# LOG4J : Example in Java

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Published 2022-12-03

This tutorial presents to you a LOG4J example in Java. This tutorial is using Eclipse and Maven. The application is a simple Java application. I suppose we have created a Java application using Maven and log4j framework is configured.

Now all we have to do is to code and to test our example of using log4j framework. For this I will create 2 java classes:

Car.java
import org.apache.log4j.Logger;
 
public class Car {
 
    private String color; 
    private int age;
    private String type;
     
    final Logger logger = Logger.getLogger(Car.class);
     
    public String getColor() {
        return color;
    }
    public void setColor(String color) {
        this.color = color;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        logger.info("The age of the Car is set !");
        this.age = age;
    }
    public String getType() {
        return type;
    }
    public void setType(String type) {
        this.type = type;
    }
}

RunTest.java
import org.apache.log4j.Logger;
 
public class RunTest {
 
    public static void main(String [] args) {
        Car car = new Car();
        car.setColor("green");
        car.setAge(4);
        car.setType("Peugeot");
 
        final Logger logger = Logger.getLogger(RunTest.class);
         
        logger.debug("This is my DEBUG text ");
        logger.error("This is my ERROR text ");
        logger.info("This is my INFO text ");
         
        System.out.println("This car is "+car.getColor()+".");
        System.out.println("This car has "+car.getAge()+" years old.");
        System.out.println("This car is a "+car.getType()+".");
    }
}

And here is the result of the log4j example into the "stdout":

2017-04-25 18:16:38 INFO Car:21 - The age of the Car is set !
2017-04-25 18:16:38 ERROR RunTest:14 - This is my ERROR text
2017-04-25 18:16:38 INFO RunTest:15 - This is my INFO text
This car is green.
This car has 4 years old.
This car is a Peugeot.

In log4j.properties file we have:

# Root logger option
log4j.rootLogger=INFO, stdout, file

# Log the messages to the console
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

# Log the messages to a log file with file rolling
log4j.appender.file=org.apache.log4j.RollingFileAppender
log4j.appender.file.File=C:\\JAVA\\log4j-application.log
log4j.appender.file.MaxFileSize=4MB
log4j.appender.file.MaxBackupIndex=10
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

Here are some things to retain:

When you log the information (with the standard Layout) you will see the object class which write the log ("Car", "RunTest").
When you log the information (with the standard Layout) you will see the line within the class where the log is generated (Car:21 -> the line is 21).
System.out.println is writing only to the console and not to the log file.