什么时候应该使用ThreadLocal变量?

它是如何使用的?


当前回答

当您希望某些状态不应该在不同线程之间共享,但每个线程在其整个生命周期内都可以访问时,ThreadLocal非常有用。

As an example, imagine a web application, where each request is served by a different thread. Imagine that for each request you need a piece of data multiple times, which is quite expensive to compute. However, that data might have changed for each incoming request, which means that you can't use a plain cache. A simple, quick solution to this problem would be to have a ThreadLocal variable holding access to this data, so that you have to calculate it only once for each request. Of course, this problem can also be solved without the use of ThreadLocal, but I devised it for illustration purposes.

也就是说,请记住ThreadLocals本质上是一种全局状态的形式。因此,它有许多其他含义,只有在考虑了所有其他可能的解决方案后才应该使用。

其他回答

您必须非常小心地使用ThreadLocal模式。有一些主要的缺点,如Phil所提到的,但其中一个没有提到的是,要确保设置ThreadLocal上下文的代码不是“可重入的”。

当设置信息的代码第二次或第三次运行时,可能会发生糟糕的事情,因为线程上的信息可能会在您没有预料到的时候开始发生变化。因此,在再次设置ThreadLocal信息之前,请注意确保没有设置它。

从本质上讲,当您需要一个变量的值依赖于当前线程,并且不方便您以其他方式将值附加到线程(例如,子类化线程)。

典型的情况是,其他框架创建了运行代码的线程,例如servlet容器,或者使用ThreadLocal更有意义,因为你的变量“在它的逻辑位置”(而不是挂在thread子类或其他哈希映射中的变量)。

在我的网站上,我有一些关于何时使用ThreadLocal的进一步讨论和示例,您可能也会感兴趣。

有些人主张使用ThreadLocal作为一种方法,在某些并发算法中,当你需要线程号时,将“线程ID”附加到每个线程上(参见Herlihy & Shavit)。在这种情况下,检查你是否真的得到了好处!

试试这个小例子,感受一下ThreadLocal变量:

public class Book implements Runnable {
    private static final ThreadLocal<List<String>> WORDS = ThreadLocal.withInitial(ArrayList::new);

    private final String bookName; // It is also the thread's name
    private final List<String> words;


    public Book(String bookName, List<String> words) {
        this.bookName = bookName;
        this.words = Collections.unmodifiableList(words);
    }

    public void run() {
        WORDS.get().addAll(words);
        System.out.printf("Result %s: '%s'.%n", bookName, String.join(", ", WORDS.get()));
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(new Book("BookA", Arrays.asList("wordA1", "wordA2", "wordA3")));
        Thread t2 = new Thread(new Book("BookB", Arrays.asList("wordB1", "wordB2")));
        t1.start();
        t2.start();
    }
}

控制台输出,如果线程BookA先执行: 结果BookA: 'wordA1, wordA2, wordA3'。 结果BookB: 'wordB1, wordB2'。 控制台输出,如果先执行线程BookB: 结果BookB: 'wordB1, wordB2'。 结果BookA: 'wordA1, wordA2, wordA3'。

什么时候?

当一个对象不是线程安全的时候,为了避免影响可伸缩性的同步,给每个线程一个对象,并保持它的线程作用域,即ThreadLocal。最常用但不是线程安全的对象之一是数据库Connection和JMSConnection。

如何?

例如,Spring框架通过将这些连接对象保存在ThreadLocal变量中,在幕后大量使用ThreadLocal来管理事务。在高层,当事务启动时,它获得连接(并禁用自动提交)并将其保存在ThreadLocal中。在进一步的db调用中,它使用相同的连接与db通信。最后,它从ThreadLocal获取连接,提交(或回滚)事务并释放连接。

我认为log4j也使用ThreadLocal来维护MDC。

Nothing really new here, but I discovered today that ThreadLocal is very useful when using Bean Validation in a web application. Validation messages are localized, but by default use Locale.getDefault(). You can configure the Validator with a different MessageInterpolator, but there's no way to specify the Locale when you call validate. So you could create a static ThreadLocal<Locale> (or better yet, a general container with other things you might need to be ThreadLocal and then have your custom MessageInterpolator pick the Locale from that. Next step is to write a ServletFilter which uses a session value or request.getLocale() to pick the locale and store it in your ThreadLocal reference.