Log4j2SpringBootLoggingSystem (log4j-spring-boot) is not compatible with spring-boot 4.0.0-RC1

[seregamorph]

The latest spring-boot release candidate 4.0.0-RC1 is not compatible with Log4j2SpringBootLoggingSystem (2.25.2).
The problem is that Log4j2SpringBootLoggingSystem extends org.springframework.boot.logging.log4j2.Log4J2LoggingSystem and this class changed the signature of it's constructor:

• the constructor is not public anymore (package visible)
• now it has two parameters, not one

As a result, the application bootstrap fails:

    java.lang.NoSuchMethodError: org.springframework.boot.logging.log4j2.Log4J2LoggingSystem: method 'void <init>(java.lang.ClassLoader)' not found
        at org.apache.logging.log4j.spring.boot.Log4j2SpringBootLoggingSystem.<init>(Log4j2SpringBootLoggingSystem.java:77)
        at org.apache.logging.log4j.spring.boot.Log4j2SpringBootLoggingSystem$Factory.getLoggingSystem(Log4j2SpringBootLoggingSystem.java:266)
        at org.springframework.boot.logging.DelegatingLoggingSystemFactory.getLoggingSystem(DelegatingLoggingSystemFactory.java:46)
        at org.springframework.boot.logging.LoggingSystem.get(LoggingSystem.java:170)

[vy]

@seregamorph, thanks so much for the report. This is a known issue for an ongoing effort – see spring-projects/spring-boot#47424. We expect to resolve the mismatch in the upcoming 2.26.0 release.

@ppkarwasz, since you authored spring-projects/spring-boot#47424, do you want to pick this one up, or shall I go for it?

[seregamorph]

Thanks for the clarification.

Please note that https://repo.maven.apache.org/maven2/org/springframework/boot/spring-boot-dependencies/4.0.0-RC1/spring-boot-dependencies-4.0.0-RC1.pom defines dependency to log4j as

<log4j2.version>2.25.2</log4j2.version>

And many (probably most) of modern spring-boot projects rely on this dependencyManagement as default. Which means that 2.26.0 should be released before spring-boot 4.0.0 GA

(I guess it's an obvious factor, just note that this may block early adoption of spring-boot 4 release candidates like happened in my project)

[ppkarwasz]

Hi @seregamorph,

We expect to resolve the mismatch in the upcoming 2.26.0 release.

@vy’s statement refers to work planned for Log4j 2.26.0, that will affect integration with Spring Boot 4.1.x and beyond. It doesn’t apply to the issue you’re encountering here.

In your case, the root cause is different. The log4j-spring-boot module, which contains the Log4j2SpringBootLoggingSystem class, was designed specifically for Spring Boot 2.x to address some shortcomings of Spring Boot's built-in Log4J2LoggingSystem. Starting with Spring Boot 3.x, this module became unnecessary because its functionality was merged into Spring Boot.

As a result:

• log4j-spring-boot is not binary compatible with Spring Boot 4.x
• It has effectively been deprecated since Spring Boot 3.x
• We currently do not plan to release a log4j-spring-boot4 module

If your project targets Spring Boot 3.x or 4.x, the recommended solution is simply to remove the log4j-spring-boot dependency and rely on the native Spring Boot Log4j2 integration, which already provides equivalent functionality.

Could you share why your project still depends on log4j-spring-boot?
Are there specific features or configuration options from that module that you feel are missing in the Spring Boot Log4j2 integration?

[seregamorph]

@ppkarwasz thanks for this explanation. Indeed, simply removing the dependency is fine. This was a leftover during migration from spring-boot 2.7 to 3.x which did not bother us at all.

This can be closed then (and it's good somebody can find an answer in this issue with similar cases in the future)