Logging with SLF4J

The Simple Logging Facade for Java (SLF4J) serves as a simple facade or abstraction for various logging frameworks. It allows you to code just depending on a one dependency namely "slf4j-api.jar", and to plug in the desire logging framework at runtime.

It is very simple to use slf4 logging in your application. You just need to create a slf4j logger and invoke its methods.

Following is a sample code,

package com.test;

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

public class Main {
    private static final Logger logger = LoggerFactory.getLogger(Main.class);

    public static void main(String[] args) {
        logger.info("Testing 123");
    }
}

You have to add the following dependency in your pom.xml file

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-api</artifactId>
    <version>1.7.13</version>
</dependency>

This is the bare minimum configuration you need to enable sl4fj logging. But if you run this code. you will get a warning similar to the below.

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

This is because, it can't find a binding in the class path, by default, if it can't find a binding in the class path, it will bind to no-op logger implementation.

Using java.util.logging

If you want to use the binding for java.util.logging in your code, you only need to add the following dependency in to your pom.file.

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-jdk14</artifactId>
    <version>1.7.13</version>
</dependency>

This will output the following,

INFO: Testing 123

Using Log4j

If you want to use the binding for log4j version 1.2 in your code, you only need to add the following dependency in to your pom.file.

<dependency>
    <groupId>org.slf4j</groupId>
    <artifactId>slf4j-log4j12</artifactId>
    <version>1.7.13</version>
</dependency>

Log4j needs an appender to log. Hence, you have to specify the log4j properties.

Create the file "log4j.properties", in resource directory of your project and add the following into it.

# Set root logger level to DEBUG and its only appender to A1.
log4j.rootLogger=DEBUG, A1

# A1 is set to be a ConsoleAppender.
log4j.appender.A1=org.apache.log4j.ConsoleAppender

# A1 uses PatternLayout.
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

This will output the following,

0    [main] INFO  com.test.Main  - Testing 123

[1] http://www.slf4j.org/
[2] http://logging.apache.org/log4j/1.2/index.html