When and how should I use a ThreadLocal variable?

Question

When should I use a ThreadLocal variable?

How is it used?

Answer 1

As was mentioned by @unknown (google), it's usage is to define a global variable in which the value referenced can be unique in each thread. It's usages typically entails storing some sort of contextual information that is linked to the current thread of execution.

We use it in a Java EE environment to pass user identity to classes that are not Java EE aware (don't have access to HttpSession, or the EJB SessionContext). This way the code, which makes usage of identity for security based operations, can access the identity from anywhere, without having to explicitly pass it in every method call.

The request/response cycle of operations in most Java EE calls makes this type of usage easy since it gives well defined entry and exit points to set and unset the ThreadLocal.