提交带有回调的可调用任务时 ExecutorService 的工作原理

Question

如何在 executorService.submit() 回调中添加自定义检查。只要标志为真，我希望我的线程不做任何事情。如果标志为假，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;
    }
}

此处，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，那么它工作正常。那么为什么 a.get() 为空？

Answer 1

您的 submitAsync 方法 return 是调用 ExecutorService.submit(Runnable) 的结果的 Future。来自 the documentation of that method:

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.

如果您想 return 一个产生值的 Future，请将 return 语句添加到您的 lambda 中，这样它将被解释为 Callable 而不是 Runnable：

    return taskToRun.call();
} catch (Exception e) {
    e.printStackTrace();
    return null;
}

Answer 2

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，这就是为什么 return 值为 null.查看此 documentation 了解更多详情。您需要将可调用对象 taskToRun 本身传递给 e1.submit() 才能获得可调用对象的 return 值。此外，将逻辑放在 Callable 本身内会更清晰。

我已经重写了示例代码以满足您的要求。

package Whosebug.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;
  }
}

提交带有回调的可调用任务时 ExecutorService 的工作原理

How ExecutorService works when submit a callable task with callback

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