The document is a presentation by Kamal Mettananda on using Log4j for logging in Java projects, detailing the significance of logging, the configuration of Log4j, and various logging components such as loggers, appenders, and layouts. It includes setup instructions, code examples, and performance considerations related to logging. The presentation also mentions the evolution of logging libraries from Log4j 1.2 to Log4j 2, highlighting the differences and improvements.

1. Logging with Log4j
KAMAL METTANANDA
SOFTWARE ARCHITECT
ACCELERITE – PERSISTENT SYSTEMS
@KAMALMETTANANDA

2. Who am I?
B.Sc. in 2004 University of Moratuwa
Masters in computer sciences 2012
10+ years of industry experience
Working as an Architect in Accelerite
Author of www.digizol.com
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3. Agenda
What is logging
• Why logging
• How to log
Log4j 1.2
• Format (Layouts)
• Handler (Appenders)
• Logger (Categories)
Log4j 2
Java logging API
Learn further
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4. Objective
Make sure ALL
brothers & sisters
know how to use log4j
in a Java project
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5. What is logging
Pile of logs in Indonesia. Photos by Rhett Butler
“Logging, or
commercial
logging,
involves
cutting trees
for sale as
timber or
pulp”
• mongabay.com
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6. What is logging
“Logging refers to the recording of
activity. Logging is a common issue for
development teams. Several frameworks
ease and standardize the process of
logging for the Java platform.”
• wikipedia
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7. Why logging
Troubleshoot by finding
What has
happened
What is
happening
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8. What to log
• Meaningful message
• State of objects
• Exceptions
• Class name
• Time of logging
• Thread/invoker
Useful
information
to
troubleshoot
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9. How to log
•Most simplest
•Not enough information
•Unable to switch on/off
System.out.print()
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10. How to log
• A lot of required information
• Simple enable/disable mechanism
• Configurable without code changes
• Multiple destinations
Use a dedicated logging library
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11. Logging libraries
Log4j v1.2
Log4j v2
JUL - Java Logging API
Logback
Commons logging
SLF4J
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12. Log4j 1.2
APACHE SOFTWARE
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13. Environment Setup
Install Java
Install Eclipse
Import Projects
• From https://github.com/lkamal/log4j-
workshop
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14. Log4j 1.2
Started in 1996
• Released in around 2000
Free and Open source
• Apache Licensed – 2.0
Latest version 1.2.17
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15. Enabling Log4j in project
• log4j-1.2.x.jarJar file
• log4j.properties OR
• log4j.xml
Configuration
file
• org.apache.log4j.Logger
Logger
instance
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16. Hello World sample
• 01_helloworld_sysout
Hello world
with Sysout
• 02_helloworld_log4j
Hello world
with log4j
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17. Log4j Components
Logger
Appender
Layout
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18. Logger instance by name
org.apache.log4j.Logger =
org.apache.log4j.Logger.getLogger(Class)
org.apache.log4j.Logger.getLogger(String)
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19. Logger Instance
public class MainLog4j {
private static Logger logger =
Logger.getLogger(MainLog4j.class);
…
}
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20. Simple log4j.properties file
# top level config=logging level, appenders
log4j.rootLogger=INFO, console
# console appender configurations
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.layout=org.apache.log4j.SimpleLayout
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21. Calculator sample
• 03_calculator_sysout
Calculator
with Sysout
• 04_calculator_log4j
Calculator
with log4j
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22. Log level - Priority
TRACE
DEBUG
INFO
WARN
ERROR
FATAL
LogFilterSize
MessagePriority
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23. Log Level - Usage
Least priority messageslogger.trace()
logger.debug()
logger.info()
logger.warn()
logger.error()
Highest priority messageslogger.fatal()
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24. Log Level - log4j.properties
log4j.rootLogger=<LEVEL>, <Appenders>
•e.g:
•DEBUG, console
•INFO , console
•ERROR , console
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25. Calculator with log levels
• 05_calculator_log4j_levels
Calculator
with log
levels
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26. log4j.properties - file content
log4j.rootLogger
Log level
Appenders
log4j.appender
Destination
Layout
log4j.logger
Package
Log Level
Appenders
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27. Layout - Format
• org.apache.log4j.SimpleLayout
• INFO - message
• org.apache.log4j.HTMLLayout
• HTML table row per log line
• org.apache.log4j.xml.XMLLayout
• XML tag per log line
• org.apache.log4j.PatternLayout
• Mostly used
log4j.appender.<appender>.layout
property value
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28. Calculator with log formatting
• 06_calculator_log4j_format
Calculator
with
xml/html logs
• 07_calculator_log4j_patterlayout
Calculator
with Pattern
Layout logs
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29. log4j.properties - file content
log4j.rootLogger
Log level
Appenders
log4j.appender
Destination
Layout
log4j.logger
Package
Log Level
Appenders
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30. Appenders – Output Handlers
• A name must be given for <appender>
• org.apache.log4j.ConsoleAppender
• System out / console
• org.apache.log4j.FileAppender
• To a given file
• org.apache.log4j.RollingFileAppender
• When one file is full – move to next
• org.apache.log4j.DailyRollingFileAppender
• Daily write to a new file
log4j.appender.<appender> property
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31. Appenders – Log Level
• Required log level can be configured by
overriding root level.
• e.g.:
• log4.appender.console.Threshold=INFO
log4j.appender.<appender>.Threshold
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32. Calculator with appenders
• 08_calculator_log4j_appe
nders
Calculator
with
multiple log
destinations
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33. log4j.properties - file content
log4j.rootLogger
Log level
Appenders
log4j.appender
Destination
Layout
log4j.logger
Name
Log Level
Appenders
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34. Logger - Categories
log4j.logger.<logger-name>
property
• Log level
• Restrict the log level for the package
• Appender
• Restrict the appender for the logs
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35. Calculator with Categories
• 09_calculator_log4j_categories
Calculator
with
different
loggers
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36. Logging Exceptions
• logger.trace(msg, exception)
• logger.debug(msg, exception)
• logger.info(msg, exception)
• logger.warn(msg, exception)
• logger.error(msg, exception)
• logger.fatal(msg, exception)
Exceptions
can be
logged
with stack
trace
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37. Calculator with exception
• 10_calculator_log4j_exception
Calculator
with
exception
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38. Log4j – Performance hit
Minimize with methods like
isDebugEnabled() isInfoEnabled()
Smaller performance hit
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39. Log4j – Performance hit
if (logger.isDebugEnabled()) {
logger.debug(“method invoked ” + result);
}
if (logger.isInfoEnabled()) {
logger.info(“method invoked ” + result);
}
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40. Log4j 1.2 Summary
Supports logging by
simple on/off
mechanism
required information
multiple destinations
Needs only 3 items
Library
Configuration file
Logger instances
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41. Log4j 2
Performance improved against log4j 1.2
Configuration via xml, not properties file
Migration tool for 1.2 to 2 available
Not by original author
• Disclaimer: I am the founder of log4j, slf4j and logback projects but
unaffiliated with log4j 2.0. As I understand it, notwithstanding its
name, log4j 2.0 is very different than log4j 1.x. As far as the user API is
concerned, log4j 2.0 is largely incompatible with log4j 1.x.
• stackoverflow.com/a/12095467/2581128
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42. JUL - Java logging API
Java itself has
a logging API
java.util.logging.Logger
But was too
late to be born
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43. Logging libraries
© zeroturnaround.com
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44. Learn further
Logback
Log4j 2
JUL - Java logging API
Commons-logging
SLF4j
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46. Thank you