The Servlet Lifecycle

The Servlet Lifecycle

Servlets are not created fresh for every request. The container manages a pool of servlet instances and controls their entire lifecycle from creation to destruction.

Three Key Phases

The lifecycle has three distinct phases: initialization, service, and destruction.

Initialization: When the container starts (or when the first request arrives, depending on configuration), it loads the servlet class, creates a single instance, and calls init(). This happens exactly once per servlet.

@Override
public void init() throws ServletException {
    // One-time setup: load config, open resources
    String dbUrl = getServletConfig().getInitParameter("dbUrl");
    this.dataSource = DataSourceFactory.create(dbUrl);
}

Service: For every incoming request, the container calls service(). The default service() method in HttpServlet inspects the HTTP method and dispatches to doGet, doPost, doPut, or doDelete.

Destruction: When the container shuts down, it calls destroy(). This is your chance to release resources - close database connections, stop background threads, flush caches.

@Override
public void destroy() {
    // Clean up resources
    if (dataSource != null) {
        dataSource.close();
    }
}

Threading Model

The container creates ONE instance of each servlet but handles concurrent requests by spawning threads. This means your servlet code must be thread-safe. Avoid mutable instance variables unless you synchronize access to them.

load-on-startup

By default, servlets are lazy-loaded on the first request. Set load-on-startup to a positive integer to make the container initialize them eagerly at deploy time:

<servlet>
    <servlet-name>init</servlet-name>
    <servlet-class>com.example.InitServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>

Lower numbers load first. This is useful for servlets that warm up caches or validate configuration on startup.

Key Points

• init() runs once when the servlet is first created.
• service() dispatches to doGet/doPost/doPut/doDelete for each request.
• destroy() runs once when the container shuts down.
• One servlet instance serves many threads - code must be thread-safe.
• Use load-on-startup to initialize critical servlets eagerly.