LOG4j allows the developer to control which log statements are output with arbitrary granularity. It is fully configurable at runtime using external configuration files.

1. log4j in 8 slides
Tarin Gamberini
www.taringamberini.com
Thanks to Ceki Gülcü for the “Short introduction to log4j” http://logging.apache.org/log4j/1.2/manual.html

2. Logger Named Hierarchy
A logger is said to be an ancestor of another logger if
its name followed by a dot is the prefix part in the
descendant logger name.
root
com.site.software
com.site.software.model
com.site.software.model.dao
com.site.software.model.dao.PersonDAOImpl
com.site.software.view
● com.site.software is an ancestor logger of the descendant
com.site.software.model.dao
● com.site.software is the parent logger of the child
com.site.software.model

3. Levels
A logger may be assigned to a level.
● Properties configuration file
log4j.rootLogger=ERROR
log4j.logger.com.site.software=INFO
● XML configuration file
● Java configuration file
Logger.getRootLogger().setLevel(Level.ERROR);
Logger.getLogger(“com.site.software”).setLevel(Level.INFO);
● levels are ordered
TRACE < DEBUG < INFO < WARN < ERROR < FATAL

4. Level Inheritance
The inherited level for a given logger L, is equal to
the first non-null level in the logger named hierarchy,
starting at L and proceeding upwards in the
hierarchy towards the root logger.
Assigned Inherited
Logger Name
Level level
root ERROR ERROR
com.site.software WARN WARN
com.site.software.model INFO INFO
com.site.software.model.dao null INFO
com.site.software.model.dao.PersonDAOImpl null INFO
com.site.software.view null WARN

5. Logging Request
A log request of level p in a logger configured (either
assigned or inherited, whichever is appropriate) with
level q, is enabled if p >= q.
package com.site.software.model.dao;
import org.apache.log4j.Logger;
public class PersonDAOImpl {
private static final Logger LOG = Logger.getLogger(PersonDAOImpl.class);
public PersonDAOImpl() {
LOG.debug("You can't see me in the log because debug < INFO");
LOG.info("You will see me in the log because info = INFO");
LOG.warn("You will see me in the log because warn > INFO");

6. Appenders
A logger may be assigned to an appender: a named
output destination your log messages are forwarded
to.
# The root logger logs to the console
log4j.rootLogger=ERROR, con
# The com.site.software logger logs to a file
log4j.logger.com.site.software=INFO, FileApp
# The con appender will log in the console
log4j.appender.con=org.apache.log4j.ConsoleAppender
#The FileApp appender will log in a file
log4j.appender.FileApp=org.apache.log4j.FileAppender

7. Appender Additivity
Each enabled logging request for a given logger L
will be forwarded to all the appenders in that logger
LA as well as all the appenders higher HA in the
logger named hierarchy.
Logger Name LA HA
root con
com.site.software null con
com.site.software.model FileApp, c con
com.site.software.model.dao d FileApp, c, con
com.site.software.view e con

8. Layout Conversion Pattern
Each appender has a layout component responsible
for formatting log messages accordingly to
conversion patterns.
log4j.appender.con=org.apache.log4j.ConsoleAppender
log4j.appender.con.layout=org.apache.log4j.PatternLayout
log4j.appender.con.layout.ConversionPattern=%d [%t] %-5p %m (%c:%L)%n
Produced logs:
2010-05-14 19:29:11,996 [main] INFO You will see me in the log because
info = INFO (com.site.software.model.dao.PersonDAOImpl:10)
2010-05-14 19:29:11,997 [main] WARN You will see me in the log because
warn > INFO (com.site.software.model.dao.PersonDAOImpl:11)

9. A lot of Appenders and Layouts
Appenders
● ConsoleAppender appends log events to System.out or System.err
● FileAppender appends log events to a file
● RollingFileAppender extends FileAppender to backup the log files when
they reach a certain size
● DailyRollingFileAppender extends FileAppender so that the underlying file
is rolled over at a user chosen frequency
● SMTPAppender sends an e-mail when a specific logging event occurs
● JMSAppender publishes log events to a JMS Topic
● JDBCAppender provides for sending log events to a database
Layouts
● PatternLayout configurable string pattern in a printf C function style
● XMLLayout appends log events as a series of log4j:event (log4j.dtd)
● HTMLLayout outputs events in a HTML table