Understanding the Servlet Lifecycle in Java Web Applications

Servlet lifecycle defines how a servlet is loaded, initialized, receives requests, responds, and is eventually destroyed. Before discussing the lifecycle, the key methods are introduced: init() , which runs once when the servlet is first loaded; service() , the core method invoked for each client request; and destroy() , called once when the servlet is taken out of service.

The lifecycle is controlled by the servlet container, beginning when the servlet class is loaded into memory and ending when it is unloaded or reloaded. The javax.servlet.Servlet interface defines the required methods that all Java servlets must implement.

Loading and Instantiating the Servlet

Tomcat loads servlets in three ways: automatically at server startup if configured, when a client first requests the servlet after server start, or when the servlet is reloaded.

During container startup, the web.xml configuration file is read to discover servlets. Each servlet must have a public no‑argument constructor, and the container creates an instance for each servlet marked for automatic loading.

Initialization

After instantiation, the container calls the servlet’s init() method, putting the servlet into an "initialized" state. Initialization can be deferred until the first request or forced at startup using the <load-on-startup> element in web.xml. If init() throws a ServletException, the servlet is discarded and will be garbage‑collected.

Request Processing

Once initialized, the servlet is ready to handle requests. Each request creates a ServletRequest and a ServletResponse object, and the container invokes the service() method. The service() method may be called many times, often concurrently, so servlets must be thread‑safe. Proper synchronization should be applied only where necessary to avoid degrading performance.

Multithreading can lead to data‑race problems when multiple threads access shared resources such as files. Developers should synchronize critical sections without over‑synchronizing, ensuring safe concurrent access while maintaining throughput.

Unloading the Servlet

When the container decides to remove a servlet—either because it is no longer needed or it is being reloaded—it calls the destroy() method. This method should release all resources allocated during init(). After destroy() completes, the servlet instance becomes eligible for garbage collection and cannot be used again.