How to implement the concurrent start of a servlet…

The following mechanisms are used for simultaneously starting a servlet with the application:

The <load-on-startup> element in web.xml:

In the deployment descriptor web.xml, for each servlet that should be loaded at application startup, the <load-on-startup> element is added.
The value of this element determines the load order of servlets. A positive integer indicates the load order (smaller numbers are loaded first). If the value is zero or negative, loading is "lazy" (upon the first request to the servlet).
If multiple servlets have the same value, the load order is not guaranteed.

<!-- web.xml -->
<web-app>
    <servlet>
        <servlet-name>MyStartupServlet</servlet-name>
        <servlet-class>com.example.MyStartupServlet</servlet-class>
        <load-on-startup>1</load-on-startup> <!-- Load at startup, first priority -->
    </servlet>
    <servlet>
        <servlet-name>AnotherStartupServlet</servlet-name>
        <servlet-class>com.example.AnotherStartupServlet</servlet-class>
        <load-on-startup>2</load-on-startup> <!-- Load at startup, after MyStartupServlet -->
    </servlet>
    <!-- ... other elements ... -->
</web-app>

@WebServlet annotation with loadOnStartup parameter:

In servlet containers supporting Servlet 3.0 and above, you can use the @WebServlet annotation directly above the servlet class.
The loadOnStartup parameter in the annotation serves the same purpose as the <load-on-startup> element in web.xml.

// com.example.MyStartupServlet.java
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(urlPatterns = "/startup", loadOnStartup = 1) // Load at startup with order 1
public class MyStartupServlet extends HttpServlet {

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        // Initialization code executed at servlet startup
        System.out.println("MyStartupServlet initialized at application startup.");
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("Hello from MyStartupServlet!");
    }
}

// com.example.AnotherStartupServlet.java
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet(urlPatterns = "/another-startup", loadOnStartup = 2) // Load at startup with order 2
public class AnotherStartupServlet extends HttpServlet {

    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);
        // Initialization code
        System.out.println("AnotherStartupServlet initialized at application startup.");
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.getWriter().println("Hello from AnotherStartupServlet!");
    }
}

When the application starts, the servlet container (e.g., Tomcat, Jetty) reads the deployment descriptor or scans annotations in servlet classes. For servlets with load-on-startup (or loadOnStartup) greater than or equal to zero, the container calls the init() method of the servlet immediately after loading and initializing the web application itself (Servlet Context). This guarantees that the servlet will be ready to operate before receiving the first request.

The main logic that should execute at application startup is implemented in the init() method of the servlet.

How to implement the concurrent start of a servlet with the application?