1. Create a simple servlet:

package web.example;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/*")
public class HelloServlet extends HttpServlet 
{           
    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException 
    {
        response.getWriter().println("Hello world!");
    }
}

2. Package the application into a Web Archive (.war):

helloapp.war
+- META-INF\
+- WEB-INF\
   +- web\example\HelloServlet.class

3. Add the application to your Liberty server:

$>  mv helloapp.war $WLP_INSTALL_DIR/usr/servers/myServer/apps/

4. Configure your server.xml to know the application and enable the Servlet 3.1 technology:

$>  cat $WLP_INSTALL_DIR/usr/servers/myServer/server.xml

<server>
  <featureManager>
    <feature>servlet-3.1</feature>
  </featureManager>
  
  <application location="helloapp.war"/>    
</server>

5. Start the server:

$>  server start myServer
Starting server myServer
Server myServer started with process ID 1234.

6. Check the console.log to verify that the application started, and what URL to find it at:

$>  tail $WLP_INSTALL_DIR/usr/servers/myServer/logs/console.log
...
[AUDIT   ] CWWKT0016I: Web application available (default_host): http://localhost:9080/helloapp/
[AUDIT   ] CWWKZ0001I: Application helloapp started in 0.272 seconds.

7. In a web browser, to go the URL http://localhost:9080/helloapp/ as indicated in the console.log. You should see the message from your servlet:

Hello world!