Servlet is like a POJO .You can create constructor in servlet. You can also use constructor for initialising purpose but it is not a useful approach as you have init() in Servlet life cycle for initialisation.

package roseindia;

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


public class ServletConstructor extends HttpServlet {

    public ServletConstructor() {
      System.out.println("Its a servlet constructor");
    }


    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    System.out.println("Hello servlet");
    }


}