在 Java 中,Runnable 接口表示一个没有返回结果的任务,而 Callable 接口表示具有返回结果的任务。
在并发编程中,异步执行任务,再获取任务结果,可以提高系统的吞吐量。Future 接口应运而生,它表示异步任务的执行结果,并提供了检查任务是否执行完、取消任务、获取任务执行结果等功能。FutureTask 是 Future 接口的基本实现,常与线程池实现类 ThreadPoolExecutor 配合使用。
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 是一个可取消的异步任务,是对 Future 接口的基本实现,具有以下功能:
- 启动或中断的任务的执行;
- 判断任务是否执行完成;
- 获取任务执行完成后的结果。
同时,FutureTask 可以用于包装 Callable 或 Runnable 对象。
由于它实现了 Runnable 接口,可以提交给 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.
// 任务的执行状态,初始为 NEW。
private volatile int state;
/** The underlying callable; nulled out after running */
// 需要执行的任务,任务执行完后为空
private Callable<V> callable;
/** The result to return or exception to throw from get() */
// 任务的执行结果,或者任务抛出的异常
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
// 执行任务的线程
private volatile Thread runner;
/** Treiber stack of waiting threads */
// 指向栈顶的指针,栈结构用于存储等待任务执行结果的线程
private volatile WaitNode waiters;
其中 state、runner、waiters 三个属性在并发时存在争用,采用 CAS 维护其准确性。
// 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 中使用 state 代表任务在运行过程中的状态。随着任务的执行,状态将不断地进行转变。
状态的说明:
- NEW: 新建状态,任务都从该状态开始。
- COMPLETING: 任务结果正在设置中(正常结果或异常信息)。
- NORMAL: 任务正常执行完成。
- EXCEPTIONAL: 任务执行过程中抛出了异常。
- CANCELLED: 任务被取消(不响应中断)。
- INTERRUPTING:任务正在被中断。
- INTERRUPTED: 任务已经中断。
状态转移过程:
NEW -> COMPLETING -> NORMAL
NEW -> COMPLETING -> EXCEPTIONAL
NEW -> CANCELLED
NEW -> INTERRUPTING -> INTERRUPTED
状态的分类:
- 任务的初始状态:NEW
- 任务的中间状态:COMPLETING、INTERRUPTING
- 任务的终止状态:NORMAL、EXCEPTIONAL、CANCELLED、INTERRUPTED
2.2 状态使用
FutureTask 中判断任务是否已取消、是否已完成,是根据 state 来判断的。
public boolean isCancelled() {
return state >= CANCELLED; // CANCELLED、INTERRUPTING、INTERRUPTED
}
public boolean isDone() {
return state != NEW;
}
可以看到:
- 被取消或被中断的任务(CANCELLED、INTERRUPTING、INTERRUPTED),都视为已取消。
- 当任务离开了初始状态 NEW,就视为任务已结束。任务的中间态很短暂,并不代表任务正在执行,而是任务已经执行完了,正在设置最终的返回结果。
根据状态值,FutureTask 可以保证已经完成的任务不会被再次运行或者被取消。
中间状态虽然是一个瞬时状态,在 FutureTask 中用于线程间的通讯。例如:
- 在 FutureTask#run 中检测到状态 >= INTERRUPTING,说明其他线程发起了取消操作,当前线程需等待对方完成中断。
- 在 FutureTask#get 中检测到状态 <= COMPLETING,说明执行任务的线程尚未处理完,当前线程需等待对方完成任务。
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; // 等待任务执行结果的线程
volatile WaitNode next; // 栈的下一个节点
WaitNode() { thread = Thread.currentThread(); }
}
FutureTask 使用链表来构造栈(Treiber stack,使用 CAS 保证栈操作的线程安全,如 SynchronousQueue 中的 TransferStack,可参考我对 SynchronousQueue 的源码阅读笔记)。
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,如果获取结果时,任务还没有执行完毕,那么获取结果的线程就在栈中挂起,直到任务执行完毕被唤醒。
3. 构造函数
赋值任务,设置任务的初始状态。
/**
* 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
}
值得注意的两个地方:
- FutureTask 创建的时候,状态为 NEW。
- 由于 FutureTask 使用 Callable 表示任务,需用 Executors#callable 方法将 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);
}
执行结果:
run!
