FutureTask 代表一个可取消的异步计算。它实现了 RunnableFuture 接口，RunnableFuture 接口继承了 Runnable 接口和 Future 接口。
Future 表示异步计算的结果。它提供了检查计算是否完成的方法，以等待计算的完成，并获取计算的结果，还提供了取消任务的方法。
FutureTask 因此既可以获取到异步计算的结果，又可以当做一个任务提交到线程池。
FutureTask<String> future =
new FutureTask<String>(new Callable<String>() {
public String call() {
return searcher.search(target);
}});
executor.execute(future);获取异步计算的结果有两种方式
/**
* 如果有必要，等待任务执行完毕，并返回计算结果.
*
* @return 计算的结果
* @throws CancellationException 计算被取消
* @throws ExecutionException 计算自己出现异常
* @throws InterruptedException 当前线程在等待时被中断
*/
V get() throws InterruptedException, ExecutionException;
/**
* 如果有必要，最多等待给定的时间去执行任务完毕，并返回计算结果（如果结果可用）.
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;get 方法是如何做到，如果任务没执行完就阻塞的呢？
FutureTask 内部定义了多个执行状态，按照状态来控制获取是否阻塞。JDK1.6 使用的 AQS 实现。
public class FutureTask<V> implements RunnableFuture<V> {
/**
* Possible state transitions:
* NEW -> COMPLETING -> NORMAL //正常结束
* NEW -> COMPLETING -> EXCEPTIONAL //执行报错
* NEW -> CANCELLED //调用了 cancel(fasle)
* NEW -> INTERRUPTING -> INTERRUPTED //调用了 cancel(true)
* 从中间状态过渡到最终状态后，无法更进一步修改
*/
private volatile int state;
private static final int NEW = 0; //初始为NEW
private static final int COMPLETING = 1; //中间状态，当 outcome 执行前就会是此状态
private static final int NORMAL = 2; //任务正常执行完毕
private static final int EXCEPTIONAL = 3; //任务执行异常
private static final int CANCELLED = 4; //任务被取消了
private static final int INTERRUPTING = 5; //中间状态，只在调用 cancel(true) 才临时出现
private static final int INTERRUPTED = 6; //任务被中断了
/** The underlying callable; nulled out after running */
//需要执行的任务
private Callable<V> callable;
/** The result to return or exception to throw from get() */
//保存计算结果
private Object outcome; // non-volatile, protected by state reads/writes
/** The thread running the callable; CASed during run() */
//当前线程
private volatile Thread runner;
/** Treiber stack of waiting threads */
//等待此计算任务结束获取结果的线程
private volatile WaitNode waiters;
static final class WaitNode {
volatile Thread thread;
volatile WaitNode next;
WaitNode() { thread = Thread.currentThread(); }
}
}一个简单的示例
/**
* Created by Tangwz on 2019/7/14
*/
public class TestFutureTask {
public static void main(String[] args) {
MyTask myTask = new MyTask();
FutureTask<String> futureTask = new FutureTask<>(myTask);
//不用线程池，也可单独使用线程
// Thread thread = new Thread(futureTask);
// thread.start();
ExecutorService executorService = Executors.newSingleThreadExecutor();
executorService.submit(futureTask);
//取消任务
// futureTask.cancel(true);
//突然关闭线程池
// executorService.shutdownNow();
try {
//此方法会阻塞，等待返回结果
String result = futureTask.get();
System.out.println("任务结果：" + result);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
executorService.shutdown();
}
static class MyTask implements Callable<String> {
@Override
public String call() throws InterruptedException {
System.out.println("模拟任务执行 1s");
Thread.sleep(1000);
return "success";
}
}
}输出结果：
模拟任务执行 1s
任务结果：success
构造一个 FutureTask 还可以通过一个 Runnable，并指定一个结果。
public FutureTask(Runnable runnable, V result) {
//使用了适配器模式
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
//执行给定的任务，并返回给定的结果
return new RunnableAdapter<T>(task, result);
}
/**
* A callable that runs given task and returns given result
*/
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}构造一个 FutureTask 后，调用 futureTask.get() 就会阻塞调用的线程直到任务执行完毕，让我们来看看 get 方法内部是怎样的。直接获取结果和设置了超时时间的获取差别不大，后者会多抛一个超时异常。
/**
* @throws CancellationException {@inheritDoc}
*/
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
//正在执行中就等待执行完毕
s = awaitDone(false, 0L);
//若已经结束了，按照状态返回结果
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 &&
//若超时时计算任务还没结束，抛出超时异常
(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
throw new TimeoutException();
//若已经结束了，按照状态返回结果
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
