我需要异步执行任务,同时丢弃任何进一步的请求,直到任务完成。
同步方法只是将任务排队并且不会跳过。我最初想使用 SingleThreadExecutor 但它也会对任务进行排队。然后,我查看了 ThreadPoolExecutor,但它读取队列来获取要执行的任务,因此将执行一个任务,并且至少有一个任务排队(其他任务可以使用 ThreadPoolExecutor.DiscardPolicy 丢弃)。
我唯一能想到的就是使用信号量来阻塞队列。我用下面的例子来展示我想要实现的目标。有没有更简单的方法?我错过了一些明显的事情吗?
import java.util.concurrent.*;
public class ThreadPoolTester {
private static ExecutorService executor = Executors.newSingleThreadExecutor();
private static Semaphore processEntry = new Semaphore(1);
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 20; i++) {
kickOffEntry(i);
Thread.sleep(200);
}
executor.shutdown();
}
private static void kickOffEntry(final int index) {
if (!processEntry.tryAcquire()) return;
executor.
submit(
new Callable<Void>() {
public Void call() throws InterruptedException {
try {
System.out.println("start " + index);
Thread.sleep(1000); // pretend to do work
System.out.println("stop " + index);
return null;
} finally {
processEntry.release();
}
}
}
);
}
}
样本输出
start 0
stop 0
start 5
stop 5
start 10
stop 10
start 15
stop 15
采用 axtavt 的答案并转换上面的示例给出了以下更简单的解决方案。
import java.util.concurrent.*;
public class SyncQueueTester {
private static ExecutorService executor = new ThreadPoolExecutor(1, 1,
1000, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>(),
new ThreadPoolExecutor.DiscardPolicy());
public static void main(String[] args) throws InterruptedException {
for (int i = 0; i < 20; i++) {
kickOffEntry(i);
Thread.sleep(200);
}
executor.shutdown();
}
private static void kickOffEntry(final int index) {
executor.
submit(
new Callable<Void>() {
public Void call() throws InterruptedException {
System.out.println("start " + index);
Thread.sleep(1000); // pretend to do work
System.out.println("stop " + index);
return null;
}
}
);
}
}