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Exam Notes for the Sun Certified Web Component Developer (SCWCD) Exam

7.0 - Designing and Developing Thread-safe Servlets

7.1. Identify which attribute scopes are thread-safe:

7.1.1. local variables: yes, thread-safe
7.1.2. instance variables: not thread-safe, since a single servlet instance may be handling multiple service requests at any given time
7.1.3. class variables: not thread-safe, since multiple servlets and/or service requests may try to access a class variable concurrently
7.1.4. request attributes: yes, thread-safe, since the request object is a local variable
7.1.5. session attributes: not thread-safe, since sessions are scoped at the web application level, hence the same session object can be accessed concurrently by multiple servlets and their service requests
7.1.6. context attributes: not thread-safe, since the same context object can be accessed concurrently by multiple servlets and their service requests

7.2. Identify correct statements about differences between multi-threaded and single-threaded servlet models

7.2.1. multi-thread model servlet:
7.2.1.1. one servlet instance per registered name
7.2.1.2. for each servlet request, the server spawns a separate thread which executes the servlet's service() method
7.2.1.3. must synchronize access to instance variables
7.2.2. single-thread model servlet:
7.2.2.1. has a pool of servlet instances per registered name (depending on the server implementation, the pool size may be configurable or not, and may be as little as one.)
7.2.2.2. guaranteed by server "that no two threads will execute concurrently in the servlet's service method"
7.2.2.3. considered thread-safe and isn't required to synchronize access to instance variables
7.2.2.4. does not prevent synchronization problems that result from servlets accessing shared resources such as static variables or objects outside the scope of the servlet (e.g. ServletContext, HttpSession)
7.2.2.5. server might end up creating more instances than the system can handle, e.g. each instance might have its own db connection, hence in total there may be more db connections than the db server can handle.

7.3. Identify the interface used to declare that a servlet must use the single thread model

7.3.1. interface javax.servlet.SingleThreadModel { // this is an empty "tag" interface }