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Exploring Java

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8.5 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 ThreadGroup can contain other ThreadGroups, in addition to 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 of the task's threads. ThreadGroups 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 thread group from interacting with threads in other thread groups. This is one way that 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 ThreadGroup that the currently running thread belongs to. To create a new ThreadGroup 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 taskPerformer is the object that performs the task; of course, it must implement Runnable. Any additional threads that myTask creates will also belong to the myTaskGroup thread group.

8.5.1 Working with the ThreadGroup

Creating thread groups isn't interesting unless you can do things to them. The ThreadGroup class exists so that you can control threads in batches. It has methods that parallel the basic Thread control methods. For example, we can stop all of the threads in a group by calling the group's stop() method. ThreadGroups also have suspend() and resume() methods that operate on all of the threads they contain. You can also mark a ThreadGroup as a "daemon"; a daemon thread group is automatically removed when all of its children are gone. If a thread group isn't a daemon, you have to call destroy() to remove it when it is empty.

We can set the maximum priority for any thread in a ThreadGroup by calling setMaximumPriority(). Thereafter, no threads can be created with a priority higher than the maximum; threads that change their priority can't set their new priority higher than the maximum.

Finally, you can get a list of all of the threads in a group. The method activeCount() tells you how many threads are in the group; the method enumerate() gives you a list of them. The argument to enumerate() is an array of Threads, which enumerate() fills in with the group's threads. (Use activeCount() to make an array of the right size.) Both activeCount() and enumerate() operate recursively on all thread groups that the group contains.

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