Java:how to pass value from class/bean to servlet

Question

hi, i am new to java, i'm having problem passing value from a class/bean (which are stored in arraylist) to servlet. any idea how can i achieve that? below is my code.

package myarraylist;

public class fypjdbClass {

String timezone;
String location;

public String getTimezone() {
    return timezone;
}
public void setTimezone(String timezone) {
    this.timezone = timezone;
}
public String getLocation() {
    return location;
}


public void setLocation(String location) {
    this.location = location;
}

public fypjdbClass() {
    super();
    ArrayList<fypjdbClass> fypjdbList = new ArrayList<fypjdbClass>();
    this.timezone = timezone;
    this.location = location;
}

public static void main(String[] args) {


    //Establish connection to MySQL database
    String connectionURL = "jdbc:mysql://localhost/fypjdb";
    Connection connection=null;
    ResultSet rs;

    try {
         // Load the database driver
        Class.forName("com.mysql.jdbc.Driver");

        // Get a Connection to the database
        connection = DriverManager.getConnection(connectionURL, "root", ""); 
        ArrayList<fypjdbClass> fypjdbList = new ArrayList<fypjdbClass>();
        //Select the data from the database
           String sql = "SELECT location,timezone FROM userclient";
        Statement s = connection.createStatement();
        //Create a PreparedStatement
        PreparedStatement stm = connection.prepareStatement(sql);
        rs = stm.executeQuery();
        //rs = s.getResultSet();
        while(rs.next()){

            fypjdbClass f = new fypjdbClass(); 

            f.setTimezone(rs.getString("timezone"));
            f.setLocation(rs.getString("location"));

            fypjdbList.add( f);

        }



        for (int j = 0; j < fypjdbList.size(); j++) {
            System.out.println(fypjdbList.get(j));

          }
        //To display the number of record in arraylist
        System.out.println("ArrayList contains " + fypjdbList.size() + " key value pair.");


        rs.close ();
        s.close ();
        }catch(Exception e){
        System.out.println("Exception is ;"+e);
        }

}
}

And this is the Servlet

package myarraylist;

public class arraylistforfypjServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

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

public static ArrayList<fypjdbClass> fypjdbList = new ArrayList<fypjdbClass>();

/**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */

protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
    //processRequest(request, response); 

    RequestDispatcher rd = request.getRequestDispatcher("/DataPage.jsp"); //You could give a relative URL, I'm just using absolute for a clear example.
    ArrayList<fypjdbClass> fypjdbList = new ArrayList<fypjdbClass>();// You can use any type of object you like here, Strings, Custom objects, in fact any object.
    request.setAttribute("fypjdbList", fypjdbList);
    rd.forward(request, response);
}

protected void doPost(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
    //processRequest(request, response);
    doGet(request,response);
}

}

i dont know if my code is right, pls advise me. thanks lot ^^

Answer 1

It looks like you are trying to load data from a database into the fypjdbList ArrayList in your servlet.

It isn't working because your servlet is not calling the database code. Your database code is in the main method of fypjdbClass; the main method is generally used by Java console or desktop applications but not in a Java servlet application.

A better way to retrieve data from the database is to create a Data Access Object (DAO). This is a Java class that contains only code to access the database. The DAO retrieves data for you, but does not store data itself (it would not contain timezone or location). The concept of a DAO is explained at http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html

Google will find you a number of tutorial on DAOs (I can't post the links here because as a new member of Stack Overflow I can only post one link!)

Writing servlets is useful when learning Java, but if you want to build a full website you will probably find it easier to use a framework like the Spring MVC (part of the Spring Framework). Spring MVC has a comprehensive step-by-step tutorial available which is very useful if you're new to Java web development.

Answer 2

You don't pass something into a servlet. You just let the servlet access something.

You should get rid of that main() method and move the database interaction code into a DAO class. I'd also give the model class (with timezone and location) a more sensitive name starting with an uppercase. So all with all, you should update the code so that it look something like the following:

Model class, the Area (name it whatever you want, as long as it makes sense), it should just represent a single entity:

public class Area {
    private String location;
    private String timezone;

    public String getLocation() { return location; }
    public String getTimezone() { return timezone; }

    public void setLocation(String location) { this.location = location; }
    public void setTimezone(String timezone) { this.timezone = timezone; }
}

Basic connection manager class, the Database, here you load the driver just once and provide a getter for the connection:

public class Database {
    private String url;
    private String username;
    private String password;

    public Database(String driver, String url, String username, String password) {
        try { 
            Class.forName(driver);
        } catch (ClassNotFoundException e) {
            throw new RuntimeException("Driver class is missing in classpath", e);
        }
        this.url = url;
        this.username = username;
        this.password = password;
    }

    public Connection getConnection() {
        return DriverManager.getConnection(url, username, password);
    }
}

DAO class, the AreaDAO, here you put all DB interaction methods:

public class AreaDAO {
    private Database database;

    public AreaDAO(Database database) {
        this.database = database;
    }

    public List<Area> list() throws SQLException {
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        List<Area> areas = new ArrayList<Area>();

        try {
            connection = database.getConnection();
            statement = connection.prepareStatement("SELECT location, timezone FROM userclient");
            resultSet = statement.executeQuery();
            while (resultSet.next()) {
                Area area = new Area();
                area.setLocation(resultSet.getString("location"));
                area.setTimezone(resultSet.getString("timezone"));
                areas.add(area);
            }
        } finally {
            if (resultSet != null) try { resultSet.close(); } catch (SQLException logOrIgnore) {}
            if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
            if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
        }

        return areas;
    }
}

Finally, in the servlet initialize the DAO once and obtain the list in the HTTP method:

public class AreaServlet extends HttpServlet {
    private AreaDAO areaDAO;

    public void init() throws ServletException {
        String driver = "com.mysql.jdbc.Driver";
        String url = "jdbc:mysql://localhost:3306/dbname";
        String username = "user";
        String password = "pass";
        Database database = new Database(driver, url, username, password);
        this.areaDAO = new AreaDAO(database);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)  throws ServletException, IOException {
        try {
            List<Area> areas = areaDAO.list();
            request.setAttribute("areas", areas);
            request.getRequestDispatcher("/WEB-INF/areas.jsp").forward(request, response);
        } catch (SQLException e) {
            throw new ServletException("Cannot retrieve areas", e);
        }
    }
}

Map this servlet on an url-pattern of /areas in web.xml so that you can invoke it by http://example.com/contextname/areas

The /WEB-INF/areas.jsp can look like something this, assuming that you want to display the areas in a table:

<table>
    <c:forEach items="${areas}" var="area">
        <tr>
            <td>${area.location}</td>
            <td>${area.timezone}</td>
        </tr>
    </c:forEach>
</table>

See also:

• Beginning and intermediate JSP/Servlet tutorials
• Advanced JSP/Servlet tutorials
• DAO tutorial (contains more advanced/flexible DAO examples)
• Hidden features of JSP/Servlet

Answer 3

Why to confuse a newbie with different patterns! @OP - Change your main() method to return data instead of void and create an instance for that class in the servlet.