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🔔 Subscribe NowWhat is Load-on-Startup in Java Servlets?
By default, a servlet is loaded and initialized by the servlet container (like Apache Tomcat) lazily—only when the first request for that servlet is received. However, using the <load-on-startup> element in your web.xml, you can instruct the container to load and initialize (call init()) the servlet automatically at the time of application deployment or server startup.
How the Load-on-Startup Order Works
The value specified inside <load-on-startup> determines the priority order in which servlets are initialized:
- Zero or Positive Integer (e.g., 0, 1, 2): Servlets are initialized in ascending order (0 first, then 1, then 2).
- Negative Integer or Not Specified: The servlet container initializes the servlet on demand when the first request arrives.
Practical Example Breakdown
In this demo, three servlets are configured in web.xml:
- HelloWorld Servlet:
<load-on-startup>0</load-on-startup>→ Initialized 1st during server startup. - ServletLifeCycle Servlet:
<load-on-startup>1</load-on-startup>→ Initialized 2nd during server startup. - CustomerInfo Servlet: No
<load-on-startup>defined → Initialized only when first requested via the form submission.
Why Use Load-on-Startup?
- Faster First Request: Eliminates initial latency for users because the servlet is already initialized in memory.
- Preloading Resources: Ideal for loading database connection pools, configuration files, and caching heavy data upfront.
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