SLF4J with Logback

It is common that most of the developers use their own logging frameworks at the time of development and that force organizations to maintain configuration for each logging framework. Normally switching from logging level from DEBUG to INFO sometimes requires a application restart in production. SLF4J is the latest logging facade helps to plug-in desired logging framework at deployment time. The article further talks about usage of the SLF4J with logback. 

SLF4J - Simple Logging Facade for Java API helps to plug-in desired logging implementation at deployment time.

Logback - helps to change logging configuration through JMX at runtime with out restarting your applications in production.

I hope this article helps to get high level overview of SLF4J/Logback and migrating your existing apps to common logging approach.  

SLF4J

This is simple logging façade to abstract the various logging frameworks such as logback, log4j, commons-logging and default java logging implementation (java.util.logging). This primarily enables the user to inlcude desired logging framework at deployment time. It is lightweight and nearly adds a zero overhead on performance.

Note that SLF4j doesn’t replace any logging framework; it is just a façade around any standard logging framework. If slf4j doesn’t find any logging framwork in classpath, by default it prints the logs in console.

Logback

This is an improved version of log4j and natively supports the slf4j, hence migrating from other logging frameworks such as log4j and java.util.logging is quite possible.

Since the logback natively supports slf4j, the combination of using slf4j with this framework is relatively faster than the slf4j with other logging frameworks. Logging configuration can be done either in xml or groovy.

*One important feature is that it exposes the configuration through JMX hence configuration (debug to info etc) can be changed via JMX console with out restarting the application.

Also, it does print the artifact version as part of the exception stacktrace that may be helpful for debugging.

java.lang.NullPointerException: null

  at com.fimt.poc.LoggingSample.<init>(LoggingSample.java:16) [classes/:na]

  at com.fimt.poc.LoggingSample.main(LoggingSample.java:23) [fimt-logging-poc-1.0.jar/:1.0]

**Reasons to prefer logback over log4j is well explainedhere

SLF4J api usage in java classes

(1)  Import the Logger and LoggerFactory from org.slf4j package

import org.slf4j.Logger;

import org.slf4j.LoggerFactory;

(2) Declare the logger class as,

private final Logger logger = LoggerFactory.getLogger(LoggingSample.class);

(3) Use debug, warn, info, error and trace with appropriate parameters. All methods by default takes the string as an input.

logger.info("This is sample info statement");

SLF4J with Logback

Include the following dependency pom.xml, it pulls its depedencies logback-core and slf4j-api in addition to logback-classic

<dependency>

  <groupId>ch.qos.logback</groupId>

  <artifactId>logback-classic</artifactId>

  <version>1.0.7</version>

  </dependency>

SLF4J can be used with existing logging framworks log4j, common-logging and java.util.logging (JUL). Required dependencies are mentioned below.

SLF4J with Log4j

Include the following dependency pom.xml, it pulls its depedencies log4jand slf4j-api in addition to slf4j-log4j12 artifact.

<dependency>

  <groupId>org.slf4j</groupId>

  <artifactId>slf4j-log4j12</artifactId>

  <version>1.7.2</version>

  </dependency>

SLF4J with JUL (java.util.logging)

Include the following dependency pom.xml, it pulls its depedency slf4j-api in addition to slf4j-jdk14 artifact.

<dependency>

  <groupId>org.slf4j</groupId>

  <artifactId>slf4j-jdk14</artifactId>

  <version>1.7.2</version>

  </dependency>

Migrating existing projects logging to logback framework

Step: 1 – Update existing project pom.xml

Add the right dependency mentioned above. Also remove the unused log4j/commons logging dependencies.

Step: 2 – Update java files with SLF4J API

Scan all java files and replace log4j or java.util.logging classes in to SLF4J api classes. This can be done using the tool java -jar slf4j-migrator-1.7.2.jar . Detailed documentation and limitation is mentioned here

This tool replaces the logging apis of log4j, commons-logging and java.util.logging in to SLF4J API classes.

Step: 3 – Convert log4j.properties to logback.xml

Logback provides a online translator which converts log4j properties in to logback.xml 

File Appenders:

Like other logging frameworks, logback implementation also supports various file appenders.

Daily file rolling:

<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">

  <FileNamePattern>logFile.%d{yyyy-MM-dd}.log</FileNamePattern>

  <!-- keep 30 days' worth of history -->

  <maxHistory>30</maxHistory>

</rollingPolicy>

Roll log files based on size:

<rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">

  <fileNamePattern>tests.%i.log.zip</fileNamePattern>

  <minIndex>1</minIndex>

  <maxIndex>10</maxIndex>

</rollingPolicy>

<triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">

  <maxFileSize>5MB</maxFileSize>

</triggeringPolicy>

Layout

<layout class="ch.qos.logback.classic.PatternLayout">

  <Pattern>%-4relative [%thread] %-5level %logger{35} - %msg%n</Pattern>

</layout>