Move generating Java code based on protobuf into a separate project

Protobuf Gradle plugin expects that Gradle `java` plugin is present in the project, but KMP deprecated compatibility with `java` plugin. Soon this will become an error.

To avoid, project error - generating Java code based on protobuf was moved into a separate Gradle subproject. 'kotlinx-serialization-protobuf' project Jvm test sources has now a normal dependency on this new subproject.

Author: Tapchicoma

build.gradle.kts

@@ -166,7 +166,12 @@ gradle.taskGraph.whenReady {
 
 // == projects lists and flags ==
 // getters are required because of variable lazy initialization in Gradle
-val unpublishedProjects get() = setOf("benchmark", "guide", "kotlinx-serialization-json-tests")
+val unpublishedProjects get() = setOf(
+    "benchmark",
+    "guide",
+    "kotlinx-serialization-json-tests",
+    "proto-test-model",
+)
 val excludedFromBomProjects get() = unpublishedProjects + "kotlinx-serialization-bom"
 val globalCompilerArgs
     get() = listOf(

formats/protobuf/build.gradle.kts

@@ -2,31 +2,16 @@
  * Copyright 2017-2024 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
  */
 import Java9Modularity.configureJava9ModuleInfo
-import com.google.protobuf.gradle.*
-import org.gradle.kotlin.dsl.protobuf
 
 plugins {
-    java // Needed for protobuf plugin only
     kotlin("multiplatform")
 
     alias(libs.plugins.serialization)
-    alias(libs.plugins.protobuf)
 
     id("native-targets-conventions")
     id("source-sets-conventions")
 }
 
-protobuf {
-    protobuf.protoc {
-        // Download from repositories
-        artifact = libs.protoc.get().toString()
-    }
-}
-
-tasks.clean {
-    delete(protobuf.protobuf.generatedFilesBaseDir)
-}
-
 kotlin {
     sourceSets {
         configureEach {
@@ -40,24 +25,12 @@ kotlin {
         }
 
         jvmTest {
-            kotlin.srcDirs(file("${protobuf.protobuf.generatedFilesBaseDir}/test/java"))
-
             dependencies {
-                implementation(libs.protobuf.java)
+                implementation(project(":kotlinx-serialization-protobuf:proto-test-model"))
                 implementation(libs.kotlintest)
             }
         }
     }
 }
 
-sourceSets.test {
-    extensions.configure<SourceDirectorySet>("proto") {
-        srcDirs("testProto", "jvmTest/resources/common")
-    }
-}
-
-tasks.compileTestKotlinJvm {
-    dependsOn("generateTestProto")
-}
-
 configureJava9ModuleInfo()

formats/protobuf/proto-test-model/build.gradle.kts

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2017-2025 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
+ */
+
+import com.google.protobuf.gradle.*
+import org.gradle.kotlin.dsl.protobuf
+
+plugins {
+    `java-library`
+    alias(libs.plugins.protobuf)
+}
+
+protobuf {
+    protobuf.protoc {
+        // Download from repositories
+        artifact = libs.protoc.get().toString()
+    }
+}
+
+java.toolchain.languageVersion.set(JavaLanguageVersion.of(jdkToolchainVersion))
+
+tasks.clean {
+    delete(protobuf.protobuf.generatedFilesBaseDir)
+}
+
+sourceSets.main {
+    extensions.configure<SourceDirectorySet>("proto") {
+        srcDirs("testProto", "../jvmTest/resources/common")
+    }
+}
+
+dependencies {
+    api(libs.protobuf.java)
+}

formats/protobuf/testProto/test_data.proto renamed to formats/protobuf/proto-test-model/testProto/test_data.proto

@@ -63,6 +63,8 @@ project(":kotlinx-serialization-json-tests").projectDir = file("./formats/json-t
 
 include(":kotlinx-serialization-protobuf")
 project(":kotlinx-serialization-protobuf").projectDir = file("./formats/protobuf")
+include(":kotlinx-serialization-protobuf:proto-test-model")
+project(":kotlinx-serialization-protobuf:proto-test-model").projectDir = file("./formats/protobuf/proto-test-model")
 
 include(":kotlinx-serialization-cbor")
 project(":kotlinx-serialization-cbor").projectDir = file("./formats/cbor")