I'm pretty happy with a logging format that I've come up with when using Log4J for output logging. To demonstrate this, let me start by showing what my Log4J log file output format look like these days:

DEBUG | 2008-09-06 10:51:44,817 | DefaultBeanDefinitionDocumentReader.java | 86 | Loading bean definitions
DEBUG | 2008-09-06 10:51:44,848 | AbstractBeanDefinitionReader.java | 185 | Loaded 5 bean definitions from location pattern [samContext.xml]
INFO | 2008-09-06 10:51:44,848 | XmlBeanDefinitionReader.java | 323 | Loading XML bean definitions from class path resource [tmfContext.xml]
DEBUG | 2008-09-06 10:51:44,848 | DefaultDocumentLoader.java | 72 | Using JAXP provider [com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl]
DEBUG | 2008-09-06 10:51:44,848 | BeansDtdResolver.java | 72 | Found beans DTD [http://www.springframework.org/dtd/spring-beans.dtd] in classpath: spring-beans.dtd
DEBUG | 2008-09-06 10:51:44,864 | DefaultBeanDefinitionDocumentReader.java | 86 | Loading bean definitions
DEBUG | 2008-09-06 10:51:45,458 | AbstractAutowireCapableBeanFactory.java | 411 | Finished creating instance of bean 'MS-SQL'
DEBUG | 2008-09-06 10:51:45,458 | DefaultSingletonBeanRegistry.java | 213 | Creating shared instance of singleton bean 'MySQL'
DEBUG | 2008-09-06 10:51:45,458 | AbstractAutowireCapableBeanFactory.java | 383 | Creating instance of bean 'MySQL'
DEBUG | 2008-09-06 10:51:45,458 | AbstractAutowireCapableBeanFactory.java | 459 | Eagerly caching bean 'MySQL' to allow for resolving potential circular references
DEBUG | 2008-09-06 10:51:45,458 | AbstractAutowireCapableBeanFactory.java | 411 | Finished creating instance of bean 'MySQL'
DEBUG | 2008-09-06 10:51:45,458 | DefaultSingletonBeanRegistry.java | 213 | Creating shared instance of singleton bean 'Oracle'
DEBUG | 2008-09-06 10:51:45,458 | AbstractAutowireCapableBeanFactory.java | 383 | Creating instance of bean 'Oracle'
DEBUG | 2008-09-06 10:51:45,458 | AbstractAutowireCapableBeanFactory.java | 459 | Eagerly caching bean 'Oracle' to allow for resolving potential circular references
DEBUG | 2008-09-06 10:51:45,473 | AbstractAutowireCapableBeanFactory.java | 411 | Finished creating instance of bean 'Oracle'
DEBUG | 2008-09-06 10:51:45,473 | DefaultSingletonBeanRegistry.java | 213 | Creating shared instance of singleton bean 'PostgreSQL'
DEBUG | 2008-09-06 10:51:45,473 | AbstractAutowireCapableBeanFactory.java | 383 | Creating instance of bean 'PostgreSQL'
DEBUG | 2008-09-06 10:51:45,473 | AbstractAutowireCapableBeanFactory.java | 459 | Eagerly caching bean 'PostgreSQL' to allow for resolving potential circular references
DEBUG | 2008-09-06 10:51:45,473 | AbstractAutowireCapableBeanFactory.java | 411 | Finished creating instance of bean 'PostgreSQL'
INFO | 2008-09-06 10:51:45,473 | SQLErrorCodesFactory.java | 128 | SQLErrorCodes loaded: [DB2, Derby, H2, HSQL, Informix, MS-SQL, MySQL, Oracle, PostgreSQL, Sybase]

As you can see, I like to split the columns of data using the "|" character, which you're not too likely to find in program output. This makes it easier to parse the files with a Perl or Ruby script when I need to do something like that, and I think it makes the files a little easier on the human eye as well.

Note that I'd really like to include the method name in this output, but the Log4J documentation says this can be a pretty expensive computational task, and you're better off not doing it. The source code line number in the fourth column gives you the exact line number in your Java class where this line of output was printed from anyway, so it's not a great loss.

My log4j.properties file

Now that you've seen the output, here's the log4j.properties file that was used to create that output format:

log4j.rootLogger=INFO, stdout

log4j.appender.stdout=org.apache.log4j.ConsoleAppender

# here's where I specify the layout and the pattern
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p | %d | %F | %L | %m%n

log4j.logger.com.devdaily.myapp=DEBUG
log4j.logger.org.springframework=DEBUG

As you can see this file is very small. If you want to use this format in your Log4J output all you really need to do is copy and paste these two lines of code into your log4j.properties file:

log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p | %d | %F | %L | %m%n

Finally, note that the name stdout needs to match the name of your appender in your Log4J properties file.