Java并发编程: 并发编程中的ExecutionException异常

一、什么是ExecutionException

在并发编程中在执行java.util.concurrent.Future实现类的get方法时，需要捕获java.util.concurrent.ExecutionException这个异常。Future.get()方法通常是要获取任务的执行结果，当执行任务的过程中抛出了异常，就会产生ExecutionException异常。

二、如何处理ExecutionException

案例分析：

package threadpool;

import org.junit.Test;

import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
import java.util.concurrent.*;
public class ExecutorCompletionServiceDemo {

    @Test
    public void TestExecutorCompletionService () throws InterruptedException {
        ThreadPoolExecutor executor = new ThreadPoolExecutor(10, 20, 1, TimeUnit.SECONDS, new ArrayBlockingQueue<>(50));
        Collection<Callable<String>> tasks = new ArrayList<>();
        tasks.add(() -> {return "1";});
        tasks.add(() -> {return "2";});
        tasks.add(() -> {return "3";});
        tasks.add(() -> {return "4";});
        tasks.add(() -> {return "5";});
        tasks.add(() -> {return "6";});
        tasks.add(() -> {return "7";});
        tasks.add(() -> {return "8";});
        tasks.add(() -> {throw new UnsupportedOperationException("暂不支持该操作");});
        tasks.add(() -> {throw new NullPointerException("NullPointerException occurred");});

        execute(executor, tasks);
    }

    private static void execute(ThreadPoolExecutor executor, Collection<Callable<String>> tasks)  {
        CompletionService<String> completionService = new ExecutorCompletionService<>(executor);
        List<Future<String>> futures = new ArrayList<>(tasks.size());
        for (Callable<String> task : tasks) {
            futures.add(completionService.submit(task));
        }

        for (int i = 0; i < futures.size(); i++) {
            try {
                Future<String> take = completionService.take();
                String s = take.get();
                System.out.println(s);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
                throw new RuntimeException(e);
            } catch (ExecutionException e) {
               throw new RuntimeException(e);
            }
        }
    }
}

在execute方法中对于对ExecutionExcetion异常可以做一下处理：
引入依赖：

        <dependency>
            <groupId>com.google.guava</groupId>
            <artifactId>guava</artifactId>
            <version>21.0</version>
        </dependency>

private static void execute(ThreadPoolExecutor executor, Collection<Callable<String>> tasks)  {
        CompletionService<String> completionService = new ExecutorCompletionService<>(executor);
        List<Future<String>> futures = new ArrayList<>(tasks.size());
        for (Callable<String> task : tasks) {
            futures.add(completionService.submit(task));
        }

        for (int i = 0; i < futures.size(); i++) {
            try {
                Future<String> take = completionService.take();
                String s = take.get();
                System.out.println(s);
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
                throw new RuntimeException(e);
            } catch (ExecutionException e) {
                Throwable cause = e.getCause() == null ? e : e.getCause();
                Throwables.propagateIfPossible(cause, RuntimeException.class);
                throw new RuntimeException(cause);
            }
        }
    }

可以看出异常信息清晰了很多，便于开发人员分析。在上面的代码中通过getCause方法获取了来获取真实的异常。接下来使用了Google Guava提供的Throwable类来进行错误的转播与检查，当异常类型为RunTimeException进行抛出，最后使用Throw new RuntimeException(cause)进行兜底。