[Bug] protobuf-java should be an optional dependency for Pulsar Java client libraries

[lhotari]

          > IMO, making protobuf-java optional is beyond the scope of this PR. It’s something worth discussing separately at a later stage.

It was always meant to be optional. It's already optional in the unshaded module. It was included as required dependency by mistake and this is why it's getting included in the shaded jar.

It don't think it's beyond the scope to fix the root cause of the issue.

Originally posted by @merlimat in #23468 (comment)

[Shawyeok]

@lhotari
I'm working on this issue, please assign this to me.

[nodece]

@lhotari @Shawyeok Today, I'm testing the Pulsar 3.0.x, I notice this changes break my test:

java.lang.NoClassDefFoundError: com/google/protobuf/MapFieldReflectionAccessor
        at org.apache.pulsar.functions.utils.FunctionConfigUtils.convert(FunctionConfigUtils.java:102)
        at org.apache.pulsar.functions.utils.FunctionConfigUtils.convert(FunctionConfigUtils.java:89)
        at org.apache.pulsar.functions.utils.FunctionConfigUtils.convert(FunctionConfigUtils.java:77)

java.lang.NoClassDefFoundError: com/google/protobuf/MapFieldReflectionAccessor
        at org.apache.pulsar.functions.utils.SourceConfigUtils.convert(SourceConfigUtils.java:97)

Is this expected?

[Shawyeok]

@lhotari @Shawyeok Today, I'm testing the Pulsar 3.0.x, I notice this changes break my test:

java.lang.NoClassDefFoundError: com/google/protobuf/MapFieldReflectionAccessor
        at org.apache.pulsar.functions.utils.FunctionConfigUtils.convert(FunctionConfigUtils.java:102)
        at org.apache.pulsar.functions.utils.FunctionConfigUtils.convert(FunctionConfigUtils.java:89)
        at org.apache.pulsar.functions.utils.FunctionConfigUtils.convert(FunctionConfigUtils.java:77)

java.lang.NoClassDefFoundError: com/google/protobuf/MapFieldReflectionAccessor
        at org.apache.pulsar.functions.utils.SourceConfigUtils.convert(SourceConfigUtils.java:97)

Is this expected?

@nodece
Does your classpath include the com.google.protobuf:protobuf-java JAR? If so, which version is it?

After #23632, users need to explicitly include the protobuf-java dependency.

[nodece]

@Shawyeok Thank you! These changes may affect some users.

Right now, The com.google.protobuf:protobuf-java dependency has been added to my project, and it works fine.