servlets

Servlets are Java programs that run on a server and extend the capabilities of a web server by responding to requests with dynamically generated content. They are implemented using the Servlet API and execute inside a servlet container or application server such as Apache Tomcat, Jetty, GlassFish, or WildFly. The most common servlet type is HttpServlet, which handles HTTP requests.

A servlet is a Java class that usually extends HttpServlet or implements the Servlet interface. It participates in a lifecycle with methods init(), service(), and destroy(). The container creates and initializes the servlet instance, calls service() to handle each request (which dispatches to doGet, doPost, etc. in HttpServlet), and finally calls destroy() when the servlet is taken out of service. Servlets share a single instance across requests, so code must be thread-safe and rely on request and response objects for per-request data.

Configuration: URL mapping is defined via web.xml or annotations like @WebServlet. They are packaged in a WAR file and deployed to a servlet container that implements the Servlet API. Servlet containers provide features such as session management, security, and resource pooling. Advanced features include asynchronous processing using AsyncContext, and the use of filters and listeners to process requests before or after they reach a servlet.

Usage and role: In modern web applications, servlets often serve as controllers in the Model-View-Controller pattern, coordinating between models and views (such as JSPs or templates) and providing REST-style endpoints for APIs. They rely on container-managed services for authentication, authorization, and resource access. The Servlet API is part of Java EE / Jakarta EE specifications; newer versions are named jakarta.servlet.