P&MSP2012 - Logging Frameworks

Planning and Managing Software Projects 2011-12

Logging frameworks

Emanuele Della Valle, Lecturer: Daniele Dell’Aglio
http://emanueledellavalle.org

Outline
 Logging framework
 Key concepts
 Declaration and Naming
 Levels
 Appenders
 Layouts
 Java logging frameworks
 Logging systems: Log4J, JUL, LogBack
 Logging systems façades: ACL, SLF4J

Planning and Managing Software Projects – Emanuele Della Valle

Why a logging framework?
 System.out/err are not enough!
 Logging frameworks overcome the System.out/err
limits
 Several importance levels for messages
 Use of different output systems (console, file, mail...)
 ...
 We will consider Log4J as logging framework (but the
concepts are similar in other systems)


Example (example1 package)
...
public int sum(int a, int b){
System.out.println(
"The input values are "+a+" and "+b);
int ret = a+b;
System.out.println("The sum is "+ret);
return ret;
}

...


Example
...
Logger logger = Logger.getLogger(Example.class);
public int sum(int a, int b){
logger.info(
//System.out.println(
int ret = a+b;
logger.info("The sum is "+ret);
//System.out.println("The sum is "+ret);
return ret;
}

...

Declaration and Naming
 Loggers are initializated through a factory method
Logger x = Logger.getLogger("Logger1");

 The logger name is unique
 Two invocations of “Logger1” return the same logger
object
 Usually logger are named with the name of the class
Logger x = Logger.getLogger(ClassX.class);

 Log4J builds and manages a tree of loggers
 Nodes and leaves are determined through logger names
 The root of the tree is called rootLogger
 The tree is a key concept for the management of the
logger system (as we will see in the next slides)


Example (example2 package)

rootLogger

pmsp

logging

example2

p1 p2

Where each logger is declared as:
Class1 s1 Class3
Logger classXLogger=
Logger.getLogger(ClassX.class)
Class2


Levels
 While developing a system is useful to print out
messages to check the correct behaviour of the
application
 When the application is deployed most of those
messages are not relevant
 Logging frameworks allow to define several levels of
importance for the messages


Level hierarchy in Log4J

FATAL

ERROR

WARN

INFO

DEBUG

TRACE


Meaning of the levels
 There are not strictly rules to verify if levels are used
correctly
 Anyway the Log4j documentation indicates a general
guide line:
• FATAL: designates very severe error events that will
presumably lead the application to abort.
• ERROR: designates error events that might still allow
the application to continue running.
• WARN: The WARN level designates potentially harmful
situations.
• INFO: designates informational messages that highlight
the progress of the application at coarse-grained
level.
• DEBUG: designates fine-grained informational events
that are most useful to debug an application.
• TRACE: designates finer-grained informational events
than the DEBUG


Configuring the levels
 Each logger has an assigned level. The level is
assigned in to possible ways
 Explicit declaration: the level associated to a logger is
declared:
 in the configuration file:
log4j.logger.<logger-name>=<level>
 or in the code:
logger.setLevel(Level.<level>);

 Implicit declaration: the level is inferred through the
logger tree
 Loggers inherit the level of their parents


Example
 In the properties file:
log4j.rootlogger=INFO INFO rootLogger

INFO pmsp

INFO logging

INFO example2

INFO p1 p2 INFO

INFO Class1 s1 INFO Class3 INFO

INFO Class2
Declared
Inferred


Example

log4j.logger.pmsp.logging.
INFO pmsp
example2.p2.Class3=ERROR

INFO logging

INFO example2

INFO p1 p2 INFO

INFO Class1 s1 INFO Class3 ERROR

INFO Class2
Declared
Inferred


Example

INFO pmsp
example2.p2.Class3=ERROR
INFO logging
example2=DEBUG
DEBUG example2

DEBUG p1 p2 DEBUG

DEBUG Class1 s1 DEBUGClass3 ERROR

DEBUG Class2
Declared
Inferred


Appenders
 Loggers can write on several output systems
 Each output system is defined by an appender
 There is a high number of available appenders in Log4J
 Console, File, DB, network, etc.
 Appenders are declared and configured in the properties
file
log4j.appenders.<name>=<appender class>
log4j.appenders.<name>.<param1>=...
log4j.appenders.<name>.<param2>=...

 Each logger is associated to one or more appenders
 The assignment is managed through the logger tree (like
the levels)


Example
#Rolling file appender: fileap
log4j.appender.fileap=org.apache.log4j.RollingFileAppender
log4j.appender.fileap.File=log.xml
log4j.appender.fileap.MaxFileSize=100KB

#Stdout appender: stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender

log4j.rootLogger=INFO,stdout
log4j.logger.psmp.logging.example2.p2.Class3=FATAL,fileap

(Class3 is associated to both stdout and fileap appenders)

More info is availabe in the Log4j documentation:
http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/AppenderSkeleton.html

Layouts
 Layouts defines the format of the log lines
 Each appender is related to one layout
log4j.appender.<name>.layout=<layout class>
log4j.appender.<name>.layout.<param1>=...
log4j.appender.<name>.layout.<param2>=...

 Some layouts built-in in Log4J are:
 DateLayout
 HTMLLayout
 XMLLayout
 PatternLayout


Example
#Rolling file appender
log4j.appender.fileap=org.apache.log4j.RollingFileAppender
log4j.appender.fileap.File=log.xml
log4j.appender.fileap.MaxFileSize=100KB
log4j.appender.fileap.layout=
org.apache.log4j.xml.XMLLayout
#Stdout appender
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=
org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=
%d [%t] %-5p %c - %m%n


Logging frameworks in Java
 Log4J is not the only available logging framework for
Java
 Other notable logging systems are:
 Java Util Logging: built-in in the Java framework, it can
be used without including external libraries in the
project. On the other hand, it has a complex level
hierarchy and less features than Log4J
 LogBack: aims to be the succesor of Log4j. This
framework is faster than Log4J and overcomes some of
its limits


Logging façades
 It is considered a bad practice to create a strong
dependance between the code and a specific logging
framework
 The best practice is to use a logging façade that
“wraps” the logging framework system
 The two most used systems in Java are
 Apache Commons Logging (formerly Jakarta Commons
Logging)
 SLF4J


SLF4J
 SLF4J abstracts the logging concepts allowing users to
decide what logging system use in their application
 SLF4J is “configured” at deploy time
 Developer should import the right libraries into the
project
 Classpath must have at most one bridge library


SLF4J

From http://www.slf4j.org/manual.html

SLF4J
 In huge projects several logger framework could be used
 In this case SLF4J allows to use migration libraries
 They bounds an existing logging framework into SLF4j

commons-logging.jar

From: http://www.slf4j.org/legacy.html

SLF4J
 In huge projects several logger framework could be used
 In this case SLF4J allows to use migration libraries
 They bounds an existing logging framework into SLF4j

From: http://www.slf4j.org/legacy.html

References
 Don't Use System.out.println! Use Log4j -
http://www.vipan.com/htdocs/log4jhelp.html
 Apache Log4J manual
http://logging.apache.org/log4j/1.2/manual.html
 SLF4J manual http://www.slf4j.org/manual.html
 Think again before adopting the commons-logging API
http://articles.qos.ch/thinkAgain.html
 Logging, which framework to choose? Log4j, Commons
Logging, LogBack, SLF4J
http://mike.vanvendeloo.net/2011/05/06/logging-
which-framework-to-choose-log4j-commons-logging-
logback-slf4j


P&MSP2012 - Logging Frameworks

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