How to use JSP on Servlet doPOst?

Question

I just started to use JSP and Servlet and i am trying to make a controller for a device. A simple page with 4 buttons. Each time when one of the buttons is clicked, the user should see something displayed(according with the pressed button). I am trying to do that by using a Servlet and JSP. I am not sure if this is a good approach, but the problem is that i am not doing this well, because when i run my application, i can see the 4 buttons, but on click them nothing happens. What i am doing wrong? I would appreciate some suggestions for another approach.

MainServlet

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

/**
 * Servlet implementation class CheckTemperature
 */
@WebServlet("/CheckTemperature")
public class MainServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;


    /**
     * @see HttpServlet#HttpServlet()
     */
    public MainServlet() {
        super();      
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html>");
        out.println("<head><title>Check tempetaure</title></head>");
        out.println("<body>");
        out.println("<input type=\"submit\" value=\"Check temperature\" name=\"button\"/>");
        out.println("<input type=\"submit\" value=\"Get AC state\" name=\"button\"/>");
        out.println("<input type=\"submit\" value=\"Turn ON AC\" name=\"button\"/>");
        out.println("<input type=\"submit\" value=\"Turn OFF AC\" name=\"button\"/>");
        out.println("</body>");
        out.println("</html>");     
    }
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        String button = request.getParameter("button");
        if(button.equalsIgnoreCase("Check temperature")){
            request.getRequestDispatcher("/WEB-INF/jsps/checkTemperature.jsp").include(request, response);
        } else if(button.equalsIgnoreCase("Get AC state")){
            request.getRequestDispatcher("/WEB-INF/jsps/acState.jsp").include(request, response);
        } else if(button.equalsIgnoreCase("Turn ON AC")){
            request.getRequestDispatcher("/WEB-INF/jsps/turnOn.jsp").include(request, response);
        } else if(button.equalsIgnoreCase("Turn OFF AC")){
            request.getRequestDispatcher("/WEB-INF/jsps/turnOff.jsp").include(request, response);
        }           
    }    
}

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>ArduinoController</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
    <description></description>
    <display-name>ACController</display-name>
    <servlet-name>MainServlet</servlet-name>
    <servlet-class>ro.dnad.controller.MainServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>MainServlet</servlet-name>
    <url-pattern>/main</url-pattern>
  </servlet-mapping>
</web-app>

checkTemperature.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<%@ page import="ro.dnad.controller.EthernetACDriver" %>

<%EthernetACDriver driver = new EthernetACDriver("localhost",8080); %>
<%double temp = driver.getTemperature() * 5; %>
<%=temp%>

acState.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<%@ page import="ro.dnad.controller.EthernetACDriver" %>

<%EthernetACDriver driver = new EthernetACDriver("localhost",8080); %>
<%=driver.ACStatus()%>

turnOff.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<%@ page import="ro.dnad.controller.EthernetACDriver" %>

<%EthernetACDriver driver = new EthernetACDriver("localhost",8080); %>
<%driver.sendMessage(3); %>
<%=driver.ACStatus()%>

turnOfn.jsp

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
pageEncoding="ISO-8859-1"%>
<%@ page import="ro.dnad.controller.EthernetACDriver" %>

<%EthernetACDriver driver = new EthernetACDriver("localhost",8080); %>
<%driver.sendMessage(2); %>
<%=driver.ACStatus()%>

score 1 · Accepted Answer · edited Apr 09 '14 at 14:22

First of all rather than using servlet for creating dynamic content, you should use JSP
For working your code you need to add each submit but inside the form. Then only submit button will work.

Code sample:

 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<html>");
        out.println("<head><title>Check tempetaure</title></head>");
        out.println("<body>");
        out.println("<form action='CheckTemperature' method='post'>");

        out.println("<input type=\"submit\" value=\"Check temperature\" name=\"button\"/>");
        out.println("<form />");
        out.println("<input type=\"submit\" value=\"Get AC state\" name=\"button\"/>");
        out.println("<input type=\"submit\" value=\"Turn ON AC\" name=\"button\"/>");
        out.println("<input type=\"submit\" value=\"Turn OFF AC\" name=\"button\"/>");
        out.println("</body>");
        out.println("</html>");     
    }

One more thing: do not generate the whole HTML source directly in servlet, instead forward to another page. — Luiggi Mendoza, Apr 09 '14 at 14:22
@LuiggiMendoza Thank you for your answer. I will try to create a JSP instead of generating the html source from servlet. — laura, Apr 09 '14 at 15:14

Abhishek Nayak · Answer 2 · 2014-04-09T17:20:26.900

when i run my application, i can see the 4 buttons, but on click them nothing happens

because you have not provided any action to perform nor the buttons are inside form.

