缓存，有时你必须计算相同的值很多时间，所以通过存储最后一组输入到一个方法和结果，你可以加快代码。通过使用线程本地存储，您可以避免考虑锁定问题。

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。

Many frameworks use ThreadLocals to maintain some context related to the current thread. For example when the current transaction is stored in a ThreadLocal, you don't need to pass it as a parameter through every method call, in case someone down the stack needs access to it. Web applications might store information about the current request and session in a ThreadLocal, so that the application has easy access to them. With Guice you can use ThreadLocals when implementing custom scopes for the injected objects (Guice's default servlet scopes most probably use them as well).

ThreadLocals are one sort of global variables (although slightly less evil because they are restricted to one thread), so you should be careful when using them to avoid unwanted side-effects and memory leaks. Design your APIs so that the ThreadLocal values will always be automatically cleared when they are not needed anymore and that incorrect use of the API won't be possible (for example like this). ThreadLocals can be used to make the code cleaner, and in some rare cases they are the only way to make something work (my current project had two such cases; they are documented here under "Static Fields and Global Variables").

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.

ThreadLocal是JVM专门提供的功能，仅为线程提供隔离的存储空间。与实例作用域变量的值一样，变量只能绑定到类的给定实例。每个对象都有其唯一的值，它们不能看到彼此的值。ThreadLocal变量的概念也是如此，在对象实例的意义上，它们是线程的本地线程，除了创建它的线程之外，其他线程看不到它。在这里看到的

import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.IntStream;


public class ThreadId {
private static final AtomicInteger nextId = new AtomicInteger(1000);

// Thread local variable containing each thread's ID
private static final ThreadLocal<Integer> threadId = ThreadLocal.withInitial(() -> nextId.getAndIncrement());


// Returns the current thread's unique ID, assigning it if necessary
public static int get() {
    return threadId.get();
}

public static void main(String[] args) {

    new Thread(() -> IntStream.range(1, 3).forEach(i -> {
        System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
    })).start();

    new Thread(() -> IntStream.range(1, 3).forEach(i -> {
        System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
    })).start();

    new Thread(() -> IntStream.range(1, 3).forEach(i -> {
        System.out.println(Thread.currentThread().getName() + " >> " + new ThreadId().get());
    })).start();

}
}

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。