How do I create zip file in Servlet for download?

The example below is a servlet that shows you how to create a zip file and send the generated zip file for user to download. The compressing process is done by the zipFiles method of this class.

For a servlet to work you need to configure it in the web.xml file of your web application which can be found after the code snippet below.

package org.kodejava.example.servlet;

import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;
import java.util.zip.ZipEntry;
import java.util.zip.ZipOutputStream;

public class ZipDownloadServlet extends HttpServlet {
    public static final String FILE_SEPARATOR = System.getProperty("file.separator");

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

    protected void doGet(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        try {
            // The path below is the root directory of data to be
            // compressed.
            String path = getServletContext().getRealPath("data");

            File directory = new File(path);
            String[] files = directory.list();

            // Checks to see if the directory contains some files.
            if (files != null && files.length > 0) {

                // Call the zipFiles method for creating a zip stream.
                byte[] zip = zipFiles(directory, files);

                // Sends the response back to the user / browser. The
                // content for zip file type is "application/zip". We
                // also set the content disposition as attachment for
                // the browser to show a dialog that will let user 
                // choose what action will he do to the sent content.
                ServletOutputStream sos = response.getOutputStream();
                response.setContentType("application/zip");
                response.setHeader("Content-Disposition", "attachment; filename="DATA.ZIP"");

                sos.write(zip);
                sos.flush();
            }
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * Compress the given directory with all its files.
     */
    private byte[] zipFiles(File directory, String[] files) throws IOException {
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ZipOutputStream zos = new ZipOutputStream(baos);
        byte bytes[] = new byte[2048];

        for (String fileName : files) {
            FileInputStream fis = new FileInputStream(directory.getPath() + 
                ZipDownloadServlet.FILE_SEPARATOR + fileName);
            BufferedInputStream bis = new BufferedInputStream(fis);

            zos.putNextEntry(new ZipEntry(fileName));

            int bytesRead;
            while ((bytesRead = bis.read(bytes)) != -1) {
                zos.write(bytes, 0, bytesRead);
            }
            zos.closeEntry();
            bis.close();
            fis.close();
        }
        zos.flush();
        baos.flush();
        zos.close();
        baos.close();

        return baos.toByteArray();
    }
}

The web.xml configuration:

<?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"
         xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
         id="WebApp_ID" version="2.5">
    <servlet>
        <servlet-name>ZipDownloadServlet</servlet-name>
        <servlet-class>org.kodejava.example.servlet.ZipDownloadServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>ZipDownloadServlet</servlet-name>
        <url-pattern>/zipservlet</url-pattern>
    </servlet-mapping>
</web-app>

How do I create a hit counter servlet?

Here we have a simple example of creating a hit counter using servlet. The servlet updates the hits counter every time a page is visited and display the number of hits as an image. The image is generated at runtime using Java’s Graphic2D and ImageIO class.

To store the number of visit data we create a table hits with a single field called counter and set the initial value of the counter to zero. Below is the HitCounterServlet servlet code.

package org.kodejava.servlet;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.OutputStream;
import java.sql.*;

@WebServlet(name = "HitCounterServlet", urlPatterns = "/hitcounter")
public class HitCounterServlet extends HttpServlet {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "";

    @Override
    public void init() throws ServletException {
        super.init();
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
        updateHitCounter();
        getHitCounterImage(request, response);
    }

    private void updateHitCounter() {
        try (Connection connection = DriverManager.getConnection(URL, USERNAME, PASSWORD)) {
            // Update the hits counter table by incrementing the
            // counter every time a user hits our page.
            String sql = "UPDATE hits SET counter = counter + 1";
            PreparedStatement stmt = connection.prepareStatement(sql);
            stmt.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    private void getHitCounterImage(HttpServletRequest request, HttpServletResponse response) throws IOException {
        String hits = "";
        try (Connection connection = DriverManager.getConnection(URL, USERNAME, PASSWORD)) {
            // Get the current hits counter from database.
            String sql = "SELECT counter FROM hits";
            PreparedStatement stmt = connection.prepareStatement(sql);
            ResultSet rs = stmt.executeQuery();
            while (rs.next()) {
                hits = rs.getString("counter");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }

        // Create an image of our counter to be sent to the browser.
        BufferedImage buffer = new BufferedImage(50, 30, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = buffer.createGraphics();
        g.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                RenderingHints.VALUE_ANTIALIAS_ON);
        g.setFont(new Font("Monospaced", Font.PLAIN, 14));
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, 50, 30);
        g.setColor(Color.BLACK);
        g.drawString(hits, 10, 20);

        response.setContentType("image/png");
        OutputStream os = response.getOutputStream();
        ImageIO.write(buffer, "png", os);
        os.close();
    }
}

To display the hits counter image on the JSP page, create an img tag with the source (src) point to our HitCounterServlet servlet url.

Visited for: <img src="http://localhost:8080/hitcounter" alt="Hit Counter"/> times.

Maven dependencies

<dependencies>
    <!--https://search.maven.org/remotecontent?filepath=javax/servlet/javax.servlet-api/4.0.1/javax.servlet-api-4.0.1.jar-->
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
        <version>4.0.1</version>
    </dependency>
    <!--https://search.maven.org/remotecontent?filepath=mysql/mysql-connector-java/8.0.26/mysql-connector-java-8.0.26.jar-->
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.26</version>
    </dependency>
</dependencies>

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How do I create a HelloWorld Servlet?

Servlet is a Java solution for creating a dynamic web application, it can be compared with the old CGI technology. Using Servlet we can create a web application that can display information from a database, receive information from a web form to be stored in the application database.

This example shows the very basic of servlet, it returns a hello world html document for the browser. At the very minimum a servlet will have a doPost() and doGet() method which handles the HTTP POST and GET request.

package org.kodejava.servlet;

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

@WebServlet(name = "HelloWorldServlet", urlPatterns = {"/hello", "/helloworld"})
public class HelloWorld extends HttpServlet {

    public HelloWorld() {
        super();
    }

    protected void doGet(HttpServletRequest req, HttpServletResponse res)
            throws IOException {
        doPost(req, res);
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse res)
            throws IOException {
        res.setContentType("text/html");

        PrintWriter writer = res.getWriter();
        writer.println("<html>");
        writer.println("<head><title>Hello World Servlet</title></head>");
        writer.println("<body>Hello World! How are you doing?</body>");
        writer.println("</html>");
        writer.close();
    }
}

When the servlet is deployed to the container we can access it from an url in a form of http://localhost:8080/app-name/helloworld.

Maven dependencies

<!--https://search.maven.org/remotecontent?filepath=javax/servlet/javax.servlet-api/4.0.1/javax.servlet-api-4.0.1.jar-->
<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

Maven Central