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clj-polylith-slf4j-logback-example

A small demo with Clojure and Polylith showing how to configure SLF4J + Logback

Background

This polylith workspace has one base: server. This base has logging configured with the following libraries:

  • io.pedestal/pedestal.log
  • org.slf4j/slf4j-api
  • ch.qos.logback/logback-classic

It also configures a development project and server project that generates a jar artifact.

In the development project, logback will use resources/logback-test.xml but in the jar file, we will use bases/server/resources/logback.xml

The most important take away from all this:

logback.xml must at the top of the resources folder. It cannot be nested under something like bases/server/resources/server/logback.xml

Requirements

  • Clojure 1.11
  • Java 21 (although it probably works with other LTS versions)
  • A working REPL (this project used nrepl + conjure)

Demo

JAR File

Using clojure.tools.build, we can generate a jar file like so

clj -T:build uberjar :project server

You should see the following output:

Compiling com.example.server.core...
Building uberjar target/server.jar...
Uberjar is built.

Run the jar like so and note how the l/debug statement is not printed based on our configuration

java -jar projects/server/target/server.jar

INFO  com.example.server.core - {:msg "hello. I show up in production.", :line 10}
ERROR com.example.server.core - {:msg "hello. I show up in production.", :line 11}
WARN  com.example.server.core - {:msg "hello. I show up in production.", :line 12}

REPL

Start a repl

clj -A:dev

You should see the following log output immediately. Note the DEBUG statement because our development configuration allows for debug logs

INFO  com.example.server.core - {:msg "hello. I show up in production.", :line 10}
ERROR com.example.server.core - {:msg "hello. I show up in production.", :line 11}
WARN  com.example.server.core - {:msg "hello. I show up in production.", :line 12}
DEBUG com.example.server.core - {:msg "hello. I show up in development.", :line 13}