Deadlock resolution.

Whenever you hit a web server / app server hang problem because of an hosted web application, you can try the following:

1. Take a thread dump of the process. You can issue "kill -3 pid". The thread dump will be available in the web server errors log file.

2. Most of the threads will be waiting to acquire a lock on particular resource say R1. There will be one thread who had acquired the lock on R1 and probably in the following state.
Doing an indefinite wait on trying to acquire a lock on another resource R2. This lock on R2 could be with another thread that is in the wait queue trying to acquire R1 (OR) R2 will never be available because of resource starvation.

3. All threads have entered into lock.wait() and no notify call to wake up these guys.

Getting threaddump of a Java Process.

Analyzing thread dump is the best way to debug a hanging process.
There are 3 ways to get it.

1. Issue kill -9 command.
For ex: If you have started a Sun Web Server process and if it is hanging because of an web-app deployed in it, you can issue command kill -9 pid where pid is the process id of the child process. Note that there will be two processes. You need to issue this on child process id.

2. Attaching jdb. Refer to the following document for more details.
http://wiki.caucho.com/Thread_dump

3. Using JHAT tool. You need to start web server with JDK 1.6 and above to use this. This can be set in the server.xml file for the variable JAVA_HOME. JHAT is a heap analyzer tool which will give you all info related to the heap of the process in the process address space.