Java - Learn Java by Examples

How do I read servlet init parameter?

By Wayan in Servlet Last modified: May 16, 2023 0 Comment

When creating a servlet we can use the @WebInitParam to define some init parameters in the servlet configuration section (@WebServlet). This init parameter can be used to define where a configuration file of our application is stored, define upload path, etc. This simple servlet below shows how to obtain these init parameters value.

package org.kodejava.servlet;

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

@WebServlet(
    name = "InitParameterServlet",
    urlPatterns = "/init-param",
    initParams = {
        @WebInitParam(name = "configPath", value = "F:/app/config"),
        @WebInitParam(name = "uploadPath", value = "F:/app/uploads")
    }
)
public class InitParameterServlet extends HttpServlet
        implements javax.servlet.Servlet {

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

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // Get application configuration path
        String configPath = getInitParameter("configPath");
        String uploadPath = getInitParameter("uploadPath");

        System.out.println("configPath = " + configPath);
        System.out.println("uploadPath = " + uploadPath);
    }
}

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

How do I read cookie in Servlet?

By Wayan in Servlet Last modified: May 16, 2023 0 Comment

package org.kodejava.servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.Cookie;
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 = "ReadCookieServlet", urlPatterns = "/read-cookie")
public class ReadCookieExample extends HttpServlet {

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

    @Override
    protected void doPost(HttpServletRequest request,
                          HttpServletResponse response) throws IOException {
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();

        Cookie[] cookies = request.getCookies();
        for (Cookie cookie : cookies) {
            writer.println("Name: " + cookie.getName() + "; Value: " + cookie.getValue());
        }
    }
}

An example result of the servlet above is:

Name: username; Value: jduke 
Name: JSESSIONID; Value: AE819A3F74E3700838ADFC99485216DF

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

How do I retrieve auto-generated keys?

By Wayan in JDBC Last modified: July 15, 2024 0 Comment

package org.kodejava.jdbc;

import java.sql.*;

public class GetGeneratedKeyExample {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            // The orders table have an ID column which value will be auto
            // generated by database. When inserting a new record a new id
            // for the primary key will be generated, and we will get the
            // generated key, so we can use it in another process. For
            // instance if we have a master detail tables where the details
            // table required an id from the master table.
            String sql = """
                    INSERT INTO purchase_order (username, order_date)
                    VALUES ('foobar', '2021-09-25')
                    """;

            Statement stmt = connection.createStatement();

            // When executing the statement we can pass the
            // Statement.RETURN_GENERATED_KEYS so that we can later extract
            // the generated key from the statement object after executing
            // the query.
            stmt.executeUpdate(sql, Statement.RETURN_GENERATED_KEYS);

            ResultSet keys = stmt.getGeneratedKeys();
            long lastKey = 1L;
            while (keys.next()) {
                lastKey = keys.getLong(1);
            }

            System.out.println("Last Key: " + lastKey);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

The structure of purchase_order table.

CREATE TABLE `purchase_order`
(
    `id`         bigint(20) unsigned NOT NULL AUTO_INCREMENT,
    `username`   varchar(50)         NOT NULL,
    `order_date` date                NOT NULL,
    PRIMARY KEY (`id`)
);

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

How do to commit or rollback transaction in JDBC?

By Wayan in JDBC Last modified: May 31, 2024 0 Comment

Executing a database manipulation command such as insert, update or delete can sometime throw exception due to invalid data. To protect the integrity of our application data we must make sure that when a transaction fails we must rollback all the executed queries.

In this example we are using MySQL database. To enable transaction capability in MySQL make sure that you are using InnoDB storage engine to create your databases and tables.

package org.kodejava.jdbc;

import java.math.BigDecimal;
import java.sql.*;

public class TransactionRollbackExample {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection conn =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {
            conn.setAutoCommit(false);

            String orderQuery = """
                    INSERT INTO purchase_order (username, order_date)
                    VALUES (?, ?)
                    """;

            try (PreparedStatement stmt = conn.prepareStatement(orderQuery,
                    PreparedStatement.RETURN_GENERATED_KEYS)) {
                stmt.setString(1, "jduke");
                stmt.setDate(2, new Date(System.currentTimeMillis()));
                stmt.execute();

                ResultSet keys = stmt.getGeneratedKeys();
                long orderId = 1L;
                if (keys.next()) {
                    orderId = keys.getLong(1);
                }

                // This is an invalid statement that will cause exception to
                // demonstrate a rollback.
                String orderDetailQuery = """
                        INSERT INTO purchase_order_detail (order_id, product_id, quantity, price)
                        VALUES (?, ?, ?, ?)
                        """;

                PreparedStatement detailStmt = conn.prepareStatement(orderDetailQuery);
                detailStmt.setLong(1, orderId);
                detailStmt.setInt(2, 1);
                detailStmt.setInt(3, 10);
                detailStmt.setBigDecimal(4, new BigDecimal("29.99"));
                detailStmt.execute();

                // Commit transaction to mark it as a success database operation
                conn.commit();
                System.out.println("Transaction commit...");
            } catch (SQLException e) {
                // Rollback any database transaction due to exception occurred
                conn.rollback();
                System.out.println("Transaction rollback...");
                e.printStackTrace();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

How do I send a cookie in Servlet?

By Wayan in Servlet Last modified: May 16, 2023 0 Comment

A cookie is a piece of information sent to a browser by a Web Server. The browser then returns that information to the Web server. This is how some Web pages remember your previous visits; for example, an e-commerce website might use a cookie to remember which items you’ve placed in your online shopping cart. Cookies can also store user preference information, login data, etc.

Here is an example to send a cookie in the HTTP response object to the client browser.

package org.kodejava.servlet;

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

@WebServlet(name = "CookieServlet", urlPatterns = "/test-cookie")
public class WriteCookieExample extends HttpServlet {

    protected void doGet(HttpServletRequest request,
                         HttpServletResponse response) throws ServletException, IOException {
        response.setContentType("text/html");

        // Send a cookie named username to the client. There are some others
        // properties that we can set before we send the cookie, such as comments,
        // domain name, max age, path, a secure flag, etc.
        Cookie cookie = new Cookie("username", "jduke");
        response.addCookie(cookie);
    }
}

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>