log4j

Sample Code

Java

import org.apache.log4j.Logger;

    /** Standard logger. */
    private static Logger logger = Logger.getLogger(MyClass.class);

    logger.debug("foo()");

    // To log an exception (e)
    logger.error("foo()", e);

Maven

Maven 2 dependency:

<dependency>
  <groupId>log4j</groupId>
  <artifactId>log4j</artifactId>
  <version>1.2.14</version>
</dependency>

Sample

Using a daily rolling file appender:

log4j.rootLogger=info, R

log4j.appender.R=org.apache.log4j.DailyRollingFileAppender
log4j.appender.R.File=logger.log
log4j.appender.R.Append=true
log4j.appender.R.DatePattern='.'yyyy-MM-dd
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%d [%p] %t %c %n%m%n

# Adjust the logging level for specific classes
#log4j.logger.com.sample.Cluster=debug

Using a rolling file appender. This might be good for a service as it uses a limited amount of disk space:

log4j.rootLogger=error, R

log4j.appender.R=org.apache.log4j.RollingFileAppender
log4j.appender.R.File=logger.log
log4j.appender.R.MaxFileSize=3MB
log4j.appender.R.MaxBackupIndex=3
log4j.appender.R.Append=true
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%d [%p] %t %c %n%m%n

Layout

http://logging.apache.org/log4j/docs/api/org/apache/log4j/PatternLayout.html PatternLayout

c	Category name		For the category name `a.b.c` the pattern `%c{2}` will output `b.c`.
C	Fully qualified class name	Slow	For the class name `org.apache.xyz.SomeClass`, the pattern `%C{1}` will output `SomeClass`.
d	Date		`%d`, `%d{ISO8601}` or `%d{ABSOLUTE}` Or a date format (but this is slow): `%d{dd MMM yyyy HH:mm:ss,SSS}`
F	File name	Slow
l	Location	Very Slow	usually consists of the fully qualified name of the calling method followed by the callers source the file name and line number between parentheses.
L	Line number	Very Slow
m	Message
M	Method name	Very Slow
n	Line separator (new line)
p	Priority
r	Time elapsed since layout creation
t	Thread name
x	Nested diagnostic context
X	Mapped diagnostic context
%	Modifier

Format modifier	left justify	minimum width	maximum width	comment
%20c	false	20	none	Left pad with spaces if the category name is less than 20 characters long.
%-20c	true	20	none	Right pad with spaces if the category name is less than 20 characters long.
%.30c	NA	none	30	Truncate from the beginning if the category name is longer than 30 characters.
%20.30c	false	20	30	Left pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning.
%-20.30c	true	20	30	Right pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning.

A nice compact layout (although the performance is not great):

log4j.appender.R.layout.ConversionPattern=[%d{HH:mm:ss}] [%p] %C{1} %m%n

Levels

Log4j by default can log messages with the following priority levels.

`trace`	finer-grained informational events than the DEBUG
`debug`	to write debugging messages which should not be printed when the application is in production.
`info`	for messages similar to the “verbose” mode of many applications.
`warn`	for warning messages which are logged to some log but the application is able to carry on without a problem.
`error`	for application error messages which are also logged to some log but, still, the application can hobble along. Such as when some administrator-supplied configuration parameter is incorrect and you fall back to using some hard-coded default value.
`fatal`	for critical messages, after logging of which the application quits abnormally.

To set the logging levels for a specific class.

Set the default rootLogger level to error or fatal.
Adjust the logging level for specific classes. See below for an example.

Levels

This properties file will copy errors to a separate file:

log4j.rootLogger=debug, R, S

log4j.appender.R=org.apache.log4j.DailyRollingFileAppender
log4j.appender.R.File=logger.log
log4j.appender.R.Append=true
log4j.appender.R.DatePattern='.'yyyy-MM-dd
log4j.appender.R.layout=org.apache.log4j.PatternLayout
log4j.appender.R.layout.ConversionPattern=%d [%p] %t %c %m%n

log4j.appender.S=org.apache.log4j.DailyRollingFileAppender
log4j.appender.S.File=logger-error.log
log4j.appender.S.Append=true
log4j.appender.S.DatePattern='.'yyyy-MM-dd
log4j.appender.S.layout=org.apache.log4j.PatternLayout
log4j.appender.S.layout.ConversionPattern=%d [%p] %t %c %m%n
log4j.appender.S.threshold=error

sample-log4j, a sample project in Subversion.

Watch

To watch for changes in the log4j configuration file:

public static void main(final String[] args) {
    /*
     * The constant, DEFAULT_CONFIGURATION_FILE, is deprecated but there is no
     * alternative in this version of log4j.
     * Later version of log4j have the org.apache.log4j.helpers.Constants class
     * which has the DEFAULT_CONFIGURATION_FILE constant.
     *
     * TODO Update to a newer version of log4j when it becomes available in the
     * Maven 2 repository.
     */
    PropertyConfigurator.configureAndWatch(org.apache.log4j.LogManager.DEFAULT_CONFIGURATION_FILE);
    logger.debug("main()");
    MyApp.initialise();
}

Configuration

To stop log4j appending each time:

log4j.appender.R.Append=false

Console

From http://logging.apache.org/log4j/docs/manual.html:

# Set root logger level to DEBUG and its only appender to A1.
log4j.rootLogger=DEBUG, A1

# A1 is set to be a ConsoleAppender.
log4j.appender.A1=org.apache.log4j.ConsoleAppender

# A1 uses PatternLayout.
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

Eclipse

To configure the location of the log4j.properties file see: ../eclipse/log4j.

Note: This is only required if the log4j.properties is not on the classpath.

Maven

To configure the location of the log4j.properties file see:

\\g1\PatrickK\documents\howto\maven\log4j.txt

Note: This is only required if the log4j.properties is not on the classpath.

Old Version (1.1.3)

Information on version 1.1.3 of log4j used by ColdFusion:

Version 1.1.3
ColdFusion logging, ../coldfusion/logging.

Tomcat

log4j on Tomcat, log4j.

Classpath

Had loads of problems with getting the software to find:

log4j.properties

In the documentation the file is supposed to be found in the classpath.

I printed the output of com.mycompany.util.SysProps and found the classpath included only the build folder and not the src folder when running under Eclipse.

My log4j.properties file was previously only in the src folder and it was working. It turns out that my original Eclipse build was copying log4j.properties into the build folder as part of the make process.

To solve the problem I just refreshed the Eclipse Package Explorer window. log4j.properties was added into the list and when I ran the project the file was copied to the build folder.