*/
//等待执行完毕，或者被中断/超时
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//此查询计算结果的任务应该结束的时间点
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
//是否入队列
boolean queued = false;
for (;;) {
//若此任务被中断，移除队列中的节点，表明此线程不能再等待计算结果了
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
//获取此时的计算状态
int s = state;
//按照状态的实际规则，状态大于 COMPLETING 代表计算结果已经放到 outcome 中
//这时可以返回状态获取结果了
if (s > COMPLETING) {
//若此线程的等待节点不为空，移除节点中的线程属性
if (q != null)
q.thread = null;
return s;
}
//若正在执行中，暂时放弃CPU资源，稍等一会儿
else if (s == COMPLETING) // cannot time out yet
Thread.yield();
//到这里说明计算任务还没开始，创建一个等待节点
else if (q == null)
q = new WaitNode();
//没有入队列，就加入到队列中
else if (!queued)
//CAS操作，失败上面还有循环，不会退出，再次设置
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
//是否设置了超时时间
else if (timed) {
nanos = deadline - System.nanoTime();
//是否已经超时
if (nanos <= 0L) {
//移除此节点
removeWaiter(q);
//因为此查询任务已超时，不管计算任务是什么状态，直接返回此时的状态
return state;
}
//阻塞一定的时间，直到被唤醒或者中断或者超时
LockSupport.parkNanos(this, nanos);
}
else
//阻塞，直到被唤醒或者中断
LockSupport.park(this);
}
}按照状态来返回结果
/**
* 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)
//正常结束状态，返回结果
return (V)x;
//如果被取消或者被中断，抛出被取消异常
if (s >= CANCELLED)
throw new CancellationException();
//s == EXCEPTIONAL，代表执行任务报错
throw new ExecutionException((Throwable)x);
}任务有没有执行完？是什么时候修改的 state 呢？
关键点就在 FutureTask 重写了的 run 方法，线程运行时就会调用 FutureTask.run。
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
//如果任务没有被取消，设置 Future 为计算的结果
public void run() {
//若此任务状态不为初始值，或者设置执行线程时失败
//都表示此任务被其他线程执行了，可以直接返回
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
//执行初始化 Future 的时候传递的 Callable
Callable<V> c = callable;
//不为空，且为初始状态才执行
if (c != null && state == NEW) {
V result;
//表明任务是否执行完毕
boolean ran;
try {
//真正的调用 run 或者 call 方法的地点
result = c.call();
ran = true;
} catch (Throwable ex) {
//捕获了执行任务抛出的异常
result = null;
ran = false;
//设置执行任务抛异常的结果
setException(ex);
}
if (ran)
//设置正常执行任务的结果
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
//利用了 volatile 的禁止指令重排，保证中断后读到的 runner 为空
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}按照任务执行的情况来设置结果
//计算任务执行抛异常设置异常结果
protected void setException(Throwable t) {
//任务可能被取消，CAS 操作
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
//CAS 成功后，结束任务，唤醒所有等待结果的线程
finishCompletion();
}
}
//计算任务执行正常结束设置结果
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
/**
* Removes and signals all waiting threads, invokes done(), and
* nulls out callable.
*/
private void finishCompletion() {
// assert state > COMPLETING;
//这里循环因为唤醒期间可能存在新的查询任务进来
for (WaitNode q; (q = waiters) != null;) {
if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
//循环唤醒所有等待线程
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;
}
}
//可扩展的方法，默认无实现
done();
callable = null; // to reduce footprint
}如何取消任务？调用 futureTask.cancel(true) 方法。
/**
* 试图取消此任务的执行。如果任务已经完成或者已取消，或者其他原因导致无法取消，cancel 返回 false；
* 当任务还没启动，调用 cancel 成功那么此任务永远不会执行（run 方法会检查状态）；
* 当任务已经启动还未完成时，mayInterruptIfRunning 决定是否停止正在执行的任务
* @param mayInterruptIfRunning 执行中的任务是否允许中断
*/
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
//是否允许中断执行中的任务
if (mayInterruptIfRunning) {
try {
Thread t = runner;
if (t != null)
//设置中断标识，也需要任务响应中断信号
t.interrupt();
} finally { // final state
//任务状态最终设置成 INTERRUPTED
UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
}
}
} finally {
finishCompletion();
}
return true;
}判断任务有无执行完成，有无被取消，就很简单
public boolean isCancelled() {
return state >= CANCELLED;
}
public boolean isDone() {
return state != NEW;
}还有一个特殊的方法 runAndReset，执行任务但是不设置计算结果，如果执行期间任务遇到错误或者被取消，那么该方法返回失败。此方法被设计用于那些本质上要执行多次的任务。
/**
* 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 {@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 {
//不获取执行结果，不设置结果到 outcome 中
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;
}
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