Mixing log4j 1.x and log4j 2
I have a new application that is being written using log4j2, to take advantage of some of its new features. One of the libraries it uses is a bit older and was built with log4j 1.x. I'm having a problem where the new code in the application writes to the log, but the code in the old library does not write to the log. I even have one class which is a subclass of something in the old library and is also called by code in the old library, and this code also will not write to the log. I have configured the application to use log4j2 version 2.0.2, and I explicitly excluded log4j when referencing the old library as a maven dependency.
How can I get the code in the old library logging into the same file as my new code?
Re-writing the old library should be thought of as a last-resort option
Answer
Are you aware that log4j2 includes a bridge for log4j-1.2? You can use it by removing the old log4j-1.2.17.jar, and include these three jars:
- log4j-api-2.x.jar
- log4j-core-2.x.jar
- log4j-1.2-api-2.x.jar
This will result in all calls that your application makes to the log4-1.2 API to be routed to the log4j2 implementation. The FAQ has a diagram that may clarify things. (Don't forget to remove the old log4j-1.2 jar from the classpath!)