Callable、Future、FutureTask基本介绍
- 直接继承Thread或者实现Runnable接口都可以创建线程,但是这两种方法都有一个问题就是:没有返回值,也就是不能获取执行完的结果。
- 因此jdk1.5就提供了Callable接口来实现这一场景,而Future和FutureTask就可以和Callable接口配合起来使用。
Callable和Runnable的区别
- Runnable不能返回一个返回值,不能抛出 checked Exception。
- Callable的call方法可以有返回值,可以声明抛出异常。
- Callable 配合的有一个 Future 类,通过 Future 可以了解任务执行情况,或者取消任务的执行,还可获取任务执行的结果,这些功能都是 Runnable 做不到的,Callable 的功能要比 Runnable 强大。
FutureTask的父类Future介绍
- Future最常用的实现是:FutureTask(由于他继承的RunnableFuture实现了Runnable和Future)
- boolean cancel (boolean mayInterruptIfRunning):取消任务的执行。参数指定是否立即中断任务执行,或者等等任务结束。
- boolean isCancelled ():任务是否已经取消,任务正常完成前将其取消,则返回 true。
- boolean isDone ():任务是否已经完成。需要注意的是如果任务正常终止、异常或取消,都将返回true。
- V get () throws InterruptedException, ExecutionException:等待任务执行结束,然后获得V类型的结果。InterruptedException 线程被中断异常, ExecutionException任务执行异常,如果任务被取消,还会抛出CancellationException。
- V get (long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException:同上面的get功能一样,多了设置超时时间。参数timeout指定超时时间,uint指定时间的单位,在枚举类TimeUnit中有相关的定义。如果计算超时,将抛出TimeoutException
Future的注意事项
- 当 for 循环批量获取 Future 的结果时容易长时间阻塞,get 方法调用时应使用 timeout 限制。
- Future 的生命周期不能后退。一旦完成了任务,它就永久停在了“已完成”的状态,不能从头再来。
Future的缺陷
- 并发执行多任务:Future只提供了get()方法来获取结果,并且是阻塞的。所以,除了等待你别无他法;
- 无法对多个任务进行链式调用:如果你希望在计算任务完成后执行特定动作,比如发邮件,但Future却没有提供这样的能力;
- 无法组合多个任务:如果你运行了10个任务,并期望在它们全部执行结束后执行特定动作,那么在Future中这是无能为力的;
- 没有异常处理:Future接口中没有关于异常处理的方法;
使用方式:把串行的任务并行化
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
public class FutureTest {
public static void main(String[] args) throws ExecutionException, InterruptedException {
FutureTask<String> ft1 = new FutureTask<>(new T1Task());
FutureTask<String> ft2 = new FutureTask<>(new T2Task());
FutureTask<String> ft3 = new FutureTask<>(new T3Task());
FutureTask<String> ft4 = new FutureTask<>(new T4Task());
FutureTask<String> ft5 = new FutureTask<>(new T5Task());
ExecutorService executorService = Executors.newFixedThreadPool(5);
executorService.submit(ft1);
executorService.submit(ft2);
executorService.submit(ft3);
executorService.submit(ft4);
executorService.submit(ft5);
System.out.println(ft1.get());
System.out.println(ft2.get());
System.out.println(ft3.get());
System.out.println(ft4.get());
System.out.println(ft5.get());
executorService.shutdown();
}
static class T1Task implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("T1:任务正在执行...");
TimeUnit.MILLISECONDS.sleep(50);
return "T1:执行完成!";
}
}
static class T2Task implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("T2:任务正在执行...");
TimeUnit.MILLISECONDS.sleep(50);
return "T2:执行完成!";
}
}
static class T3Task implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("T3:任务正在执行...");
TimeUnit.MILLISECONDS.sleep(50);
return "T3:执行完成!";
}
}
static class T4Task implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("T4:任务正在执行...");
TimeUnit.MILLISECONDS.sleep(50);
return "T4:执行完成!";
}
}
static class T5Task implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("T5:任务正在执行...");
TimeUnit.MILLISECONDS.sleep(50);
return "T5:执行完成!";
}
}
}
- 运行结果

FutureTask的状态源码分析
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的构造方法源码分析
public FutureTask(Callable<V> callable) {
if (callable == null)
throw new NullPointerException();
this.callable = callable;
this.state = NEW;
}
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW;
}
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, 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;
}
}
FutureTask的run方法源码分析
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 = null;
int s = state;
if (s >= INTERRUPTING)
handlePossibleCancellationInterrupt(s);
}
}
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL);
finishCompletion();
}
}
private void finishCompletion() {
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;
q = next;
}
break;
}
}
done();
callable = null;
}
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL);
finishCompletion();
}
}
private void handlePossibleCancellationInterrupt(int s) {
if (s == INTERRUPTING)
while (state == INTERRUPTING)
Thread.yield();
}
FutureTask的get方法源码分析
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
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);
}
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long = 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)
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);
}
}
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);
}
结束语
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