.getMessage());
e.printStackTrace();} finally {endGate.countDown();
}
}
}).start();
endGate.await();}
执行后果:
Thread-0 执行工作开始
Thread-1 获取工作后果开始
Thread-2 勾销工作开始
Thread-2 勾销工作完结 true
Thread-0 执行工作完结
Thread-1 获取工作后果失败 null
java.util.concurrent.CancellationException
at java.util.concurrent.FutureTask.report(FutureTask.java:121)
at java.util.concurrent.FutureTask.get(FutureTask.java:192)
at com.sumkor.pool.FutureTest$6.run(FutureTest.java:129)
at java.lang.Thread.run(Thread.java:745)
阐明:
- 线程 A 启动工作一段时间后,线程 B 来获取工作后果,进入期待。
- 随后线程 C 勾销工作,将线程 A 中断(线程 A 不会抛异样,因为 FutureTask#cancel 先一步批改了 state 导致 FutureTask#setException 中 CAS 失败)。
- 此时线程 B 在期待中被唤醒(由线程 C 唤醒,查看到 state 为 INTERRUPTED)并抛出异样 CancellationException。
7. 总结
- FutureTask 实现了 Runnable 和 Future 接口,是一个可勾销的异步工作。
- FutureTask 中的工作具备 7 种状态,多个线程之间通过该状态来操作工作,如判断工作是否已实现、勾销工作、获取工作后果。
- FutureTask 中只有工作不是 NEW 状态,就示意工作曾经执行结束或者不再执行了,并没有示意“工作正在执行中”的状态。
- FutureTask 中应用链表和 CAS 机制构建一个并发平安的栈,用于存储期待获取工作后果的线程。
- FutureTask 在期待获取工作后果时,依旧会阻塞主线程,违反了异步的初衷。JDK 8 引入了 CompletableFuture,利用回调机制来做到异步获取工作后果。
作者:Sumkor
链接:https://segmentfault.com/a/11…