Java中的活动线程组是什么？

Question

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?

Answer 1

In Java there is a group abstraction around a group of threads, so it is easier to manage a group of threads. See eg Java: Thread Group

Every Java thread is a member of a thread group. Thread groups provide a mechanism for collecting multiple threads into a single object and manipulating those threads all at once, rather than individually.

For example, you can start or suspend all the threads within a group with a single method call. Java thread groups are implemented by the ThreadGroup(in the API reference documentation) class in the java.lang package.

Answer 2

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.

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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.