Along with threads, locks, and monitors, a basic abstraction offered by the threading system is thread groups. Thread groups were originally envisioned as a mechanism for isolating applets for security purposes.
Given that thread groups don’t provide any security functionality to speak of, what functionality do they provide? Not much. They allow you to apply certain Thread primitives to a bunch of threads at once.
In an ironic twist, the ThreadGroup API is weak from a thread safety standpoint. To get a list of the active threads in a thread group, you must invoke the enumerate method, which takes as a parameter an array large enough to hold all the active threads. The activeCount method returns the number of active threads in a thread group, but there is no guarantee that this count will still be accurate once an array has been allocated and passed to the enumerate method. If the thread count has increased and the array is too small, the enumerate method silently ignores any threads for which there is no room in the array.
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.
Prior to release 1.5, there was one small piece of functionality that was available only with the ThreadGroup API: the ThreadGroup.uncaughtException method was the only way to gain control when a thread threw an uncaught exception. This functionality is useful, for example, to direct stack traces to an application- specific log. As of release 1.5, however, the same functionality is available with Thread’s setUncaughtExceptionHandler method.
To summarize, 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).
Reference: Effective Java 2nd Edition by Joshua Bloch