Spring nullable annotation generates unknown enum constant warning

This warning is caused by the javax.annotation.meta.When enum not being available to your projects runtime (org.springframework.lang.Nullable references this enum but it is not made available automatically). You need to bring in a JSR305 implementation to fix this warning.

the Google find bugs repo includes a JSR305 implementation that should fix the problem:
https://mvnrepository.com/artifact/com.google.code.findbugs/jsr305

since you are using gradle, add the dependency to your build.gradle script:

...
dependencies {
    ...

     // https://mvnrepository.com/artifact/com.google.code.findbugs/jsr305
    implementation 'com.google.code.findbugs:jsr305:3.0.2'

    ...
}
...

do a clean build and the error should go away

If you do not want to use the com.google.code.findbugs group’s artifact you can try another from this list:
https://mvnrepository.com/search?q=JSR305

references:

• Strange Eclipse IDE error javax.annotation.meta.When #Java
• What happened to Java’s @OverridingMethodMustCallSuper?
• https://github.com/sbrannen/runtime-annotations/blob/master/src/main/java/org/example/SpringApplication.java