How do I use Apache Commons Lang ToStringBuilder class?

By Wayan in Apache Commons, Commons Lang 0 Comment

The toString() method defined in the java.lang.Object can be overridden when we want to give more meaningful information about our object. We can simply return any information of the object in the toString() method, for instance the value of object’s states or fields.

The Apache Commons Lang library offers a good utility for creating this toString() information. Here I give a simple example using the ToStringBuilder class.

package org.kodejava.commons.lang;

import org.apache.commons.lang3.builder.ToStringBuilder;
import org.apache.commons.lang3.builder.ToStringStyle;

public class ToStringBuilderDemo {
    private Long id;
    private String firstName;
    private String lastName;

    public static void main(String[] args) {
        ToStringBuilderDemo demo = new ToStringBuilderDemo();
        demo.id = 1L;
        demo.firstName = "First Name";
        demo.lastName = "Last Name";

        System.out.println(demo);
    }

    public String toString() {
        return new ToStringBuilder(this, ToStringStyle.MULTI_LINE_STYLE)
            .append("id", id)
            .append("firstName", firstName)
            .append("lastName", lastName)
            .toString();
    }
}

The ToStringStyle class allows us to choose the styling for our toString() method when we print it out. Here are the available styles that we can use.

  • ToStringStyle.DEFAULT_STYLE
  • ToStringStyle.JSON_STYLE
  • ToStringStyle.MULTI_LINE_STYLE
  • ToStringStyle.NO_CLASS_NAME_STYLE
  • ToStringStyle.NO_FIELD_NAMES_STYLE
  • ToStringStyle.SHORT_PREFIX_STYLE
  • ToStringStyle.SIMPLE_STYLE

The result of the code above is:

org.kodejava.commons.lang.ToStringBuilderDemo@8efb846[
  id=1
  firstName=First Name
  lastName=Last Name
]

Below are example results of the other ToStringStyle:

  • ToStringStyle.DEFAULT_STYLE
org.kodejava.commons.lang.ToStringBuilderDemo@d716361[id=1,firstName=First Name,lastName=Last Name]
  • ToStringStyle.JSON_STYLE
{"id":1,"firstName":"First Name","lastName":"Last Name"}
  • ToStringStyle.NO_CLASS_NAME_STYLE
[id=1,firstName=First Name,lastName=Last Name]
  • ToStringStyle.NO_FIELD_NAMES_STYLE
org.kodejava.commons.lang.ToStringBuilderDemo@d716361[1,First Name,Last Name]
  • ToStringStyle.SHORT_PREFIX_STYLE
ToStringBuilderDemo[id=1,firstName=First Name,lastName=Last Name]
  • ToStringStyle.SIMPLE_STYLE
1,First Name,Last Name

If you want to make the code event more simple by using the ToStringBuilder.reflectionToString() method to generate the string for the toString() method to return. Using this method the ToStringBuilder will the hard job of finding information about our class and return the string information.

Maven Dependencies

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.14.0</version>
</dependency>

Maven Central

How do I count number of online users?

By Wayan in Servlet 18 Comments

When you have a web application you might want to know how many users are currently online or connected to your website. If you have visited some web online forums you can see; usually on the first page; the list of their online users or maybe just the number of currently online users.

How do we know / count how many sessions or users are currently connected to our website. Do you care to know? Let’s see what Java Servlet API offers us on this matter.

Servlet API has an interface javax.servlet.http.HttpSessionListener, an implementation of this interface will have the ability to be notified by the servlet engine at anytime when a new session was created or destroyed.

This interface has two methods to be implemented; these methods are sessionCreated(HttpSessionEvent se) and sessionDestroyed(HttpSessionEvent se). These methods will be called as a notification that a new session was created, and the session was about to be destroyed respectively.

Now let’s create our session listener. The code below is what our class is going to be implemented.

package org.kodejava.servlet;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSession;
import java.util.List;
import java.util.ArrayList;

@WebListener
public class SessionCounter implements HttpSessionListener {
    public static final String COUNTER = "SESSION-COUNTER";
    private final List<String> sessions = new ArrayList<>();

    public void sessionCreated(HttpSessionEvent event) {
        System.out.println("SessionCounter.sessionCreated");
        HttpSession session = event.getSession();
        sessions.add(session.getId());
        session.setAttribute(SessionCounter.COUNTER, this);
    }

    public void sessionDestroyed(HttpSessionEvent event) {
        System.out.println("SessionCounter.sessionDestroyed");
        HttpSession session = event.getSession();
        sessions.remove(session.getId());
        session.setAttribute(SessionCounter.COUNTER, this);
    }

    public int getActiveSessionNumber() {
        return sessions.size();
    }
}

To display information of current online users we need to create a simple JSP page. This JSP file will get the number of online user from HttpSession attribute named counter that we set in our listener above.