call = haha
4. Runnable 实现
4.1 FutureTask#run
代码流程:
- 校验任务是否可执行:任务已执行或其他线程已获取执行权,则无法执行。
- 调用 Callable#call 执行任务。
- 若任务执行失败,使用 setException 方法设置异常。
- 若任务执行成功,使用 set 方法设置返回结果。
- 最后,清除对当前线程的记录,判断是否等待中断。
注意,在任务执行结束后,属性 runner、callable 都会被清空。
java.util.concurrent.FutureTask#run
public void run() {
// state != NEW 说明任务已经执行完毕,不再重复执行
// 将 runner 属性设置为当前线程,若设置失败说明其他线程已获取执行权
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,设置异常
}
if (ran)
set(result); // 执行成功,设置结果
}
} 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
任务执行成功之后,调用该方法。
用于设置任务状态、设置任务执行结果、唤醒栈中等待任务执行结果的线程。
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();
}
}
状态变化:NEW -> COMPLETING -> NORMAL
由于 state 属性是 volatile,这里 putOrderedInt 和 putIntVolatile 是等价的,保证可见性。
为什么这里使用 lazySet 而没有用 CAS :
- 在并发情况下,只有一个线程执行 CAS 将 state 从 NEW 修改为 COMPLETING 会成功,其他线程均失败。
- 因此随后只有一个线程继续修改 state 为 NORMAL,不存在争用,无需使用 CAS。
4.1.2 FutureTask#setException
任务执行发生异常,调用该方法。
除了设置任务状态不同,其他与 FutureTask#set 相同。
状态变化: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
执行完毕,唤醒等待线程。
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 成功,遍历链表唤醒所有节点;2. CAS 失败,重新读取 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(); // 预留方法
callable = null; // to reduce footprint
}
4.1.4 FutureTask#handlePossibleCancellationInterrupt
在 FutureTask#cancel 方法中,会先将 state 设为 INTERRUPTING,再中断 runner 线程,最后将 state 设为 INTERRUPTED。
所以在 FutureTask#run 的 finally 块中如果检查到 state == INTERRUPTING
,说明其他线程发起了 cancel(true) 操作,这里需要等待其他线程中断当前线程。直到检测到 state != INTERRUPTING
,说明其他线程已完成中断当前线程操作。
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) // 其他线程中断当前线程之后,会设置 state 为 INTERRUPTED,使这里结束循环
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
支持周期性执行任务:
- 执行任务成功,不用返回任务结果,也不用改变任务状态(保持为 NEW),下次可以再次执行任务。
- 执行任务失败,则设置异常结果,并修改任务状态(不为 NEW),下次无法再次执行任务。
- 取消执行任务,则等待其他线程中断当前线程,并修改任务状态(不为 NEW),下次无法再次执行任务。
/**
* designed for use with tasks that intrinsically execute more // 设计用来支持定时任务
* 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); // 修改 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 则允许下次再执行 runAndReset
}
5. Future 实现
5.1 Future#get
获取任务执行的结果:
- 如果任务未完成(NEW、COMPLETING),取结果的线程会阻塞(或自旋)。
- 如果任务执行出错(EXCEPTIONAL),抛出 ExecutionException
- 如果任务被取消了(CANCELLED、INTERRUPTING、INTERRUPTED),抛出 CancellationException
- 如果线程等待被中断,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); // 自旋或阻塞等待任务完成
return report(s); // 获取任务执行结果或抛出异常
}
5.1.1 FutureTask#awaitDone
等待任务完成(任务执行完成、任务执行出现异常、任务取消执行),若当前线程发生中断、超时则停止等待。
在自旋中进行判断:
- 若当前线程已中断,则将节点出栈,抛出 InterruptedException。
- 若 state > COMPLETING,说明任务已经完成,返回当前 state。
- 若 state == COMPLETING,说明任务即将完成,当前线程继续自旋。
若 state < COMPLETING,需要将当前线程入栈等待:
- 无超时时间,一直等待直到被其他线程唤醒(FutureTask#run 或 FutureTask#cancel)或发生中断(Thread#interrupt);
- 有超时时间,阻塞直到超时、被唤醒、发生中断。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()) { // 检查并清除中断状态
removeWaiter(q); // 已中断,将节点出栈
throw new InterruptedException();
}
int s = state;
if (s > COMPLETING) { // 其他线程已完成任务,结束等待
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(); // 创建节点,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(内部均调用 FutureTask#finishCompletion)
}
}
5.1.2 FutureTask#report
当前线程等待完毕,获取任务的执行结果,或者抛出异常。
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)
在一定的时间之内,等待获取任务执行的结果。
/**
* @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(); // 等待超时了,任务还没有执行完,则抛出 TimeoutException
