Java Programming II (old course)

The getSession() method of HttpServletRequest is used: HttpSession session = request.getSession(); (or request.getSession(true)). It returns the current session, creating a new one if none exists.

The delegation event model is the Java AWT/Swing event-handling model in which a source component generates an event and delegates its handling to one or more registered listener objects. A listener is registered with the source (e.g. button.addActionListener(listener)) and its handler method is called when the event occurs.

A component is an object with a visual representation that can be displayed on the screen and interact with the user, such as a button, label, text field or checkbox. In AWT, java.awt.Component is the abstract base class of all such user-interface elements.

In an applet (AWT), a radio button is created with Checkbox objects that belong to the same CheckboxGroup, so only one can be selected at a time:

CheckboxGroup cbg = new CheckboxGroup();
Checkbox male = new Checkbox("Male", cbg, true);
Checkbox female = new Checkbox("Female", cbg, false);
add(male);
add(female);

(In Swing, JRadioButton objects are grouped using ButtonGroup.)

String[] columns = {"ID", "Name", "Age"};
Object[][] data = {{1, "Ram", 20}, {2, "Sita", 21}};
JTable table = new JTable(data, columns);

For MySQL:

Connection con = DriverManager.getConnection(
    "jdbc:mysql://localhost:3306/studentdb", "root", "password");

The driver is loaded beforehand with Class.forName("com.mysql.cj.jdbc.Driver");.

A Document Type Definition (DTD) defines the structure and the legal elements and attributes of an XML document. It is used to validate that an XML document follows the agreed rules, so that different applications can exchange data in a common format.

The Servlet API (packages javax.servlet and javax.servlet.http, or jakarta.servlet in newer versions) is needed to compile a servlet. Its classes come in the servlet-api.jar supplied with Tomcat, which must be on the classpath: javac -cp servlet-api.jar MyServlet.java.

MouseListener (package java.awt.event) is used to receive and handle mouse events on a component. It declares five methods: mouseClicked(), mousePressed(), mouseReleased(), mouseEntered() and mouseExited(). A class implements it and is registered with addMouseListener().

ServletException (javax.servlet.ServletException), thrown when a general servlet problem occurs, and UnavailableException (a subclass of ServletException), thrown when a servlet is temporarily or permanently unavailable. Servlet methods such as doGet() also declare IOException.

A session keeps data about a user across several requests. The servlet below counts how many times the same user has visited, storing the count in the HttpSession.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class SessionDemo extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        HttpSession session = request.getSession(true);   // create or get session
        Integer count = (Integer) session.getAttribute("visits");
        if (count == null) {
            count = 1;
        } else {
            count = count + 1;
        }
        session.setAttribute("visits", count);             // store state

        out.println("<html><body>");
        out.println("<h3>Session ID: " + session.getId() + "</h3>");
        out.println("<h3>You have visited this page " + count + " time(s).</h3>");
        out.println("</body></html>");
    }
}

Mapping in web.xml:

<servlet>
    <servlet-name>SessionDemo</servlet-name>
    <servlet-class>SessionDemo</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>SessionDemo</servlet-name>
    <url-pattern>/session</url-pattern>
</servlet-mapping>

Each reload by the same browser increases the count, because the session ID (cookie JSESSIONID) identifies the stored state. session.invalidate() ends the session.

A Swing program with radio buttons: the user selects one option and the choice is displayed.

