Thread safety

[Rajesh Pathak]

Following is a question from the JR mock test itself.
Which of the following are considered ThreadSafe?
class variables, request attributes, context attributes, local variables.
My perception of thread safety is, that variable/attribute which cannot be acted upon by two threads simultaneously...
Can anyone throw some light on its answer and explanations.
Thanks.

[Anthony Villanueva]

There is only one servlet instance per registered servlet. Normally, when concurrent requests are handled by the web server, threads are spawned which executes the service() method of the servlet. Instance variables are not thread safe, but local variables within the service() method are. The request object is also "localized" per thread.
According to the Servlet specs, when a servlet implements the SingleThreadModel interface (which is an empty "tag" interface) and concurrent requests are handled by the web server, the container may create a new servlet instance per request. It is possible that the container will pool these servlet instances. it is also possible that the container will serialize, i.e. queue, the requests per instance. It is possible that the container implementation is a combination of these.
Since there are now multiple servlet instances handling each request, the instance variables are now thread safe.
In either case, however, if any servlet static variables are defined, these will NOT be thread safe unless you explicitly make it so with custom code. The same argument goes for any external resources like context attributes.

[Bruce Jin]

I think request attributes and local variables are thread safe.

[Rajesh Pathak]

Thanks for the info.