Compiling gRPC,with both Kotlin and Java using Maven (various packages not found) - kotlin

Question

I have a mixed kotlin/java project and I wanted to add gRPC to it. I configured the gRPC plugin as usual:
<extensions>
<extension>
<groupId>kr.motd.maven</groupId>
<artifactId>os-maven-plugin</artifactId>
<version>1.6.2</version>
</extension>
</extensions>
<plugins>
<plugin>
<groupId>org.xolstice.maven.plugins</groupId>
<artifactId>protobuf-maven-plugin</artifactId>
<version>0.6.1</version>
<configuration>
<protocArtifact>
com.google.protobuf:protoc:3.11.2:exe:${os.detected.classifier}
</protocArtifact>
<pluginId>grpc-java</pluginId>
<pluginArtifact>
io.grpc:protoc-gen-grpc-java:1.25.0:exe:${os.detected.classifier}
</pluginArtifact>
</configuration>
<executions>
<execution>
<goals>
<goal>compile</goal>
<goal>compile-custom</goal>
</goals>
</execution>
</executions>
</plugin>
...
but when building from scratch
mvn clean && mvn package
the kotlin code would never find the generated sources. When building first from IntelliJ it would work fine - IntelliJ knows what to do.
I tried configuring build-helper-maven-plugin to include the generated-sources directories that the protobuf compiler and stub generator use, but that didn't seem to have any effect (and in fact seems unnecessary).

Answer 1

The answer it turns out is because of the order plugins execute. Generally for kotlin-java hybrid projects you want to compile the kotlin first so that the java compiler knows how to find kotlin classes. The problem is, if you do that the kotlin compiler doesn't see the protoc-generated java classes yet. It's a circular dependency.
It took me a while to figure it out, but to fix it you just have to tell the kotlin compiler where to find the protoc-generated sources like this:
<plugin>
<groupId>org.jetbrains.kotlin</groupId>
<artifactId>kotlin-maven-plugin</artifactId>
<version>${kotlin.version}</version>
<executions>
<execution>
<id>compile</id>
<goals> <goal>compile</goal> </goals>
<configuration>
<sourceDirs>
<sourceDir>${project.basedir}/src/main/java</sourceDir>
<sourceDir>${project.build.directory}/generated-sources/protobuf/java/</sourceDir>
<sourceDir>${project.build.directory}/generated-sources/protobuf/grpc-java/</sourceDir>
</sourceDirs>
</configuration>
</execution>
...
</plugin>
It took me entirely too long to figure that out, so I'm posting this as a self-answered question to perhaps save somebody else some time.
I saw a number of related questions but none exactly the same. Good luck.