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!

文档说得很好:“每个访问[线程局部变量]的线程(通过它的get或set方法)都有它自己的、独立初始化的变量副本”。

当每个线程必须有自己的某个副本时，可以使用一个。默认情况下，数据在线程之间共享。

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").

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。

Threadlocal提供了一种非常简单的零成本实现对象可重用性的方法。

我遇到过这样一种情况，在每次更新通知时，多个线程都在创建可变缓存的映像。

我在每个线程上使用Threadlocal，然后每个线程只需要重置旧映像，然后在每次更新通知时从缓存中再次更新它。

来自对象池的通常可重用对象具有与之相关的线程安全成本，而此方法没有。