Effective Java 73 Avoid thread groups

Thread groups were originally envisioned as a mechanism for isolating applets for security purposes. They never really fulfilled this promise, and their security importance has waned to the extent that they aren't even mentioned in the standard work on the Java security model [Gong03].  

They allow you to apply certain Thread primitives to a bunch of threads at once. Several of these primitives have been deprecated, and the remainder are infrequently used.  

The API that lists the subgroups of a thread group is similarly flawed. While these problems could have been fixed with the addition of new methods, they haven't, because there is no real need: thread groups are obsolete.  

Summary

Thread groups don't provide much in the way of useful functionality, and much of the functionality they do provide is flawed. Thread groups are best viewed as an unsuccessful experiment, and you should simply ignore their existence. If you design a class that deals with logical groups of threads, you should probably use thread pool executors (Item 68).