CompletableFuture with Runnable-delegation - 在委托中忽略异常 class

Question

我在使用 CompletableFuture 将我的代码转换为非阻塞代码时遇到问题。为了尽量减少问题的范围，我创建了一个 sample code，它在我使用 CompletableFuture 时表现不同。问题是 CompletableFuture 吞下了来自 Runnable-delegation 的异常。

我在 Runnable 和 ExecutorService 之上使用委托来提供我的原始应用程序所需的一些包装器代码。

示例代码：

MyRunnable：我的示例运行nable，它总是抛出异常。

public class MyRunnable implements Runnable {

    @Override
    public void run() {
        System.out.println("This is My Thread throwing exception : " + Thread.currentThread().getName());
        throw new RuntimeException("Runtime exception from MyThread");
    }
}

DelegatingRunnable - 这是委托运行nable，它委托并包装传递给它的 Runnable 的逻辑，以及异常处理的占位符。

public class DelegatingRunnable implements Runnable {

    private Runnable delegate; 

    public DelegatingRunnable(Runnable delegate) {
        this.delegate = delegate;
    }

    @Override
    public void run() {
        System.out.println("Delegating Thread start : " + Thread.currentThread().getName());
        try {
            // Some code before thread execution
            delegate.run();
            // Some code after thread execution
        } catch (Exception e) {
            // While using CompletableFuture, could not catch exception here
            System.out.println("###### Delegating Thread Exception Caught : " + Thread.currentThread().getName());
            //throw new RuntimeException(e.getMessage());
        } catch (Throwable t) {
            System.out.println("!!!!!!! Delegating Thread Throwable Caught : " + Thread.currentThread().getName());
        }
        System.out.println("Delegating Thread ends : " + Thread.currentThread().getName());
    }

}

DelegatingExecutorService - 这委托执行方法。它只是用 DelegatingRunnable 包裹运行nable。

public class DelegatingExecutorService extends AbstractExecutorService {

    private ExecutorService executor;

    public DelegatingExecutorService(ExecutorService executor) {
        this.executor = executor;
    }

    @Override
    public void execute(Runnable command) {
        executor.execute(new DelegatingRunnable(command));
    }

    // Othere delegating methods

}

MainClass - 我正在使用两种方法。 Way1 - 使用不带 CompletableFuture 的 ExecutorService。 Way2 - 使用 CompletableFuture

public class MainClass {

    public static void main(String[] arg) {
        //way1();
        way2();
    }

    public static void way2() {
        System.out.println("Way:2 # This is main class : " + Thread.currentThread().getName());

        ExecutorService executor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors()+1);
        DelegatingExecutorService executorService = new DelegatingExecutorService(executor);

        CompletableFuture.runAsync(new MyRunnable(), executorService)
            .whenComplete((res, ex) -> {
                if (ex != null) {
                    System.out.println("whenComplete - exception  : " + Thread.currentThread().getName());
                } else {
                    System.out.println("whenComplete - success  : " + Thread.currentThread().getName());
                }
            });

        executor.shutdown();
        System.out.println("main class completed : " + Thread.currentThread().getName());
    }

    public static void way1() {
        System.out.println("Way:1 # This is main class : " + Thread.currentThread().getName());
        ExecutorService executor = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors()+1);

        DelegatingExecutorService executorService = new DelegatingExecutorService(executor);

        executorService.execute(new MyRunnable());

        executor.shutdown();
        System.out.println("main class completed : " + Thread.currentThread().getName());
    }
}

问题： 当我运行 way1()时，输出是

    Way:1 # This is main class : main
    Delegating Thread start : pool-1-thread-1
    This is My Thread throwing exception : pool-1-thread-1
    ###### Delegating Thread Exception Caught : pool-1-thread-1
    main class completed : main
    Delegating Thread ends : pool-1-thread-1

你可以注意到 'DelegatingRunnable' 的 catch 块可以捕获这里的异常，这是从 MyRunnable 引发的。但是，如果我使用 CompletableFuture 使用 way2()，MyRunnable 的异常不会在 DelegatingRunnable 下咳嗽，尽管我看到它在 CompletableFuture 的 'whenComplete' 回调下咳嗽。

way2的输出是

    Way:2 # This is main class : main
    Delegating Thread start : pool-1-thread-1
    This is My Thread throwing exception : pool-1-thread-1
    Delegating Thread ends : pool-1-thread-1
    whenComplete - exception  : main
    main class completed : main

您可以注意到 CompletableFuture 在内部使用相同的 DelegatingExecutionService 和 DelegatingRunnable。我不明白为什么 DelegatingRunnable 在这种情况下无法捕获异常。

（为什么我要使用 CompletableFuture？-这只是一个示例代码，用于解释我所面临的确切问题。但总的来说，我需要使用 CompletableFuture 以非阻塞方式最终创建任务链）

Answer 1

在 CompletableFuture 的源代码中，您可以看到它将给定的 Runnable 包装在一个 AsyncRun 类型的对象中，该对象本身实现了 Runnable。此 AsyncRun 将传递给执行程序的 execute 方法。当 inner/original Runnable 抛出异常时，它会被 AsyncRun 的代码捕获并且 CompletableFuture 完成为失败但异常将 not 被重新抛出。

这就是为什么您的包装器 (DelegatingRunnable) 永远不会看到异常的原因。

CompletableFuture with Runnable-delegation - 在委托中忽略异常 class

CompletableFuture with Runnable-delegation - exception is ignored in delegating class

java

java-8

completable-future