java - Retrieve values from JDBC and use JSTL tags to call the methods

Question

Below is the code which i have written to retrieve values from the database (I have added the whole code so it will be easier for you to understand what i am trying to say here):

package ipscheme;

import java.sql.*;

public class AddRecords {

Connection con = new DBConnection().getConnection();
ResultSet  resultSet = null;  

public String [] populateSelect() throws SQLException {

    Statement statement = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);

    resultSet = statement.executeQuery("SELECT * FROM ipsections");

    resultSet.last();
    int size = resultSet.getRow();
    resultSet.beforeFirst();

    String [] sectionName = new String[size];

    int j = 0;
    while (resultSet.next()){
        sectionName[j] = resultSet.getString("section_name");
        j = j + 1;
    }

    resultSet.close();
    statement.close();
    con.close();

    return sectionName;

}

}

And down here is the JSP Code which i have used to populate values from the database into a selectbox.

<select name="sltSection">
 <% 
    AddRecords added = new AddRecords();
    String sectionNm  [] = added.populateSelect();
        for(int i=0; i<sectionNm.length; i++){ %>   
    <option>
        <% out.print(sectionNm[i]); %>
    </option>
 <% } %>
</select>

The above code works fine without any compilation errors. As you can see i have used Java code in my .jsp page to fulfill the task, which is a wrong thing to do.

I need to implement this code according to the MVC and therefore i should use JSTL tags to implement the code to .jsp? if so how do i do that? How do i instantiate Object from the class AddRecords and call its methods? Can someone please help me. Thanks in advance!

Answer 1

Since it looks you're new to Java Web Programming, you can implement a full MVC scenario using JSP (View), Servlet (Controller) and Entities, DAOs and Service layer (Model). This is what you have:

Model:

• DBConnection as a database access class (Data access layer)
• AddRecords as your DAO class (Data access layer)

• No class as Service class (Business logic layer). For this example, let's have a RecordService class for this:

  public class RecordService {
      public RecordService() {
      }
      //since it has no real business logic, it will serve as facade
      public String[] getRecords() {
          AddRecords addRecords = new AddRecords();
          return addRecords.populateSelect();
      }
  }
  

Controller:

• No class for Controller yet. Since I assume you're using plain Java EE, you would use a Servlet (implementation based on Servlets StackOverflow wiki):

  @WebServlet("/RecordServlet")
  public class RecordsServlet extends HttpServlet {
      @Override
      protected void doGet(HttpServletRequest request, HttpServletResponse response)
          throws ServletException, IOException {
          //it will fire on a GET request (like accessing directly to the URL
          //from the browser)
          //here you should load the data for the View (JSP)
          loadData(request);
          //forward to show the view
          request.getRequestDispatcher("hello.jsp").forward(request, response);
      }
  
      @Override
      protected void doPost(HttpServletRequest request, HttpServletResponse response)
          throws ServletException, IOException {
          //it will fire on a POST request (like submitting the form using POST method)
          String selectedRecord = request.getParameter("selectedRecord");
          System.out.println(selectedRecord);
          request.setAttribute("selectedRecord", selectedRecord);
          loadData(request);
          //forward to show the view
          request.getRequestDispatcher("hello.jsp").forward(request, response);
      }
  
      //method to load the data available to select
      private void loadData(HttpServletRequest request) {
          RecordService recordService = new RecordService();
          String[] records = recordService.getRecords();
          //set the data as attribute on the request to be available on the View
          request.setAttribute("records", records);
      }
  }
  

View:

• You already have a JSP file. Since you haven't posted the name, let's call it hello.jsp. I will just adap your actual JSP code using JSTL:

  <!-- at the beginning of the JSP, call the JSTL library -->
  <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
  
  <form action="RecordServlet" method="POST">
      Please select an element:
      <select id="selectedRecord" name="selectedRecord">
          <c:forEach items="${records}" var="record">
          <option value="${record}">${record}</option>
          </c:forEach>
      </select>
      <br />
      <input type="submit" value="Show selected record" />
      <c:if test="${not empty selectedRecord}">
          <br />
          You've selected ${selectedRecord}!
      </c:if>
  </form>
  

Now, to run the example, build the project and access to it using http://localhost:8080/YourWebProjectName/RecordServlet. Also, you can set in the web.xml file:

<welcome-file-list>
    <welcome-file>RecordServlet</welcome-file>
</welcome-file-list>

And just run the web project.

Some notes:

• Use relevant names for the class/methods, I used RecordXXX since you had this AddRecord class (it should be RecordDAO or something more useful to code readers like UserDAO or YourEntityDAO).
• Distribute your classes through different packages. Every package should contain only relevant classes to its scope i.e. edu.home.dao for DAO classes, edu.home.service for service/business logic classes, and on...