Support protoc Kotlin code generation

Author: PhilKes

bom/application/pom.xml

@@ -6249,6 +6249,12 @@
                     </exclusion>
                 </exclusions>
             </dependency>
+            <dependency>
+                <groupId>com.google.protobuf</groupId>
+                <artifactId>protobuf-kotlin</artifactId>
+                <version>${protobuf-kotlin.version}</version>
+                <scope>compile</scope>
+            </dependency>
             <dependency>
                 <groupId>com.google.protobuf</groupId>
                 <artifactId>protoc</artifactId>

docs/src/main/asciidoc/grpc-generation-reference.adoc

@@ -165,6 +165,17 @@ quarkus.generate-code.grpc.scan-for-proto-exclude."<groupId>:<artifactId>"=foo/p
 
 Note that `:` characters in the property keys must be escaped.
 
+== Kotlin code generation
+
+`protoc` also supports https://protobuf.dev/reference/kotlin/kotlin-generated/[generating Kotlin code] in addition to the generated Java code.
+
+By default, the Kotlin code generation is enabled if the dependency `io.quarkus:quarkus-kotlin` is present.
+
+To explicitly en-/disable this feature, set the `quarkus.generate-code.grpc.kotlin.generate` property in your `application.properties` file to `true` or `false`.
+
+
+IMPORTANT: When using Gradle, you also have to include `com.google.protobuf:protobuf-kotlin` as a `compileOnly` dependency in your project.
+
 == Skipping code generation
 
 You can skip gRPC code generation using:

extensions/grpc/codegen/src/main/java/io/quarkus/grpc/codegen/GrpcCodeGen.java

@@ -23,6 +23,7 @@
 import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.Predicate;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -66,8 +67,11 @@ public class GrpcCodeGen implements CodeGenProvider {
 
     private static final String USE_ARG_FILE = "quarkus.generate-code.grpc.use-arg-file";
 
+    private static final String GENERATE_KOTLIN = "quarkus.generate-code.grpc.kotlin.generate";
+
     private Executables executables;
     private String input;
+    private boolean hasQuarkusKotlinDependency;
 
     @Override
     public String providerId() {
@@ -95,6 +99,7 @@ public Path getInputDirectory() {
     @Override
     public void init(ApplicationModel model, Map<String, String> properties) {
         this.input = properties.get("quarkus.grpc.codegen.proto-directory");
+        this.hasQuarkusKotlinDependency = containsQuarkusKotlin(model.getDependencies());
     }
 
     @Override
@@ -162,6 +167,10 @@ public boolean trigger(CodeGenContext context) throws CodeGenException {
                         "--grpc_out=" + outDir,
                         "--java_out=" + outDir));
 
+                if (shouldGenerateKotlin(context.config())) {
+                    command.add("--kotlin_out=" + outDir);
+                }
+
                 if (shouldGenerateDescriptorSet(context.config())) {
                     command.add(String.format("--descriptor_set_out=%s", getDescriptorSetOutputFile(context)));
                 }
@@ -296,6 +305,11 @@ private boolean isGeneratingFromAppDependenciesEnabled(Config config) {
                 .filter(value -> !"none".equals(value)).isPresent();
     }
 
+    private boolean shouldGenerateKotlin(Config config) {
+        return config.getOptionalValue(GENERATE_KOTLIN, Boolean.class).orElse(
+                hasQuarkusKotlinDependency);
+    }
+
     private boolean shouldGenerateDescriptorSet(Config config) {
         return config.getOptionalValue(GENERATE_DESCRIPTOR_SET, Boolean.class).orElse(FALSE);
     }
@@ -526,6 +540,16 @@ private static void writePluginExeCmd(Path pluginPath, BufferedWriter writer) th
         writer.newLine();
     }
 
+    private static boolean containsQuarkusKotlin(Collection<ResolvedDependency> dependencies) {
+        return dependencies.stream().anyMatch(new Predicate<ResolvedDependency>() {
+            @Override
+            public boolean test(ResolvedDependency rd) {
+                return rd.getGroupId().equalsIgnoreCase("io.quarkus")
+                        && rd.getArtifactId().equalsIgnoreCase("quarkus-kotlin");
+            }
+        });
+    }
+
     private static class Executables {
 
         final Path protoc;

integration-tests/gradle/src/main/resources/grpc-multi-module-no-java/application/build.gradle.kts

@@ -11,6 +11,7 @@ dependencies {
     implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
     implementation("io.quarkus:quarkus-arc")
     implementation("io.quarkus:quarkus-grpc")
+    compileOnly("com.google.protobuf:protobuf-kotlin")
 
     testImplementation("io.quarkus:quarkus-junit5")
     testImplementation("io.rest-assured:rest-assured")

integration-tests/gradle/src/main/resources/kotlin-grpc-project/build.gradle

@@ -15,6 +15,7 @@ repositories {
 }
 
 dependencies {
+    compileOnly 'com.google.protobuf:protobuf-kotlin'
     implementation 'io.quarkus:quarkus-kotlin'
     implementation 'io.quarkus:quarkus-grpc'
     implementation 'io.quarkus:quarkus-smallrye-graphql'

integration-tests/gradle/src/main/resources/kotlin-grpc-project/src/main/kotlin/org/acme/ExampleResource.kt

@@ -6,11 +6,12 @@ import jakarta.ws.rs.Produces
 import jakarta.ws.rs.core.MediaType
 
 import io.quarkus.example.HelloMsg
+import io.quarkus.example.helloMsg
 
 @Path("/hello")
 class ExampleResource {
 
     @GET
     @Produces(MediaType.TEXT_PLAIN)
-    fun hello() = "hello" + HelloMsg.Status.TEST_ONE.getNumber()
+    fun hello() = "hello" + helloMsg { status = HelloMsg.Status.TEST_ONE }.statusValue
 }

pom.xml

@@ -84,6 +84,7 @@
         <grpc-jprotoc.version>1.2.2</grpc-jprotoc.version>
         <protoc.version>3.25.5</protoc.version>
         <protobuf-java.version>${protoc.version}</protobuf-java.version>
+        <protobuf-kotlin.version>4.29.3</protobuf-kotlin.version>
         <proto-google-common-protos.version>2.53.0</proto-google-common-protos.version>
 
         <!-- Used in the build parent and test BOM (for the junit 5 plugin) and in the BOM (for the API) -->