The Servlet API From Zero

Learn the foundation under every Java web framework: what a servlet is, the servlet container and its thread-per-request lifecycle, handling HTTP by hand with HttpServlet, URL mapping and the front-controller pattern, filters and the filter chain, and sessions. Spring's DispatcherServlet, JAX-RS, and middleware all live on top of this — see it bare.

1. What a Servlet Is A servlet is a Java object that handles an HTTP request and produces a response — the base unit every Java web framework is built on. Here's the mental model and a first one.
2. The Servlet Container & Lifecycle How the container runs a servlet: init/service/destroy, one shared instance serving every request on its own thread, and why that shared instance is exactly why your servlets (and Spring controllers) must be stateless.
3. Handling Requests with HttpServlet Override doGet/doPost on HttpServlet, read params, headers, and the raw body from the request, write status, headers, and JSON to the response — the unglamorous truth beneath @GetMapping.
4. Mapping & the Front-Controller Pattern How the container decides which servlet handles which URL, why one-servlet-per-URL doesn't scale, and the front-controller pattern — one servlet routing everything — that is exactly what Spring's DispatcherServlet does.
5. Filters & the Chain A filter intercepts every request before your servlet and the response after, without the servlet knowing. Chained together, filters are the root of what every framework calls middleware.
6. Sessions & State HTTP forgets you between requests, yet apps keep you logged in. Here's the mechanism: a session id in a cookie, server-side state behind it — and the stateless token alternative.
7. From Servlets to Frameworks The payoff: see the servlet under every Java web framework. DispatcherServlet, @GetMapping, Spring Security, middleware, and JAX-RS all map back to the lifecycle, front controller, and filter chain you now know.