ThreadLocal Best Practices

• Be careful to initialize the ThreadLocal object whenever the thread starts working, or you might end up connecting a logged-off user's data to another user's thread. When you're using ThreadLocals in a situation in which a filter initializes the object with user data, make sure there are no other entries to your servlet application—except through the filter.
• Always remember that the ThreadLocal data is connected to the thread, not the user or anything else. A good idea is to delete the ThreadLocal data when it's no longer used.
• Use InheritableThreadLocal to pass parent ThreadLocal data to child threads.

References

• Managing Data with the ThreadLocal Class
• Java Thread Local – How to Use and Code Sample
• Threading lightly, Part 3: Sometimes it's best not to share