return report(s);
}
5.3 Future#cancel
尝试取消任务的执行:
- 如果任务已完成或已取消,则取消操作会失败,返回 false。
- 如果任务还未执行,则取消操作会成功,返回 true。
如果任务正在执行,方法的参数就会指示线程是否需要中断:
- mayInterruptIfRunning 为 true,则当前正在执行的任务会被中断;
- mayInterruptIfRunning 为 false,则允许正在执行的任务继续运行,直到它执行完。
状态变化:
NEW -> CANCELLED
NEW -> INTERRUPTING -> INTERRUPTED
public boolean cancel(boolean mayInterruptIfRunning) {
// 如果任务还没有启动(NEW),则修改任务状态(INTERRUPTING or CANCELLED),修改成功则进入下一步
// 如果任务状态不是 NEW,则直接返回。说明任务已完结(已完成、已取消、出现异常),无法取消,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,说明任务状态为 INTERRUPTING or CANCELLED
// mayInterruptIfRunning 为 true 说明需要中断执行任务的线程,为 false 允许任务继续执行完
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,无需使用 CAS
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // INTERRUPTING -> INTERRUPTED
}
}
} finally {
finishCompletion(); // 唤醒等待线程
}
return true;
}
6. 示例
使用三个线程依次执行:提交任务、等待任务、取消任务。
观察执行结果,理解并发情况下多个线程之间如何使用 Future 进行交互。
/**
* 三个线程依次执行:提交任务、等待任务、取消任务
* 在任务未执行完的时候,取消任务。
*
* @author Sumkor
* @since 2021/4/28
*/
@Test
public void cancel() throws InterruptedException {
// 定义任务
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); // 等待任务提交
CountDownLatch endGate = new CountDownLatch(3); // 等待线程执行完
// 提交任务
new Thread(new Runnable() {
@Override
public void run() {
try {
submitGate.countDown();
System.out.println(Thread.currentThread().getName() + " 执行任务开始");
futureTask.run();
System.out.println(Thread.currentThread().getName() + " 执行任务结束");
} finally {
endGate.countDown();
}
}
}).start();
// 等待任务
new Thread(new Runnable() {
@Override
public void run() {
try {
submitGate.await();
Thread.sleep(1000);// 等待 futureTask.run() 执行一段时间后再获取结果
System.out.println(Thread.currentThread().getName() + " 获取任务结果开始");
String result = futureTask.get();
System.out.println(Thread.currentThread().getName() + " 获取任务结果结束 " + result);
} catch (Exception e) {
System.out.println(Thread.currentThread().getName() + " 获取任务结果失败 " + e.getMessage());
e.printStackTrace();
} finally {
endGate.countDown();
}
}
}).start();
// 取消任务
new Thread(new Runnable() {
@Override
public void run() {
try {
submitGate.await();
Thread.sleep(2000);// 等待 futureTask.get() 执行一段时间后再取消任务
System.out.println(Thread.currentThread().getName() + " 取消任务开始");
boolean cancel = futureTask.cancel(true);
System.out.println(Thread.currentThread().getName() + " 取消任务结束 " + cancel);
} catch (Exception e) {
System.out.println(Thread.currentThread().getName() + " 取消任务失败 " + e.getMessage());
e.printStackTrace();
} finally {
endGate.countDown();
}
}
}).start();
endGate.await();
}
执行结果:
Thread-0 执行任务开始
Thread-1 获取任务结果开始
Thread-2 取消任务开始
Thread-2 取消任务结束 true
Thread-0 执行任务结束
Thread-1 获取任务结果失败 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)
说明:
- 线程 A 启动任务一段时间后,线程 B 来获取任务结果,进入等待。
- 随后线程 C 取消任务,将线程 A 中断(线程 A 不会抛异常,因为 FutureTask#cancel 先一步修改了 state 导致 FutureTask#setException 中 CAS 失败)。
- 此时线程 B 在等待中被唤醒(由线程 C 唤醒,检查到 state 为 INTERRUPTED)并抛出异常 CancellationException。
7. 总结
- FutureTask 实现了 Runnable 和 Future 接口,是一个可取消的异步任务。
- FutureTask 中的任务具有 7 种状态,多个线程之间通过该状态来操作任务,如判断任务是否已完成、取消任务、获取任务结果。
- FutureTask 中只要任务不是 NEW 状态,就表示任务已经执行完毕或者不再执行了,并没有表示“任务正在执行中”的状态。
- FutureTask 中使用链表和 CAS 机制构建一个并发安全的栈,用于存储等待获取任务结果的线程。
- FutureTask 在等待获取任务结果时,依旧会阻塞主线程,违背了异步的初衷。JDK 8 引入了 CompletableFuture,利用回调机制来做到异步获取任务结果。
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