将ThreadLocal与ExecutorService一起使用是否有危险? [英] Is it dangerous to use ThreadLocal with ExecutorService?

问题描述

我在下面的博客中介绍了ThreadLocals的概念:

I was going through the concept of ThreadLocals on the below blog :

https://www.baeldung.com/java-threadlocal

它说不要将ThreadLocal与ExecutorService一起使用"

It says that "Do not use ThreadLocal with ExecutorService"

下面说明了使用ThreadLocals的示例.

It illustrates below example for using ThreadLocals.

public class ThreadLocalWithUserContext implements Runnable {

    private static ThreadLocal<Context> userContext 
      = new ThreadLocal<>();
    private Integer userId;
    private UserRepository userRepository = new UserRepository();

    @Override
    public void run() {
        String userName = userRepository.getUserNameForUserId(userId);
        userContext.set(new Context(userName));
        System.out.println("thread context for given userId: "
          + userId + " is: " + userContext.get());
    }

    // standard constructor
}

文章末尾提到:

如果我们要使用ExecutorService并向其提交Runnable，则使用ThreadLocal会产生不确定的结果-因为我们不能保证每次给定userId的每个Runnable操作都将由同一线程处理它被执行.

If we want to use an ExecutorService and submit a Runnable to it, using ThreadLocal will yield non-deterministic results – because we do not have a guarantee that every Runnable action for a given userId will be handled by the same thread every time it is executed.

因此，我们的ThreadLocal将在不同的userId之间共享.这就是为什么我们不应该将TheadLocal与ExecutorService一起使用.仅当我们完全控制哪个线程将选择要执行的可运行操作时，才应使用它.

Because of that, our ThreadLocal will be shared among different userIds. That’s why we should not use a TheadLocal together with ExecutorService. It should only be used when we have full control over which thread will pick which runnable action to execute.

这个解释对我来说是个保镖.我试图为此专门在网上进行一些研究，但没有得到太多帮助，请一些专家详细解释上述解释吗?是作者观点还是真正的威胁?

This explanation was a bouncer to me. I tried to do some research online for this point specifically but I could not get much help, can some expert please elaborate on the above explanation? Is it authors view or a real Threat?

推荐答案

要注意的要点是您的Runnable的多个运行可能在不同的线程上执行.可执行程序服务可以由单个线程支持，但也可以由线程池支持.在Runnable的后续执行中，其他线程将访问其他ThreadLocal.

The point of that caution is that multiple runs of your Runnable may execute on different threads. An executor service can be backed by a single thread but it may just as well be backed by a pool of threads. On subsequent executions of your Runnable, a different thread will be accessing a different ThreadLocal.

因此，您当然可以使用Runnable内的rel ="nofollow noreferrer"> ThreadLocal .但这不太可能有用，因为通常ThreadLocal的目的是保留一个值一段时间.相反，Runnable通常应该是短命的.

So you certainly can use ThreadLocal within a single run of the Runnable. But it is not likely to be useful, as generally the purpose of a ThreadLocal is to hold a value for a while. In contrast, a Runnable should generally be short-lived.

所以，不，通常，将ThreadLocal与线程池一起使用是没有意义的.

So, no, generally it does not make sense to use a ThreadLocal with a thread pool.

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