是否存在使用当前线程的ExecutorService？

我所追求的是一种兼容的方式来配置是否使用线程池。理想情况下，其余代码完全不会受到影响。我可以使用具有1个线程的线程池，但这不是我想要的。有任何想法吗？

ExecutorService es = threads == 0 ? new CurrentThreadExecutor() : Executors.newThreadPoolExecutor(threads);

// es.execute / es.submit / new ExecutorCompletionService(es) etc

这是一个非常简单的实现Executor（不要ExecutorService介意），它仅使用当前线程。从“ Java并发实践”（必读）中窃取了该信息。

public class CurrentThreadExecutor implements Executor {
    public void execute(Runnable r) {
        r.run();
    }
}

ExecutorService 是一个更复杂的界面，但可以使用相同的方法进行处理。

您可以使用番石榴MoreExecutors.newDirectExecutorService()，MoreExecutors.directExecutor()也可以不需要ExecutorService。

如果包括Guava在内的应用程序过于繁琐，则可以实现几乎相同的功能：

public final class SameThreadExecutorService extends ThreadPoolExecutor {
  private final CountDownLatch signal = new CountDownLatch(1);

  private SameThreadExecutorService() {
    super(1, 1, 0, TimeUnit.DAYS, new SynchronousQueue<Runnable>(),
        new ThreadPoolExecutor.CallerRunsPolicy());
  }

  @Override public void shutdown() {
    super.shutdown();
    signal.countDown();
  }

  public static ExecutorService getInstance() {
    return SingletonHolder.instance;
  }

  private static class SingletonHolder {
    static ExecutorService instance = createInstance();    
  }

  private static ExecutorService createInstance() {
    final SameThreadExecutorService instance
        = new SameThreadExecutorService();

    // The executor has one worker thread. Give it a Runnable that waits
    // until the executor service is shut down.
    // All other submitted tasks will use the RejectedExecutionHandler
    // which runs tasks using the  caller's thread.
    instance.submit(new Runnable() {
        @Override public void run() {
          boolean interrupted = false;
          try {
            while (true) {
              try {
                instance.signal.await();
                break;
              } catch (InterruptedException e) {
                interrupted = true;
              }
            }
          } finally {
            if (interrupted) {
              Thread.currentThread().interrupt();
            }
          }
        }});
    return Executors.unconfigurableScheduledExecutorService(instance);
  }
}

Java 8样式：

Executor e = Runnable::run;

我写了一个ExecutorService基于的AbstractExecutorService。

/**
 * Executes all submitted tasks directly in the same thread as the caller.
 */
public class SameThreadExecutorService extends AbstractExecutorService {

    //volatile because can be viewed by other threads
    private volatile boolean terminated;

    @Override
    public void shutdown() {
        terminated = true;
    }

    @Override
    public boolean isShutdown() {
        return terminated;
    }

    @Override
    public boolean isTerminated() {
        return terminated;
    }

    @Override
    public boolean awaitTermination(long theTimeout, TimeUnit theUnit) throws InterruptedException {
        shutdown(); // TODO ok to call shutdown? what if the client never called shutdown???
        return terminated;
    }

    @Override
    public List<Runnable> shutdownNow() {
        return Collections.emptyList();
    }

    @Override
    public void execute(Runnable theCommand) {
        theCommand.run();
    }
}

为了测试目的，我不得不使用相同的“ CurrentThreadExecutorService”，尽管所有建议的解决方案都不错（尤其是提到Guava方式的解决方案），但我想出了与Peter Lawrey 在这里建议的类似的东西。

正如Axelle Ziegler 在此处提到的那样，不幸的是，Peter的解决方案实际上无法正常工作，因为ThreadPoolExecutor在maximumPoolSize构造函数参数上引入了检查（即maximumPoolSize，不能是<=0）。

为了避免这种情况，我做了以下工作：

private static ExecutorService currentThreadExecutorService() {
    CallerRunsPolicy callerRunsPolicy = new ThreadPoolExecutor.CallerRunsPolicy();
    return new ThreadPoolExecutor(0, 1, 0L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), callerRunsPolicy) {
        @Override
        public void execute(Runnable command) {
            callerRunsPolicy.rejectedExecution(command, this);
        }
    };
}

您可以使用RejectedExecutionHandler在当前线程中运行任务。

public static final ThreadPoolExecutor CURRENT_THREAD_EXECUTOR = new ThreadPoolExecutor(0, 0, 0, TimeUnit.DAYS, new SynchronousQueue<Runnable>(), new RejectedExecutionHandler() {
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
        r.run();
    }
});

您只需要其中之一。