java - 当提交带有回调的可调用任务时，ExecutorService 是如何工作的

Question

如何在 executorService.submit() 回调中添加自定义检查。 我希望我的线程在标志为 true 时不要执行任何操作。如果 flag 为 false 那么它应该照常工作。

import org.apache.commons.lang3.concurrent.BasicThreadFactory;
import java.util.concurrent.Callable;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class test{
    private static volatile boolean flag= false;
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        Callable<Boolean> callableTask = () -> {
            return isFlag();
        };
        Future a = submitAsync(callableTask);
        System.out.println("tartest"+a.get());
    }
    public static  Future submitAsync(Callable taskToRun) {
        ExecutorService e1 =  Executors.newSingleThreadExecutor(new BasicThreadFactory.Builder().namingPattern("").build());
        return e1.submit(() -> {
            try {
                if (flag) {
                    return;
                }
                taskToRun.call();
            } catch (Exception e) {
                System.out.println("garbage ---");
            }
        });
        // return e1.submit(taskToRun);
    }
    public static boolean isFlag() {
        return true;
    }
}

这里，a.get() 返回 null。如果我用 e1.submit(taskToRun) 替换 e1.submit(....) ，它在给定的代码片段中被注释，那么它的工作正常。那么为什么 a.get() 为 null？

Answer 1

return e1.submit(() -> {
    try {
        if (flag) {
            return;
        }
        taskToRun.call();
    } catch (Exception e) {
        System.out.println("garbage ---");
    }
});

在此代码段中，传递给 ExecutorService e1 上的 submit 方法的 lambda 被解释为 Runnable，这就是返回值为空。检查这个documentation更多细节。您需要将可调用的 taskToRun 本身传递给 e1.submit() 才能获取可调用的返回值。此外，将逻辑放在 Callable 本身内部会更清晰。

我重写了示例代码来满足您的要求。

package stackoverflow.test;

import org.apache.commons.lang3.concurrent.BasicThreadFactory;
import java.util.concurrent.Callable;
import java.util.concurrent.Future;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class test{

  private static volatile boolean flag = true;

  public static void main(String[] args) throws ExecutionException, InterruptedException {

    Callable<Boolean> callableTask = () -> {
      if (flag) {
        System.out.println("Flag is true, returning without executing callable logic !");
        return false;
      }
      System.out.println("Flag is false, executing callable logic !");
      return isFlag();
    };

    Future a = submitAsync(callableTask);
    System.out.println("test " + a.get());
  }

  private static  Future<Boolean> submitAsync(Callable<Boolean> taskToRun) {
    ExecutorService e1 =  Executors.newSingleThreadExecutor(new BasicThreadFactory.Builder().namingPattern("").build());

    return e1.submit(taskToRun);
  }

  private static boolean isFlag() {
    return true;
  }
}