How to configure Log4j in Java program without XML or Properties File - Example

Sometimes configuring Log4j using XML or properties file looks annoying, especially if your program is not able to find them because of some classpath issues, wouldn't it be nice if you can configure and use Log4j without using any configuration file e.g. XML or properties. Well, Log4j people have thought about it and they provide a BasicConfigurator class to configure log4j programmatically, though this is not as rich as their XML and properties file version is, but it's really handy for quickly incorporating Log4j in your Java program.

One of the reason programmer prefer to use System.out.println() over Log4j, of course for testing purposes, because it doesn't require any configuration, you can just use it, without bothering about XML or properties file configuration, but the most programmer will agree that they would prefer to use Log4j over println statements, even for test programs if it's easy to set them up.

By the way, for production use prefer SLF4J over Log4J.  In this Java tutorial, we will see a nice little tip to use Log4j right away by configuring in a couple of lines. In fact, you can configure Log4J in just one line, if you intend to use their basic configuration which set's the log level as DEBUG.

Log4J configuration without an XML file

Here is the code to configure Log4J in one line without using any external configuration file i.e. Log4j.xml or Log4j.properties. The first one uses an XML file to configure this logging library, while the second one uses Java specific properties files, which are essentially text files. This code leverage configuration options defined in BasicConfigurator class.

import org.apache.log4j.BasicConfigurator;

import org.apache.log4j.Level;

import org.apache.log4j.Logger;

/**

  * Java program to configure log4j without using XML or properties file.

  * By using BasicConfigurator class, you can configure Log4j in one line.

  * @author

  */

public class Log4JConfigurator {

    private static final Logger logger = Logger.getLogger(Log4JConfigurator.class);

  

    public static void main(String args[]) {

      

      

        BasicConfigurator.configure(); //enough for configuring log4j

      

        Logger.getRootLogger().setLevel(Level.WARN); //changing log level

      

      

        logger.error("Critical message, almost fatal");

        logger.warn("Warnings, which may lead to system impact");

        logger.info("Information");

        logger.debug("Debugging information ");

           

    }    

  

}

Output:

0 [main] ERROR Log4JConfigurator  - Critical message, almost fatal

0 [main] WARN Log4JConfigurator  - Warnings, which may lead to system impact

Couple of things to note about basic Log4j Configuration :

1) BasicConfigurator display log messages into console by using PatternLayout with pattern "%-4r [%t] %-5p %c %x - %m%n" to display log messages.

2) Root logger is set to use DEBUG log level, which means it will display all messages. If you look at our example, we have reset the log level to WARN, which means only ERROR and WARN level messages will be displayed into the console, and INFO and DEBUG level messages will be suppressed. 

This is one of the important logging tips in Java to remember because the amount of logging is inversely proportional to performance, the more you log, the slower your program will be. Use ERROR or WARN level on production and DEBUG during development

3) You can reset configuration by calling BasiConfigurator.reset() method.

Isn't it great to use Log4j just like this, I really liked it. Yes, I still prefer the System.out.println() statement for the simplest task but like to use Log4j more often now. Though it still makes sense to use XML based logger for production use, Log4j with basic configuration is just fine for temporary or tutorial purposes.