ExecutorService的shutdown（）不会等到所有线程都完成 [英] ExecutorService's shutdown() doesn't wait until all threads will be finished

问题描述

我有一个代码，其中4个线程同时运行。我想等待，直到所有这4个线程将完成。

I have a code where 4 threads run at the same time. I want to wait until all these 4 threads will be finished. And only after that to continue the app flow.

我尝试了两种方法：

1. Thread＃join（） ，这种方法正常工作。 join（）后的代码只有在所有主题完成后才会执行。


2. ExecutorService＃shutdown（） ，这种技术允许执行 shutdown（）之后的代码，即使并非所有线程都完成。 li>
   

1. Thread#join(), this approach works as expected. The code, which comes after join() is executed only after all threads are finished.
2. ExecutorService#shutdown(), this technique allows executing code, which comes after shutdown() even if not all threads are finished.

程式码范例：

ExecutorService service = Executors.newFixedThreadPool(cpuCoresNum);

for (int i = 0; i < cpuCoresNum; i++) {

    service.submit(() -> {
        try {
            foo(); // some long execution function
        } catch (Exception e) {
            e.printStackTrace();
        }
    });
}

service.shutdown();

System.out.println("We're done! All threads are finished!");

我的问题：

• 为什么 submit（）和 shutdown（）所有线程将完成并打印«我们完成了！所有主题已完成！紧接着 service.shutdown（）; ？

• Why submit() and shutdown() don't wait until all threads will be finished and prints «We're done! All threads are finished!» right after call of service.shutdown();?

推荐答案

答案可在 ExecutorService.shutdown（） Javadoc：

The answer is available in the ExecutorService.shutdown() Javadoc:

此方法不会等待先前提交的任务完成执行。

This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

如果你想等待线程完成工作，你有以下选项：

If you want to wait for the threads to finish work you have the following options:

• get 由 submit（）返回的实例 并在每个未来实例


• 上调用 get（） 关闭在服务调用 awaitTermination > service ，直到它返回 true


• ，而不是调用提交 服务将 Runnable 实例添加到 java.util.List ，并将此列表传递给服务调用的 invokeAll 方法



• get Future instances returned by submit() and call get() on every Future instance
• after calling shutdown on service call awaitTermination on service until it returns true
• instead of calling submit on service add your Runnable instances to a java.util.List and pass this list to the invokeAll method called on service

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