Last week, I wrote a post that described how to hack the Maven dependency resolution system. I admit it was a dirty hack, it's even in the post name.

But I got it wrong. Thanks Stéphane Nicoll for pointing it out:

Even though I linked the documentation, I misread it. Here's my mea culpa.

As Stéphane states, you can exclude the default dependency from the parent spring-boot-starter starter:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter</artifactId>
  <exclusions>
    <exclusion>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-logging</artifactId>
    </exclusion>
  </exclusions>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

Let's check the mvn dependency:tree output:

[INFO] com.example:demo:jar:0.0.1-SNAPSHOT
[INFO] +- org.springframework.boot:spring-boot-starter:jar:2.4.1:compile          // 2
[INFO] |  +- org.springframework.boot:spring-boot:jar:2.4.1:compile
[INFO] |  |  \- org.springframework:spring-context:jar:5.3.2:compile
[INFO] |  +- org.springframework.boot:spring-boot-autoconfigure:jar:2.4.1:compile
[INFO] |  +- jakarta.annotation:jakarta.annotation-api:jar:1.3.5:compile
[INFO] |  +- org.springframework:spring-core:jar:5.3.2:compile
[INFO] |  |  \- org.springframework:spring-jcl:jar:5.3.2:compile
[INFO] |  \- org.yaml:snakeyaml:jar:1.27:compile
[INFO] +- org.springframework.boot:spring-boot-starter-log4j2:jar:2.4.1:compile   // 1
[INFO] |  +- org.apache.logging.log4j:log4j-slf4j-impl:jar:2.13.3:compile
[INFO] |  |  +- org.slf4j:slf4j-api:jar:1.7.30:compile
[INFO] |  |  \- org.apache.logging.log4j:log4j-api:jar:2.13.3:compile
[INFO] |  +- org.apache.logging.log4j:log4j-core:jar:2.13.3:compile
[INFO] |  +- org.apache.logging.log4j:log4j-jul:jar:2.13.3:compile
[INFO] |  \- org.slf4j:jul-to-slf4j:jar:1.7.30:compile
[INFO] +- org.springframework.boot:spring-boot-starter-actuator:jar:2.4.1:compile // 2
[INFO] |  +- org.springframework.boot:spring-boot-actuator-autoconfigure:jar:2.4.1:compile
[INFO] |  |  +- org.springframework.boot:spring-boot-actuator:jar:2.4.1:compile
[INFO] |  |  +- com.fasterxml.jackson.core:jackson-databind:jar:2.11.3:compile
[INFO] |  |  |  +- com.fasterxml.jackson.core:jackson-annotations:jar:2.11.3:compile
[INFO] |  |  |  \- com.fasterxml.jackson.core:jackson-core:jar:2.11.3:compile
[INFO] |  |  \- com.fasterxml.jackson.datatype:jackson-datatype-jsr310:jar:2.11.3:compile
[INFO] |  \- io.micrometer:micrometer-core:jar:1.6.2:compile
[INFO] |     +- org.hdrhistogram:HdrHistogram:jar:2.1.12:compile
[INFO] |     \- org.latencyutils:LatencyUtils:jar:2.0.3:runtime

The Log4J2 starter is present as expected
None of the starters brings in the SLF4J dependency

Besides that specific point, there are a couple of unrelated lessons:

Remember to check the documentation
Remember to check the documentation once more
Be surrounded by people who are willing to help correct you
Admit when you're wrong
Most importantly, it's less damaging to your pride to ask for feedback before posting than to publish such a fixing post after

To go further:

Configure Log4j for Logging

Originally published at A Java Geek on December 20^th 2020

Feedback on the Log4J2 hack in Spring Boot