Thread safe issues in Servlets

After browsing thru the book Head First Servlets and JSP , I have to say the book is quite good. Even though I have no interest in the Sun Certified Web Component Developer Exam, the way it presents the material is quite different than what I am used to.
The JavaCamp site has some things to be aware of while developing servlets. Things like sessions and global class instance files not being thread safe. This site had some information on the topic as well. Agaiin, its notes for the SCWCD exam.
The Online version of the book Java Servlet programming is a good reference. Lastly, as this article states : "thread safety is an issue you must contend with regardless of your application type or project size"

In the servlet model (multithreaded) , container maintains a thread pool (threads waiting to execute some code).  The way it work is that there is a dispatcher thread that manages these threads in the pool (worker threads).

When container receives a request for a servlet, the dispatcher thread picks picks up a worker thread from the pool, and the worker thread then executes the service() method of the servlet.

If the container receives a new request while the thread is still working, it will pick up the new thread from the pool. It does not matter if the new thread is for the same servlet, the container just does its job of executing multiple threads.

The single thread model is used for tasks such as things that must be excuted sequentially and not simultaneously.  Basically, the specification says that it will not exceute the service() routine in more than one thread at the same time. However, the container can create multiple instances of a servlet if the servlet implements the single thread model. The single thread model should be avoided if possible.

Thread safety tips:

• local variables: each thread gets its own copy of the local variables.
• instance variables: not thread safe. There is only one copy of the instance variable per instance of the servlet. Use instance vriables for variables that can be initialized at servlet startup and can be used througout life of the servlet (i.e connection variable).
• class variables: not thread safe also. Use static on variables where value remains constant.
• context scope (getServletContext()): not thread safe
• session: not thread safe also. try synchronized (session){}
• request:  thread safe. Paased as a a parm of the service() method.