Usage of pre-release SLF4J 1.8 breaks compatibility with majority of libraries that use SLF4J

[antjori]

• Framework version: 1.5
• Implementations: Spring Boot 2.3.1

Scenario

A simple Kotlin + Gradle + Springboot2 application that exposes a GET endpoint.

Expected behavior

On startup the OOB Springboot 2 logs should be displayed without any SLF4J warnings:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.1.RELEASE)

2020-06-15 10:27:40.138  INFO 46658 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2020-06-15 10:27:40.163  INFO 46658 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2020-06-15 10:27:40.163  INFO 46658 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.36]
2020-06-15 10:27:40.286  INFO 46658 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2020-06-15 10:27:40.286  INFO 46658 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1597 ms
2020-06-15 10:27:40.480  INFO 46658 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-06-15 10:27:40.719  INFO 46658 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-06-15 10:27:40.740  INFO 46658 --- [           main] ApplicationKt  : Started ApplicationKt in 4.021 seconds (JVM running for 5.401)

Actual behavior

On startup - both local or remote - the following warning is logged, followed by the Spring logo:

SLF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.
SLF4J: Class path contains SLF4J bindings targeting slf4j-api versions prior to 1.8.
SLF4J: Ignoring binding found at [jar:file:/Users/gregor/.gradle/caches/modules-2/files-2.1/ch.qos.logback/logback-classic/1.2.3/7c4f3c474fb2c041d8028740440937705ebb473a/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#ignoredBindings for an explanation.

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.1.RELEASE)

There's also a similar issue reported: #80.

Apparently, this is happening due to the bump of slf4j-api dependency version from 1.7.25 to 1.8.0-beta2.

If it's okay, I'll follow up with a PR that sets slf4j-api version to 1.7.30.

[sapessi]

Release 1.5.1 is making its way to maven central and should be available later today. 🚀