Learning log4j for Java beginners with a sample set of projects using log4j 1.2. This was done for Level 2 computer engineering students at University of Moratuwa 2015. I have hosted the samples in github (https://github.com/lkamal/log4j-workshop ), so that you will be able to download and try yourself.

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