Not sure if thread is dying

Question

I have a small chat application passing sockets to seperate threads. The application is set to be a daemon thread so I can close the application that launches it.
I added small checks throughout the application to see the number of active threads.
There are two things happenning that I don't understand:
1. When I first start the application there are always 61 (including newly created daemon) threads running. I assume these are necessary background threads running in the servlet container (Tomcat 5) and the JVM.
2. When I log in with one client (creating a new socket and therefore new thread on the server's end) the thread count goes to 62. As I log in with other clients the thread count increases accordingly. The strange thing is that when I exit the client application therefore ending the run() method of that thread the first client to exit always seems to have its thread stay alive. By that I mean if I add 3 clients taking the thread count from 61 to 64, then close one client the thread count stays at 64. But then if I close another client then the thread count goes down to 63. The other strange thing is that if I close all clients the thread count remains at 62 (instead of 61 where it should be) and if I then log in another client the thread count remains at 62 but increases with every new client after that. It seems that the first client thread stays active and somehow new sockets can just replace that thread. Since none of the code is different if it's the first socket or the 20th....how can this first one not die and then even get replaced.
Sorry for the long description....I really am not that well versed in threads. If anyone could shine some light on this problem (or even offer a suggestion as how to better monitor whether the threads are dying or not) I would really appreciate it. Thanks in advance.

Answer 1

You should experiment/test a little more. Dig around until you can find out what's going on. It's not hard.
For example, try naming all of your threads. This will allow you to see which threads are yours and which belong to the system. Then, print out the names of only the threads you created, from time to time. You might do this by matching on the name, or perhaps by assigning all of your threads to a single thread group. You also might want to print a message when your thread terminates (that is, at the very bottom of its run() method), for each thread.
By the way, jconsole is a very useful JVM monitoring tool. It allows you to connect to your application and see which threads are running and what they are doing (among other things). It comes with the Sun JDK.
Jim