Learning Java, 4th Edition

Thread Groups

The ThreadGroup class allows us to deal with threads wholesale: we can use it to arrange threads in groups and deal with the groups as a whole. A thread group can contain other thread groups in addition to individual threads, so our arrangements can be hierarchical. Thread groups are particularly useful when we want to start a task that might create many threads of its own. By assigning the task a thread group, we can later identify and control all the task’s threads. Thread groups are also the subject of restrictions that can be imposed by the Java Security Manager, so we can restrict a thread’s behavior according to its thread group. For example, we can forbid threads in a particular group from interacting with threads in other groups. This is one way web browsers can prevent threads started by Java applets from stopping important system threads.

When we create a thread, it normally becomes part of the thread group to which the currently running thread belongs. To create a new thread group of our own, we can call the constructor:

    ThreadGroup
            
    myTaskGroup = new ThreadGroup("My Task Group");

The ThreadGroup constructor takes a name, which a debugger can use to help you identify the group. (You can also assign names to the threads themselves.) Once we have a group, we can put threads in the group by supplying the ThreadGroup object as an argument to the Thread constructor:

    Thread myTask = new Thread( myTaskGroup, taskPerformer );

Here, myTaskGroup is the thread group, and ...