Question

Why is javax.servlet.SingleThreadModel deprecated?

Answer 1

The javadoc says why. SingleThreadModel was designed to be an easy solution to low-load concurrency, but it didn’t even manage that:

Note that SingleThreadModel does not
solve all thread safety issues. For
example, session attributes and static
variables can still be accessed by
multiple requests on multiple threads
at the same time, even when
SingleThreadModel servlets are used.
It is recommended that a developer
take other means to resolve those
issues instead of implementing this
interface, such as avoiding the usage
of an instance variable or
synchronizing the block of the code
accessing those resources.

If it can’t achieve what it was designed for, it should not be used.