Effective Logging in Java/JEE

It all started when i was sitting with a colleague to resolve some application issue, when i noticed something interesting. He was merging the code and my eyes caught the attention of this class “org.apache.log4j.MDC”. This led to few discoveries on the way which  follow here:

What is MDC?

MDC stands for Mapped Diagnostic Context. It helps you to distinguish inter-leaving logs from multiple sources. Let me explain in detail. When we have multiple user-requests coming in for a given servlet, each request of an user is serviced using a thread. This leaves multiple users logging to the same log file and the log statements get inter-mixed. Now, to filter out logs of a particular user, we need to append the user-id to the log statements so that we can grep(search) them in the log file, to make some sense of it.

An obvious way of logging, is to append the user-id in the log statements i.e. log.info(userId+” logged something “);

A non-invasive way of logging is to use MDC. With MDC, you put the user-id in a context-map which is attached to the thread (of each user request) by the logger.

MDC is thread-safe and uses a Map internally to store the context information.[Courtesy : Kalyan Dabburi]

How to use MDC?

a. Configure the information, which needs to be logged (user-id in this case) in the log4j.xml as part of ConversionPattern.

= %d %i - %m - %X{user-id}%n

b. In your respective class, before you start processing the user request, place the actual user-id in the context(MDC).


c. Remove the context information from MDC at the end of the processing.


References :

Logback implementation

Log4J implementation


What is NDC ? Which one to use MDC or NDC?

NDC stands for Nested Diagnostic Context. It is a stack-based implementation of attaching context information. For all purposes, use MDC over NDC, as MDC is memory efficient. For a detailed comparison, click here.

Which logging framework to use? Log4J or SLF4J or logback?

For all new application development, use logback. logback is a run-time implementation of SLF4J. If you have an existing application with Log4J, it is still worth-while to switch to logback. For a detailed explanation, click here.

To understand the evolution of logging in Java and JEE world, refer to this article by Micheal Andrews.


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