知道ThreadGroup在Java中有用吗？

Question

The Java Programming Language uses ThreadGroup to manage the threads access rights to each other:

A thread group represents a set of threads. In addition, a thread group can also include other thread groups. The thread groups form a tree in which every thread group except the initial thread group has a parent.

A thread is allowed to access information about its own thread group, but not to access information about its thread group's parent thread group or any other thread groups.

I think I understand it, but is this useful in any way? I wonder whether revealing the current ThreadGroup structure and its elements, thus the threads and other ThreadGroup, hold any information from a debugging perspective.

Answer 1

I have found it handy although it's quite bugged. It can be useful for providing management content b/c the threads created will be added to the parent thread's ThreadGroup (and can be efficiently enumerated). It can be used to prevent new Thread spawning (but it's not documented or intended).

Unfortunately ThreadGroup has quite a few bugs, incl. destroy() method.

In short, if you are not going to write middleware, don't bother. If you need to enumerate all the threads in an application use Thread.getAllStackTraces().keySet() , the method has a security hole though but I am not discussing this part.

Edit: I missed the debugging aspect: ThreadGroups are actually useful for debugging purposes if the debugging tools allows for tree-view and grouping by the ThreadGroup (for example eclipse does so). Provided you use it properly, all your threads in the currently debugged module/mbean/etc should be in the same group. It allows easier tracking of thread leaks (compared to looking at hundred other threads)

Answer 2

ThreadGroup is generally considered a failed experiment. If your code needs to interact with the Java PlugIn or Java WebStart environments, then knowing about it is useful. Otherwise, I wouldn't really bother.

Answer 3

First, this is pretty much moot. Very few applications make use of thread groups, so application threads nearly always end up in the same thread group.

Second, I think that restriction you quoted is referring to sandboxed applications. According to other parts of the ThreadGroup javadoc, the ability to enumerate the threads in a thread thread group is controlled by the security manager.

Thirdly, if an application is sandboxed, it is always a good idea to restrict its knowledge of stuff outside of the sandbox. If not, well there's not much to hide.

Finally, if you are using a debugger it will have access to the root thread group, and hence won't be hampered by this restriction ... if it really exists. Similarly, the "main" thread will be in the root thread group.

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As @Tom Hawtin says, thread groups are regarded as a failed experiment, especially since the methods that they are designed to support (Thread.stop, Thread.destroy, etc) have been deprecated on account of their being unsafe.

Answer 4

ThreadGroups can make thread management a little easier, in a case when you must deal with large number of threads...

Btw, article explaining ThreadGroups at JavaWorld: http://www.javaworld.com/javaworld/jw-08-2002/jw-0802-java101.html