Logging with Log4j: Java Example

Log4j Overview

Log4j is java based fast reliable and flexible logging framework distributed under the Apache Software License. log4j is highly configurable through external configuration files at runtime.

Benefit of Logging

Logging is an important component of the software development. A well-written logging code offers quick debugging, easy maintenance, and structured storage of an application's runtime information.

log4j has three main components:

1. loggers: Responsible for capturing logging information.
2. appenders: Responsible for publishing logging information to various preferred destinations.
3. layouts: Responsible for formatting logging information in different styles.

Log4j Logging levels

OFF          The highest possible rank and is intended to turn off logging.
ALL          All levels including custom levels.
FATAL      Very Severe errors that cause premature termination.
ERROR    Errors or unexpected conditions that may allow application to continue running.
WARN      Situations that are undesirable or unexpected, but not necessarily "wrong".
INFO         Informational messages that highlight the progress of the application.
DEBUG    Detailed information on the flow that are most useful to debug an application.
TRACE     Most detailed information than the DEBUG.

Configuration

Log4j can be configured using a properties file or xml file externally. In this example we'll use properties file.

Log4j Sample Project

We're going to create a simple example project of Log4j using Eclipse IDE to log the information in Console, File and Database. Below is the preview of logging in file

File Preview of Logging using Log4j

To create the Log4j project we'll follow below steps:

Step 1: Create a maven project

Open Eclipse IDE
Click on File -> New -> Project
Expand Maven and Select Maven Project and click on the Next button

Select a wizard for Maven Project

Check the checkbox Create a simple project and click on the Next button

Create a Simple Maven Project

Specify the Group Id and Artifact Id as shown in the below image and click on the Finish button

Specify Group Id and Artifact Id for Maven Project

Your Maven Project has been created.

Step 2: Add Maven Dependencies

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>com.blogspot.sonevalley</groupId>
 <artifactId>Log4jSampleProject</artifactId>
 <version>0.0.1-SNAPSHOT</version>

 <dependencies>

  <dependency>
   <groupId>log4j</groupId>
   <artifactId>log4j</artifactId>
   <version>1.2.17</version>
  </dependency>

  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <version>5.1.6</version>
  </dependency>

 </dependencies>

</project>

Step 3: Create Packages and Files

Now you have to create a Main Class and a properties file as shown in the below image

Log4j Project Explorer

MainApp.java

package com.blogspot.sonevalley;

import org.apache.log4j.Logger;

public class MainApp {

 final static Logger logger = Logger.getLogger(MainApp.class);

 public static void main(String args[]) {

  MainApp app = new MainApp();
  app.generateLog("myLogger");
 }

 private void generateLog(String parameter) {

  if (logger.isDebugEnabled()) {
   logger.debug("This is debug : " + parameter);
  }

  if (logger.isInfoEnabled()) {
   logger.info("This is info : " + parameter);
  }

  if(logger.isTraceEnabled()) {
   logger.trace("This is trace : " + parameter);
  }
  
  logger.warn("This is warn : " + parameter);
  logger.error("This is error : " + parameter);
  logger.fatal("This is fatal : " + parameter);
 }
}

log4j.properties

The log4j.properties file is a log4j configuration file. By default, the LogManager looks for a file named log4j.properties in the CLASSPATH.

# Root logger option
log4j.rootLogger = DEBUG, stdout, file, DB

# ============================================================================================
# Redirect log messages to console

# Define the console appender
log4j.appender.stdout=org.apache.log4j.ConsoleAppender

log4j.appender.stdout.Target=System.out

# Define the layout for console appender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

# Set the console layout pattern.
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

# ============================================================================================
# Redirect log messages to a log file, support file rolling when the file reaches MaxFileSize.

# Define the file appender
log4j.appender.file=org.apache.log4j.RollingFileAppender

# Set log file path
log4j.appender.file.File=C:\\log4j-application.log

# Set max size of the log file
log4j.appender.file.MaxFileSize=5MB

# Set max backup index
log4j.appender.file.MaxBackupIndex=10

# Define the layout for file appender
log4j.appender.file.layout=org.apache.log4j.PatternLayout

# Set the file layout pattern.
log4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

# ============================================================================================
# Store log messages to a DB

# Define the DB appender
log4j.appender.DB=org.apache.log4j.jdbc.JDBCAppender

# Set JDBC URL
log4j.appender.DB.URL=jdbc:mysql://localhost/my_db

# Set Database Driver
log4j.appender.DB.driver=com.mysql.jdbc.Driver

# Set database user name and password
log4j.appender.DB.user=root
log4j.appender.DB.password=root

# Set the SQL statement to be executed.
log4j.appender.DB.sql=INSERT INTO LOGS VALUES('%d{yyyy-MM-dd HH:mm:ss}','%p','%c','%t','%L','%m')

# Define the layout for DB appender
log4j.appender.DB.layout=org.apache.log4j.PatternLayout

Step 3: Run the Project

To run the project right click on MainApp.java in the Project Explorer and select
Run As -> Java Application

Log4j Console Logging

Log4j File Logging

Log4j DB Logging

Clone or Download Project from GitHub

https://github.com/sushilkrs/Miscellaneous-Examples.git 

Look for Log4jSampleProject Under Miscellaneous-Examples Repository

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