适用于“主”线程的Java执行器

Question

我想从程序的“主”线程（类似于Android中的主循环程序）创建一个Executor ，然后运行直到它处理了提交给它的所有内容：

public class MyApp {
  private static Callable<Integer> task = () -> {
    // ... return an int somehow ...
  };

  public static void main(String[] args) throws ExecutionException, InterruptedException {
    ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(1));
    Thread main = Thread.currentThread();
    ExecutorService executorService = Executors.newSingleThreadExecutor(r -> main);

    service.submit(task).addListener(() -> {
      /// ... do something with the result ...
    }, executorService);

    executorService.awaitTermination(100, TimeUnit.SECONDS);
  }
}

但是我得到了一个IllegalThreadState异常：

SEVERE: RuntimeException while executing runnable MyApp$$Lambda$20/0x00000008000a6440@71f06a3c with executor java.util.concurrent.Executors$FinalizableDelegatedExecutorService@47add263
java.lang.IllegalThreadStateException
    at java.base/java.util.concurrent.ThreadPoolExecutor.addWorker(ThreadPoolExecutor.java:926)
    at java.base/java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1343)
    at java.base/java.util.concurrent.Executors$DelegatedExecutorService.execute(Executors.java:687)
    at com.google.common.util.concurrent.AbstractFuture.executeListener(AbstractFuture.java:1137)
    at com.google.common.util.concurrent.AbstractFuture.complete(AbstractFuture.java:957)
    at com.google.common.util.concurrent.AbstractFuture.set(AbstractFuture.java:726)
    at com.google.common.util.concurrent.TrustedListenableFutureTask$TrustedFutureInterruptibleTask.afterRanInterruptibly(TrustedListenableFutureTask.java:131)
    at com.google.common.util.concurrent.InterruptibleTask.run(InterruptibleTask.java:133)
    at com.google.common.util.concurrent.TrustedListenableFutureTask.run(TrustedListenableFutureTask.java:78)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:834)

我可以在新线程上启动ExecutorService ，然后等待它，但这似乎很浪费。

有没有什么好方法可以从当前线程创建Executor ，并等待它处理已提交给它的所有内容？

Answer 1

您可能正在寻找类似CompletionService东西：您向此任务提交任务，并且它按完成顺序将其返回。

ExecutorService service = Executors.newFixedThreadPool(1);
ExecutorCompletionService<Integer> comp = new ExecutorCompletionService<>(service);

comp.submit(task);
// ... I assume you want to submit N tasks.

for (int i = 0; i < N; ++i) {
  Future<Integer> future = comp.take();
  Integer result = future.get();

  // ... do something with the result ...
}

项目完成后，将立即对其进行处理。 如果您能够等到拥有所有东西之后再进行一次处理，则可以使用Guava的Futures.allAsList ：

ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(1));
List<Callable<Integer>> tasks = ...;

List<ListenableFuture<Integer>> futures = new ArrayList<>();
for (Callable<Integer> task : tasks) {
  futures.add(service.submit(task));
}

ListenableFuture<List<Integer>> resultsFuture = Futures.allAsList(futures);
List<Integer> results = resultsFuture.get();

// Process the results.

Answer 2

使用番石榴的MoreExecutors.newDirectExecutorService()

这将确保提交的代码将在ThreadPool的同一线程中执行。 我知道，它不是主线程，但至少您不会像想要的那样仅为侦听器创建其他新线程。

import com.google.common.util.concurrent.*;
import org.junit.jupiter.api.Test;
import java.util.concurrent.*;

class ExecutorTest {
  private static Callable<Integer> task = () -> {
    System.out.println("in call: " + Thread.currentThread().getName());
    TimeUnit.SECONDS.sleep(1);
    return 0;
  };
  @Test
  void test() throws InterruptedException {
    ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(1));
    ExecutorService executor = MoreExecutors.newDirectExecutorService();
    service.submit(task).addListener(() -> {
      System.out.println("in listener: " + Thread.currentThread().getName());
    }, executor);
    executor.awaitTermination(2, TimeUnit.SECONDS);
  }
}

Answer 3

这里记录的方法invokeAll()等待所有任务完成
你可以这样做

executorService.invokeAll();

请按照此帖子获取更多说明

Answer 4

您正在尝试在主线程中执行回调。 直接的方法是调用get() ：

YourResult result = service.submit(task).get();
/* Do something with your result... */

如果要异步执行回调，则可能对CompletionStage感兴趣

CompletableFuture.runAsync(task).thenAccept(r -> /* do something with result */);