There is method java.lang.ThreadGroup.activeGroupCount() returns an estimate of the number of active groups in a thread group. In response to this question, the active thread is defined. But what does active thread group mean?
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Answer
As you noted, the terminology “active thread group” appears in the javadoc for ThreadGroup::activeGroupCount.
An active thread group is a ThreadGroup containing at least one active thread.
An active thread is one for which Thread::isAlive returns true. In other words, it has been started and has not yet terminated.
Note that thread groups are are only really suitable for debugging; see What is the benefit of ThreadGroup in java over creating separate threads?. For example, the enumerate method has this javadoc caveat:
“Due to the inherent race condition in this method, it is recommended that the method only be used for debugging and monitoring purposes.”
This also applies to the “count” methods.