Java.Servlet.What is 1 in loadOnStartup = 1 ?

The 1 in loadOnStartup = 1 (or <load-on-startup>1</load-on-startup>) is an integer priority that tells the servlet container:

👉 “Load this servlet when the application starts — and do it in this order relative to other servlets.”

🔢 What Does the Number Mean?

✅ Non-negative Integer (0, 1, 2, etc.)

• Tells the container to load the servlet eagerly, at application startup.
• Lower numbers load first.
• Multiple servlets? Order of loading is based on ascending numbers.

Value	Effect
0	Load immediately (high priority)
1	Load after 0
5	Load later

So if you have:

<load-on-startup>1</load-on-startup>  // InitServlet
<load-on-startup>2</load-on-startup>  // MetricsServlet

Then InitServlet is initialized before MetricsServlet.

❌ Negative or Absent Value

• Servlet is loaded lazily — only when it receives its first request.
• This is the default behavior if load-on-startup is not specified.

🚀 Use Cases for loadOnStartup

Scenario	Why Set Priority?
DB connection pool initialization	Ensure DB is ready before user requests
Logging system bootstrapping	Start logs early
Caching data	Preload and warm up cache
Background schedulers	Start worker threads early

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✅ Example with Annotation

@WebServlet(urlPatterns = "/init", loadOnStartup = 1)
public class InitServlet extends HttpServlet {
    @Override
    public void init() throws ServletException {
        System.out.println("🚀 InitServlet started at app startup!");
    }
}