JavaServer Pages, 3rd Edition

Servlet Containers

A servlet container is the connection between a web server and the servlets. It provides the runtime environment for all the servlets on the server as defined by the servlet specification, and is responsible for loading and invoking those servlets when the time is right. The container typically loads a servlet class when it receives the first request for the servlet, gives it a chance to initialize itself, and then asks it to process the request. Subsequent requests use the same, initialized servlet until the server is shut down. The container then gives the servlet a chance to release resources and save its state (for instance, information accumulated during its lifetime).

There are many different types of servlet containers. Some containers are called add-ons , or plug-ins , and are used to add servlet support to web servers without native servlet support (such as Apache and IIS). They can run in the same operating-system process as the web server or in a separate process. Other containers are standalone servers. A standalone server includes web server functionality to provide full support for HTTP in addition to the servlet runtime environment. Containers can also be embedded in other servers, such as a climate-control system, to offer a web-based interface to the system. A container bundled as part of an application server can distribute the execution of servlets over multiple hosts. The server can balance the load evenly over all containers, and some servers ...