Augmenting Log4J stack traces with class versions

If you have multiple versions of your code in production, it is extremely useful for the log to include the version of the classes when producing a stacktrace, otherwise it is very hard to match the lines in the stacktrace with the lines of the source code (sidenote: there is an optimization in the Sun JVM where - if an exception is thrown "too much" - the JVM stops providing stacktrace - see this article about it and to learn how to disable this feature).

If you are using Log4J as your logging framework, with a little magic you can turn the following stacktrace:

17:47:40,208 ERROR [main] [TestLog4jExtendedStacktrace] An exception has occurred!
java.lang.IllegalArgumentException: Test exception
 at ...Callee.called(TestLog4jExtendedStacktrace.java:144)
 at ...Caller.call(TestLog4jExtendedStacktrace.java:136)
 ...

Into this (note the class versions at the bottom):

java.lang.IllegalArgumentException: Test exception
 at ...Callee.called(TestLog4jExtendedStacktrace.java:144)
 at ...Caller.call(TestLog4jExtendedStacktrace.java:136)
 ...

Callee: $Revision: 56 $
Caller: $Revision: 56 $

The main mechanism is as follows:

• Each class contains a static String field called "VCS_VERSION". This field is set (trough the magic keyword substitution - see the documentation for SVN, CVS) to the last version when the file was committed to your VCS.
• When a stacktrace is sent to the logger, it will look at the classes and if they have the "VCS_VERSION" field, it will output it at the end (it doesn't annotate the stacktrace itself, because some tools - like IDEs - depend on the stacktrace having a certain format for them to be able to process it - like adding one-click "go-to-line" shortcuts)

As usual, the source can be found in the repository.

Some more implementation details: the current way of modifying the stacktrace is quite hackish (adding a filter and trough reflection modifying the passed in LoggingEvent object). However it has the advantage of being usable without modifying your config files (ie. if you have many config files, but can modify it in only one place in the code, this is a good solution. More "proper" alternatives would be to implement a "wrapper" appender (like AsyncAppender) or a wrapper around Layout, however both of these require you to modify your configuration files.