Quickstart with SLF4J

In this tutorial we will configure SLF4J logging with log4j implementation.

SLF4J is a facade of abstractions for various logging frameworks(log4j, java.util.logging, commons logging, logback). It is really easy to swap between logging frameworks when you use SLF4J. Here is what you have to add to your Maven pom.xml to use SLF4J with log4j implementaion:

<properties>
 <slf4j.version>1.6.6</slf4j.version>
</properties>
<dependencies>    
 <!-- Logging -->
 <dependency>
  <groupId>log4j</groupId>
  <artifactId>log4j</artifactId>
  <version>1.2.17</version>
 </dependency>
 <dependency>
  <groupId>org.slf4j</groupId>
  <artifactId>slf4j-api</artifactId>
  <version>${slf4j.version}</version>
 </dependency>
 <dependency>
  <groupId>org.slf4j</groupId>
  <artifactId>slf4j-log4j12</artifactId>
  <version>${slf4j.version}</version>
 </dependency>
</dependencies>

Run

mvn install

Now we need to configure log4j as usual. There are two ways to do it: via xml or via properties file. Let's use properties file. We need to create log4j.properties in the classpath. Here is the code of the file:

# Root logger option
log4j.rootLogger=INFO, file, stdout
 
# Direct log messages to a log file
log4j.appender.file=org.apache.log4j.DailyRollingFileAppender
log4j.appender.file.DatePattern = '.'yyyy-MM-dd
log4j.appender.file.Append = true
log4j.appender.file.File=logs/main.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n
 
# Direct log messages to stdout
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{dd-MM-yy HH:mm:ss,SSS} %5p [%c{1}:%M:%L] - %m%n

According to this configuration we defined the lowest log priority to INFO and set the output to Console and to the logs/main.log file(logs/main.log file will be rolled by days automatically).

Now we can test how the logging works:

package uay.test;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class App 
{
    private static Logger log = LoggerFactory.getLogger(App.class);
    public static void main( String[] args )
    {
     log.info("test");
    }
}

If we run the code the Console output would be:

01-12-12 23:12:24,840  INFO [App:main:11] - test

The file output would be:

23:12:24,840  INFO App:11 - test

Thus, we have configured the code to use log4j via SLF4J. As a further improvement of code you should think about using annotations to make the code look like this:

@Log
private Logger log;

This tutorial will definitely help you with it. Later I will show how to implement this annotation even easier using Spring Framework.