Gradle: unexpected compiler warning with Log4j `2.25.3`

[ppkarwasz]

This issue is a continuation of the long-running -Werror -Xlint:all saga.

When all of the following apply:

• the build uses Gradle,
• the compiler is JDK 8 through 16,
• compilation is run with -Werror -Xlint:all,
• the code references a Log4j API class annotated with a biz.aQute.bnd.annotation
  (for example PropertiesUtil),

the build fails with a compiler warning promoted to an error.

The failure is caused by a bytecode-level mismatch: Log4j 2.25.3 exposes biz.aQute.bnd.annotation:7.1.0 via Gradle Module Metadata, but that artifact is built for Java 17 (class file version 61) and is therefore incompatible with JDK 8/11 compilers when -Xlint:classfile is enabled.

Example failure:

> Task :compileJava FAILED
.../log4j-api-2.25.3.jar(org/apache/logging/log4j/util/ProviderUtil.class): warning:
Cannot find annotation method 'value()' in type 'aQute.bnd.annotation.spi.ServiceConsumer':
bad class file:
.../biz.aQute.bnd.annotation-7.1.0.jar(aQute/bnd/annotation/spi/ServiceConsumer.class)
    class has wrong version 61.0, should be 52.0
    Please remove or make sure it appears in the correct subdirectory of the classpath.

Workarounds

Several workarounds are available:

1. Use a newer JDK to compile the project.
   Even recent JDKs (including JDK 25) can still target Java 8 bytecode via --release. The JDK used for compilation does not dictate the output bytecode level.

2. Disable the classfile lint category by adding -Xlint:-classfile.
   This category rarely provides actionable information and is the root cause of the failure here. See
   Upgrading to 2.24.1 produces a compile time warning: BaselineIgnore not found #3110 (comment) for additional context.

3. Force a compatible annotation version by adding
   biz.aQute.bnd.annotation:6.4.1 to the application’s compileOnly configuration and enforcing that version in Gradle resolution.

Planned fix

This will be addressed in a future Log4j API release by downgrading the exported biz.aQute.bnd.annotation version to 6.4.1 in the Gradle Module Metadata (via the Gradle Module Metadata Maven Plugin), ensuring compatibility with Java 8 and Java 11 consumers.

[vy]

Force a compatible annotation version

+1

While implementing this change, please document the motivation and add a reference to here.

[ppkarwasz]

I opened bndtools/bnd#7005 to ask for the next biz.aQute.bnd.annotation release to use a Java 8 baseline, if possible.