Thread: Why is HttpServlet serialized

Why is HttpServlet serializable? Do we need to serialize the controllers in our web-application? Pros of it?

This question comes up every now and then, and you'll find a number of discussions on this if you search for "httpservlet serializable". It's good that you're asking this question though.

The general opinion is HttpServlet is marked Serializable to give the servlet container the option to "passivate" the servlet (remove from memory and write the servlet into the file system). It seems that this can be useful for some containers to passivate servlets before a restart. There are also suggestions that this allows the container to put servlets that are not in use into "hibernation" thereby freeing up memory, as well as to duplicate servlets instances to other containers in a clustered environment.

Personally I'm not sure how this can be useful when servlets should be thin (and so quick to be instantiated as needed, and do not take up much heap space). They should also be stateless (state information is stored in the Session object, not in the servlet) so that multiple instances of the same servlet in a cluster of servlet containers should function identically to each other, and so there's no need to duplicate servlet instances in the cluster.