I recently ran into an interesting issue when I added the slf4j-timbre dependency to a project. As soon as the dependency was added the project would fail to build and I'd see the following error:

Caused by: java.io.FileNotFoundException: Could not locate clojure/tools/reader/impl/ExceptionInfo__init.class or clojure/tools/reader/impl/ExceptionInfo.clj on classpath.

The slf4j-timbre library does not depend on clojure.tools.reader , and at first glance there's nothing in it that should've caused problems. I did notice that the library depends on com.taoensso/timbre 4.1.4 that in turn depends on com.taoensso/encore 2.18.0 , and it uses on an older version of clojure.tools.reader .

At this point I thought the solution would be simple. I'd just include the latest version of encore and everything should work fine. However that didn't turn out to be the case.

I decided to take another look at slf4j-timbre to see what else might be happening. At this point I noticed that it uses :aot :all in the project configuration. This causes the library to be compiled to Java classes as opposed to being distributed at source. This is necessary since the library has to implement the SLF4J interface and has to provide a Java class in its implementation.

When the namespace that references Timbre is compiled then any namespaces it depends on are also compiled and packaged in the jar . These compiled classes will take precedence over the source dependencies when the library is included in the project.

So, even though I was explicitly including the version of encore that uses the latest clojure.tools.reader , the compiled version packaged in slf4j-timbre would end up being used causing the exception above. As far as I can tell there's no way to overwrite these in the project configuration.

Implications for Luminus

Unfortunately, Luminus dependencies require both a SLF4J compliant logger and the latest clojure.tools.reader . While I think Timbre is an excellent library, it's just not the right fit at the moment.

Luckily, clojure.tools.logging provides a SLF4J compliant API for Clojure logging. The latest release of Luminus uses clojure.tools.logging along with the log4j library as the default logging implementation. It's a mature library that has excellent performance and provides a plethora of logging appenders.

Since log4j can be configured using a properties file, it fits the Luminus approach of using 12 factor style configuration. The library will look for a file called log4j.properties on the classpath to get its default configuration. Luminus packages this file in the resources folder with the following configuration:

### stdout appender log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.Target=System.out log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=[%d][%p][%c] %m%n ### rolling file appender log4j.appender.R=org.apache.log4j.RollingFileAppender log4j.appender.R.File=./log/app-name.log log4j.appender.R.MaxFileSize=100KB log4j.appender.R.MaxBackupIndex=20 log4j.appender.R.layout=org.apache.log4j.PatternLayout log4j.appender.R.layout.ConversionPattern=[%d][%p][%c] %m%n ### root logger sets the minimum logging level ### and aggregates the appenders log4j.rootLogger=DEBUG, stdout, R

As you can see the configuration is very straight forward, it's also well documented. The default configuration can be overridden at runtime by setting the :log-config environment variable. You can now create a custom logging configuration on the target system and then set an environment variable to point to it as seen below:

export LOG_CONFIG=prod-log.properties

I think that the new approach provides a solid solution for most situations with minimal changes from the existing behavior.

Final Thoughts

The moral of the story is that you want to be very careful when using AOT in libraries. Whenever possible it is best to avoid it, and if you do have to use it then try to find the minimal subset of the namespaces that absolutely have to be compiled.