What is the benefit of ThreadGroup in java over creating separate threads?

Using ThreadGroup can be a useful diagnostic technique in big application servers with thousands of threads. If your threads are logically grouped together, then when you get a stack trace you can see which group the offending thread was part of (e.g. "Tomcat threads", "MDB threads", "thread pool X", etc), which can be a big help in tracking down and fixing the problem.

Somewhat complimentary to the answer provided (6 years ago or so). But, while the Concurrency API provides a lot of constructs, the ThreadGroup might still be useful to use. It provides the following functionality:

1. Logical organisation of your threads (for diagnostic purposes).
2. You can interrupt() all the threads in the group. (Interrupting is perfectly fine, unlike suspend(), resume() and stop()).
3. You can set the maximum priority of the threads in the group. (not sure how widely useful is that, but there you have it).
4. Sets the ThreadGroup as a daemon. (So all new threads added to it will be daemon threads).
5. It allows you to override its uncaughtExceptionHandler so that if one of the threads in the group throws an Exception, you have a callback to handle it.
6. It provides you some extra tools such as getting the list of threads, how many active ones you have etc. Useful when having a group of worker threads, or some thread pool of some kind.

Don't use ThreadGroup for new code. Use the Executor stuff in java.util.concurrent instead.