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Java 101: Understanding Java threads, Part 4: Thread groups, volatility, and thread-local variables
Final concepts for improving Java application programming with Java threads
Page 5 of 8
A about to wait.
B about to wait.
A interrupted.
A terminating.
B interrupted.
B terminating.
A is interrupted and terminates before B is interrupted and terminates.
You'll occasionally want to enumerate all threads or subgroups that comprise a thread group. The next section explores that activity.
Enumerate threads and subgroups
Part 1 introduced you to Thread's activeCount() and enumerate(Thread [] thdarray) methods. activeCount() calls ThreadGroup's int activeCount() method via the current thread's group reference to return an estimate of the active threads in the current thread's group
and subgroups. enumerate(Thread [] thdarray) calls ThreadGroup's int enumerate(Thread [] thdarray) method via the current thread's group reference. enumerate(Thread [] thdarray) is just one of four enumeration methods in ThreadGroup:
int enumerate(Thread [] thdarray)copies intothdarrayreferences to every active thread in the current thread group and all subgroups.int enumerate(Thread [] thdarray, boolean recurse)copies intothdarrayreferences to every active thread in the current thread group only ifrecurseis false. Otherwise, this method includes active threads from subgroups.int enumerate(ThreadGroup [] tgarray)copies intotgarrayreferences to every active subgroup in the current thread group.int enumerate(ThreadGroup [] tgarray, boolean recurse)copies intotgarrayreferences to every active subgroup in the current thread group only ifrecurseis false. Otherwise, this method includes all active subgroups of active subgroups, active subgroups of active subgroups of active subgroups, and so on.
You can use ThreadGroup's activeCount and enumerate(Thread [] thdarray) methods to enumerate all program threads. First, you find the system thread group. Then you call ThreadGroup's activeCount() method to retrieve an active thread count for array-sizing purposes. Next, you call ThreadGroup's enumerate(Thread [] thdarray) method to populate that array with Thread references, as Listing 4 demonstrates:
Listing 4. EnumThreads.java
// EnumThreads.java
class EnumThreads
{
public static void main (String [] args)
{
// Find system thread group
ThreadGroup system = null;
ThreadGroup tg = Thread.currentThread ().getThreadGroup ();
while (tg != null)
{
system = tg;
tg = tg.getParent ();
}
// Display a list of all system and application threads, and their
// daemon status
if (system != null)
{
Thread [] thds = new Thread [system.activeCount ()];
int nthds = system.enumerate (thds);
for (int i = 0; i < nthds; i++)
System.out.println (thds [i] + " " + thds [i].isDaemon ());
}
}
}
When run, EnumThreads produces the following output (on my platform):
Thread[Reference Handler,10,system] true
Thread[Finalizer,8,system] true
Thread[Signal Dispatcher,10,system] true
Thread[CompileThread0,10,system] true
Thread[main,5,main] false
Apart from a main thread, all other threads belong to the system thread group.
Tip: You can easily determine a thread group's parent group by calling ThreadGroup's ThreadGroup getParent() method. For all thread groups, save system, this method returns a nonnull reference. For system, this method returns null. You can also find out if a thread group is the parent, grandparent, and so forth of another thread
group by calling ThreadGroup's boolean parentOf(ThreadGroup tg) method. That method returns true if a thread, whose reference you use to call parentOf(ThreadGroup tg), is a parent (or other ancestor) of the group that tg references—or is the same group as the tg-referenced group. Otherwise, the method returns false.
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More from JavaWorld
Resources
- Download this article's source code and resource files.
- Read every article in the Understanding Java threads series (Jeff Friesen, JavaWorld.com, 2002):
- For a glossary specific to this article, homework, tips, and more, see the Java 101 study guide that accompanies this article.
- For more information about exceptions and the run() method, read "Handling Uncaught Exceptions" ( Glen McCluskey, Java Developer Connection, January 2001).
- The Programming Java Threads in the Real World series uses common programming scenarios to introduce readers to programming Java threads (Alan Holub, JavaWorld.com, 1998-2001):
- Part 1: A Java programmer's guide to threading architectures
- Part 2: The perils of race conditions, deadlock, and other threading problems
- Part 3: Roll-your-own mutexes and centralized lock management
- Part 4: Condition variables and counting semaphores—filling in a few chinks in Java's threading model)
- Part 5: Has Sun abandoned run anywhere? PlusThreads and Swing, timers, and getting around stop(), suspend(), and resume() deprecation)
- Part 6: The Observer pattern and mysteries of the AWTEventMulticaster
- Part 7: Singletons, critical sections, and reader/writer locks
- Part 8: Threads in an object-oriented world, thread pools, implementing socket accept loops