Monday, September 22, 2014

Java SLF4J: Dynamic Log Level

In short

Problem: SLF4J doesn't support to set the log level at runtime.
Solution: Use Lidalia Extensions to SLF4J

The whole story

My Java projects that aren't older than 2008 all use the same logging setup: SLF4J for the interface, Logback for the implementation. It's a widely accepted standard with benefits.

Today I had the need to dynamically set the log level at run time. Just like in Log4j logger.log(priority, message). But there's no such or similar method in the API. With one eyebrow raised I started typing into Google:

Ah, good, apparently I'm not the only one. There must be a simple solution. Stackoverflow already has the matching question. But this time the answer is a surprise:
"There is no way to do this with slf4j."
That's not a statement a programmer comes across often. I shared my reaction with this popular user comment:
"WTF? SLF4J is heralded as the future of java's logging, and this simply use case is not supported?"
The SLF4J API team has arguments not to include these methods.

I wasn't too much interested in the architectural design decisions. I just needed a quick and clean solution. And here is a nice one:

Lidalia Extensions to SLF4J "An extension to SLF4J allowing logging at a level determined at run, rather than compile, time via a Level enum."

It's small, available from Maven central, adds exactly the missing functionality, and works.




public class Example {

    private static final Logger logger = LoggerFactory.getLogger(Example.class);

    public static void main(String[] args) {
        Level level = Level.valueOf(args[0]);
        logger.log(level, "Logged at a level configured at runtime");

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