<%@ page import="org.kodejava.servlet.SessionCounter" %>
<html>
<head>
    <title>Session Counter</title>
</head>
<body>
<%
    SessionCounter counter = (SessionCounter) session.getAttribute(
            SessionCounter.COUNTER); 
%>

Number of online user(s): <%= counter.getActiveSessionNumber() %>
</body>
</html>

The final step to make the listener working is to register it in the web.xml file. Below is the example how to register the listener in web.xml.

<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <display-name>Servlet Examples</display-name>
    <session-config>
        <session-timeout>1</session-timeout>
    </session-config>
</web-app>

Maven dependencies

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

Maven Central

How do I create and write data into text file?

By Wayan in Core API, IO 0 Comment

Here is a code example for creating text file and put some texts in it. This program will create a file called write.txt. To create and write a text file we do the following steps:

File file = new File("write.txt");
FileWriter fileWriter = new FileWriter(file);
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

Below is the complete code snippet:

package org.kodejava.io;

import java.io.*;

public class WriteTextFileExample {
    public static void main(String[] args) {
        File file = new File("write.txt");

        try (Writer writer = new BufferedWriter(new FileWriter(file))) {
            String contents = "The quick brown fox" + 
                System.getProperty("line.separator") + "jumps over the lazy dog.";

            writer.write(contents);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

To read a text file see the following example: How do I read a text file using BufferedReader?.

How do I read a text file using BufferedReader?

By Wayan in Core API, IO 0 Comment

The code snippet below is an example of how to read a text file using BufferedReader class from the java.io package. This snippet read a text file called README.md and print out its content.

To create an instance of java.io.BufferedReader we do the following steps:

File file = new File("README.md");
FileReader fileReader = new FileReader(file));
BufferedReader bufferedReader = new BufferedReader(fileReader);

Let’s see the complete code snippet.

package org.kodejava.io;

import java.io.*;

public class ReadTextFileExample {
    private static final String LINE_SEPARATOR = System.getProperty("line.separator");

    public static void main(String[] args) {
        File file = new File("README.md");
        try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
            StringBuilder contents = new StringBuilder();
            String text;
            while ((text = reader.readLine()) != null) {
                contents.append(text).append(LINE_SEPARATOR);
            }

            System.out.println(contents.toString());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

You can also try to use the following example to read a file, How do I read text file content line by line using commons-io?. To create and write a text file see the following example: How do I create and write data into text file?.

How do I read a configuration file using java.util.Properties?

By Wayan in Core API, Util Package 0 Comment

When we have an application that used a text file to store a configuration, and the configuration is typically in a key=value format then we can use java.util.Properties to read that configuration file.

Here is an example of a configuration file called app.config:

app.name=Properties Sample Code
app.version=1.0

The code below show you how to read the configuration.

package org.kodejava.util;

import java.io.*;
import java.net.URL;
import java.util.Objects;
import java.util.Properties;

public class PropertiesExample {
    public static void main(String[] args) {
        Properties prop = new Properties();
        try {
            // the configuration file name
            String fileName = "app.config";
            ClassLoader classLoader = PropertiesExample.class.getClassLoader();

            // Make sure that the configuration file exists
            URL res = Objects.requireNonNull(classLoader.getResource(fileName),
                "Can't find configuration file app.config");

            InputStream is = new FileInputStream(res.getFile());

            // load the properties file
            prop.load(is);

            // get the value for app.name key
            System.out.println(prop.getProperty("app.name"));
            // get the value for app.version key
            System.out.println(prop.getProperty("app.version"));

            // get the value for app.vendor key and if the
            // key is not available return Kode Java as
            // the default value
            System.out.println(prop.getProperty("app.vendor","Kode Java"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

The code snippet will print these results:

Properties Sample Code
1.0
Kode Java