Question

I want to create a Servlet that processes input to a serial device and for that reason I want to make sure that exactly one instance of Servlet exists in the container at a time (irrespective of whether the container makes only one instance I have to make sure of it) and also the access to the serial port is either synchronised or serialised.

Any suggestions?

Answer 1

You don’t need the servlet to be a singleton, you only need to be able to control access to the serial port. In fact even if you could enforce a single instance of the servlet class, the spec enables multiple users to access the servlet concurrently.

You could instead write a class that handles access to the port, encapsulating control by only allowing a single thread to access at a time. You’d then need to decide how you wanted concurrent requests to the servlet to behave (block, return some sort of ‘serial port in use’ error message, etc).