Servlets are usually used for more complex database applications.
The simple Hello World app below doesn't use a database.

Many application will use servlets instead of JSP and others will use servlets in conjunction with JSP. Experienced JSP programmers use servlets in conjunction with JSP to create clearer and simpler applications. The servlets handle Java processing: form handing, calculation and database queries. JSP formats the results.

Using Servlets, you can collect input from users through web page forms, present records from a database or another source, and create web pages dynamically.

Apache Tomcat is a popular tool for developing server side apps. It is an open source implementation of several Java EE (Enterprise Edition) specifications, the Java Servlet, JavaServer Pages, Java Expression Language and Java WebSocket technologies.

See Server-Side Development on Mac OS X: Servlets and Tomcat - Developer.com

// Import required java libraries - I haven't tested this
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
// Extend HttpServlet class
public class HelloServlet extends HttpServlet {
 
  private String message;
  public void init() throws ServletException
  {
      // Do required initialization
      message = "Hello World";
  }
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
            throws ServletException, IOException
  {
      // Set response content type
      response.setContentType("text/html");
     // Actual logic goes here.
      PrintWriter out = response.getWriter();
      out.println("<h1>" + message + "</h1>");
  }
  
  public void destroy()
  {
      // do nothing.
  }
}