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class OptionDemo extends JFrame implements ActionListener {
    JRadioButton r1, r2, r3;
    JLabel result;

    OptionDemo() {
        setLayout(new FlowLayout());
        r1 = new JRadioButton("Java");
        r2 = new JRadioButton("Python");
        r3 = new JRadioButton("C++");
        ButtonGroup bg = new ButtonGroup();      // only one can be selected
        bg.add(r1); bg.add(r2); bg.add(r3);
        r1.addActionListener(this);
        r2.addActionListener(this);
        r3.addActionListener(this);
        result = new JLabel("No option selected");
        add(r1); add(r2); add(r3); add(result);
        setSize(300, 120);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setVisible(true);
    }

    public void actionPerformed(ActionEvent e) {
        result.setText("You selected: " + e.getActionCommand());
    }

    public static void main(String[] args) {
        new OptionDemo();
    }
}

Assumption: MySQL database college with table student(id INT, name VARCHAR(50), address VARCHAR(50)).

import java.sql.*;

public class DisplayRecords {
    public static void main(String[] args) {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            Connection con = DriverManager.getConnection(
                "jdbc:mysql://localhost:3306/college", "root", "password");
            Statement st = con.createStatement();
            ResultSet rs = st.executeQuery("SELECT * FROM student");

            System.out.println("ID\tName\tAddress");
            while (rs.next()) {
                System.out.println(rs.getInt("id") + "\t"
                    + rs.getString("name") + "\t" + rs.getString("address"));
            }
            rs.close();
            st.close();
            con.close();
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }
}

Steps: load the driver, open a connection, create a Statement, run the query to get a ResultSet, loop with rs.next() to read each row, then close the resources.

The servlet reads an integer from a request parameter n (e.g. /fact?n=5) and prints its factorial.

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class FactorialServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        try {
            int n = Integer.parseInt(request.getParameter("n"));
            if (n < 0) {
                out.println("<h3>Factorial is not defined for negative numbers.</h3>");
            } else {
                long fact = 1;
                for (int i = 2; i <= n; i++) {
                    fact *= i;
                }
                out.println("<html><body>");
                out.println("<h3>Factorial of " + n + " = " + fact + "</h3>");
                out.println("</body></html>");
            }
        } catch (NumberFormatException e) {
            out.println("<h3>Please supply an integer as parameter n.</h3>");
        }
    }
}

For n=5 the output is Factorial of 5 = 120. (long is accurate up to $n=20$.)

index.jsp:

<%@ page language="java" contentType="text/html; charset=UTF-8" %>
<html>
<head>
    <title>Tomcat</title>
</head>
<body>
    <h2><% out.println("Apache Tomcat"); %></h2>
</body>
</html>

Place it in the web application folder (e.g. webapps/myapp/), start Tomcat and open http://localhost:8080/myapp/index.jsp. The page displays Apache Tomcat. The scriptlet <% ... %> runs Java code; the expression <%= "Apache Tomcat" %> would give the same result.

Difference between JSP and Servlet

Use of web.xml

web.xml is the deployment descriptor of a web application, kept in WEB-INF/. It tells the container how to deploy the app: it maps URLs to servlets, holds initialization parameters, listeners, filters, welcome files, error pages and session timeout.

Example:

<web-app>
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>HelloServlet</servlet-class>
        <init-param>
            <param-name>college</param-name>
            <param-value>ABC College</param-value>
        </init-param>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
    <session-config>
        <session-timeout>30</session-timeout>
    </session-config>
</web-app>

A request to /hello is handled by HelloServlet.

Difference between Applet and Swing

BorderLayout manager

BorderLayout divides a container into five regions: NORTH, SOUTH, EAST, WEST and CENTER. Each region holds at most one component. NORTH and SOUTH stretch across the width, EAST and WEST take the height between them, and CENTER takes all the remaining space. It is the default layout of JFrame and Frame. If no region is given the component goes to CENTER.

JFrame f = new JFrame("BorderLayout");
f.setLayout(new BorderLayout());
f.add(new JButton("North"), BorderLayout.NORTH);
f.add(new JButton("South"), BorderLayout.SOUTH);
f.add(new JButton("East"), BorderLayout.EAST);
f.add(new JButton("West"), BorderLayout.WEST);
f.add(new JButton("Center"), BorderLayout.CENTER);
f.setSize(300, 200);
f.setVisible(true);