Also you have mapped your Servlet two times :

using @WebServlet("/CheckTemperature")
and using url-pattern in web.xml, choose one.

you learning in old way, i.e.

Avoid scriptlets(<% %>, <%= %>) instead use JSTL or other tag library's.
Instead of writing Java codes in jsp, use Model/Service class for keeping bussiness logic's separate from view. then you can easily DEBUG your code, while not in jsp.
don't hard code to generate html codes in servlets. instead create static HTML/jsp page .with a form by giving action value as Servlet url-pattern and method type as GET/POST, then after filling details submit the from, handle the form in servlet then, finally forward response to jsp/html page.

Solution:

Servlet:

/**
* Servlet implementation class CheckTemperature
*/
@WebServlet("/CheckTemperature")
public class MainServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

  /**
   * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
   */
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
      PrintWriter out = response.getWriter();
      String button = request.getParameter("button");

      EthernetACDriver driver = new EthernetACDriver("localhost",8080);

      if(button.equalsIgnoreCase("Check temperature")){
         double temp = driver.getTemperature() * 5;
         request.setAttribute("temp", temp);// add temperature value to request
         RequestDispatcher dispatcher =
                 request.getRequestDispatcher("/WEB-INF/jsps/checkTemperature.jsp");
         dispatcher.forward(request,response);
      } else if(button.equalsIgnoreCase("Get AC state")){
         request.setAttribute("acStatus", driver.ACStatus()); // add AC status value to request
         RequestDispatcher dispatcher =
                 request.getRequestDispatcher("/WEB-INF/jsps/acState.jsp");          
         dispatcher.forward(request,response);
      } else if(button.equalsIgnoreCase("Turn ON AC")){
         driver.sendMessage(3);
         request.setAttribute("acTurnOnMsg", driver.ACStatus());// add AC turn on status value to request        
         RequestDispatcher dispatcher =
                 request.getRequestDispatcher("/WEB-INF/jsps/turnOn.jsp");                
         dispatcher.forward(request,response);
      } else if(button.equalsIgnoreCase("Turn OFF AC")){
         driver.sendMessage(3);
         request.setAttribute("acTurnOffMsg", driver.ACStatus()); //AC turn off status value to request
         RequestDispatcher dispatcher =
                 request.getRequestDispatcher("/WEB-INF/jsps/turnOff.jsp");      
         dispatcher.forward(request,response);
      }           
  }    
}

and in JSP use expression like: add

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

in top of jsp.

then

checkTemperature.jsp:

${temp} // get temperature value from request.

acState.jsp:

${acStatus} //get AC status value from request

turnOff.jsp:

${acTurnOnMsg} //get AC turn on status value from request

turnOfn.jsp:

${acTurnOffMsg} //get AC turn on status value from request

EDIT: JSP:

<c:url value="/CheckTemperature" var="postUrl"/>
<form action="${postUrl}" method="POST">
    <input type="submit" value="Check temperature" name="button"/>
    <input type="submit" value="Get AC state" name="button"/>
    <input type="submit" value="Turn ON AC" name="button"/>
    <input type="submit" value="Turn OFF AC" name="button"/>
</form>

get JSTL here

Rembo, i created a jsp file for the doGet method and update the doPost and the others jsp files as you suggested, the main page gets to be displayed, but when i click a button i get HTTP 404 error. — laura, Apr 09 '14 at 17:08
@laura no need of `url-pattern` and servlet registration in web.xml, if you have annotated with `@WebServlet`, just specify the jsp you have form element as welcome-file in web.xml. will work fine. — Abhishek Nayak, Apr 09 '14 at 17:34
I get this error now: The absolute uri: http://java.sun.com/jsp/jstl/core cannot be resolved in either web.xml or the jar files deployed with this application — laura, Apr 09 '14 at 17:46
you have forgot to add JSTL jar in CLASSPATH, add it. have a look [here](http://stackoverflow.com/questions/8701307/the-absolute-uri-http-java-sun-com-jsp-jstl-core-cannot-be-resolved-in-either) — Abhishek Nayak, Apr 09 '14 at 17:47
i have JSTL jar in the BUILDPATH and this is bad..is not in the classpath — laura, Apr 09 '14 at 18:02

How to use JSP on Servlet doPOst?

2 Answers2