Java How to make sure a servlet is loaded at the application startup?

To make sure a servlet is loaded at the application startup in Java, you can specify the load-on-startup element in the servlet's web.xml deployment descriptor.

The load-on-startup element specifies the order in which the servlet should be loaded relative to other servlets that have the same element value. A positive integer indicates that the servlet should be loaded before servlets with lower load-on-startup values, and a negative integer or 0 (the default value) indicates that the servlet should be loaded after servlets with lower load-on-startup values or when the servlet is first requested.

Here is an example of how you can specify the load-on-startup element in the web.xml deployment descriptor to make sure a servlet is loaded at the application startup:

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

In this example, the MyServlet servlet is specified to be loaded at the application startup with a load-on-startup value of 1, which means that it will be loaded before servlets with lower load-on-startup values.

Keep in mind that the load-on-startup element only applies to servlets that are defined in the web.xml deployment descriptor. If you are using a Servlet 3.0 container or higher, you can also use the @WebServlet annotation to specify the loadOnStartup attribute and specify that a servlet should be loaded at the application startup. For example:

@WebServlet(name = "MyServlet", urlPatterns = {"/myservlet"}, loadOnStartup = 1)
public class MyServlet extends HttpServlet {
    // Servlet code goes here
}

In this example, the MyServlet servlet is annotated with the @WebServlet annotation and the loadOnStartup attribute is set to 1, which means that the servlet will be loaded at the application startup.