Question

按照示例代码我正在注入一个睡眠100毫秒的biconsumer作为一组可完成的未来的完成动作。我已经使用whenCompleteAsync方法，通过单独使用executorService。 executorService是ThreadPoolExecutor，核心池大小为5，最大大小为5，队列长度为1。

public class CompleteTest {
    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(5, 5, 10,
                TimeUnit.SECONDS, new ArrayBlockingQueue<>(1));

        ArrayList<CompletableFuture<String>> list = new ArrayList<>();

        for (int i = 0; i <100; i++) {
            CompletableFuture<String> stringCompletableFuture = new CompletableFuture<>();
            stringCompletableFuture.whenCompleteAsync((e, a) -> {
                System.out.println("Complete " + e);
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e1) {e1.printStackTrace();}
            }, executorService);

            list.add(stringCompletableFuture);
        }

        for (int i = 0; i < list.size(); i++) {
            list.get(i).complete(i + "");
        }
    }
}

当我运行代码时，即使我正在完成100个期货，也只会打印6个输出。这是5个核心线程和1个排队的线程。剩下的会怎么样？如果其他runnable由于队列已满而无法提交给执行程序服务，那么不应该有异常。？

输出

Complete 0
Complete 1
Complete 2
Complete 3
Complete 4
Complete 5

Answer 1

抛出异常，异常完成CompletableFuture，而不是您正在跟踪的任何内容。

您正在使用构造函数实例化和初始化ThreadPoolExecutor，该构造函数使用仅抛出异常的默认RejectedExecutionHandler。我们知道如果RejectedExecutionException无法接受任务，则会引发ExecutorService。那么添加的任务在哪里以及抛出的异常在哪里？

目前，所有链接都在whenCompleteAsync内发生。当您拨打该号码时，您需要向接收者CompletableFuture，stringCompletableFuture添加一个从属。完成stringCompletableFuture后（在这种情况下成功），它将创建一个新的CompletableFuture（它返回）并尝试在给定的BiConsumer上安排给定的ExecutorService 。

由于ExecutorService的队列没有空格，它将调用RejectedExecutionHandler，它将抛出RejectedExecutionException。该异常在那时被捕获并用于completeExceptionally将返回的CompletableFuture。

换句话说，在for循环中，捕获CompletableFuture返回的whenCompleteAsync，存储它，并打印出其状态。

ArrayList<CompletableFuture<String>> list = new ArrayList<>();
ArrayList<CompletableFuture<?>> dependents = new ArrayList<>();
for (int i = 0; i <100; i++) {
    CompletableFuture<String> stringCompletableFuture = new CompletableFuture<>();
    CompletableFuture<?> thisWillHaveException = stringCompletableFuture.whenCompleteAsync((e, a) -> {
        System.out.println("Complete " + e);
        try {
            Thread.sleep(100);
        } catch (InterruptedException e1) {e1.printStackTrace();}
    }, executorService);
    dependents.add(thisWillHaveException);
    list.add(stringCompletableFuture);
}

for (int i = 0; i < list.size(); i++) {
    list.get(i).complete(i + "");
}
Thread.sleep(2000);
dependents.forEach(cf -> {
    cf.whenComplete((r, e) -> {
        if (e != null)
            System.out.println(cf + " " + e.getMessage());
    });
});

你会注意到他们都是（除了之前成功打印的6个）以RejectedExecutionException特殊完成。

...
java.util.concurrent.CompletableFuture@2d8e6db6[Completed exceptionally] java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.CompletableFuture$UniWhenComplete@3f91beef rejected from java.util.concurrent.ThreadPoolExecutor@4eec7777[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
java.util.concurrent.CompletableFuture@23ab930d[Completed exceptionally] java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.CompletableFuture$UniWhenComplete@1a6c5a9e rejected from java.util.concurrent.ThreadPoolExecutor@4eec7777[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
java.util.concurrent.CompletableFuture@4534b60d[Completed exceptionally] java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.CompletableFuture$UniWhenComplete@37bba400 rejected from java.util.concurrent.ThreadPoolExecutor@4eec7777[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]

如何捕获CompletableFuture的whenCompleteAsync调用中抛出的RejectedExecutionException？

1 个答案: