Add Kotlinx Serialization support to BindingReflectionHintsRegistrar

Closes gh-28635

Author: sdeleuze

spring-core/spring-core.gradle

@@ -3,6 +3,7 @@ import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
 description = "Spring Core"
 
 apply plugin: "kotlin"
+apply plugin: "kotlinx-serialization"
 
 // spring-core includes asm, javapoet and repackages cglib, inlining all into the
 // spring-core jar. cglib itself depends on asm and is therefore further transformed by
@@ -68,6 +69,7 @@ dependencies {
 	testImplementation("io.projectreactor:reactor-test")
 	testImplementation("io.projectreactor.tools:blockhound")
 	testImplementation("org.skyscreamer:jsonassert")
+	testImplementation("org.jetbrains.kotlinx:kotlinx-serialization-json")
 	testFixturesImplementation("com.google.code.findbugs:jsr305")
 	testFixturesImplementation("org.junit.platform:junit-platform-launcher")
 	testFixturesImplementation("org.junit.jupiter:junit-jupiter-api")

spring-core/src/main/java/org/springframework/aot/hint/support/BindingReflectionHintsRegistrar.java

@@ -35,8 +35,10 @@
 import org.springframework.aot.hint.MemberCategory;
 import org.springframework.aot.hint.ReflectionHints;
 import org.springframework.aot.hint.TypeHint.Builder;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
+import org.springframework.util.ClassUtils;
 
 /**
  * Register the necessary reflection hints so that the specified type can be
@@ -55,6 +57,8 @@ public class BindingReflectionHintsRegistrar {
 	private static final Consumer<ExecutableHint.Builder> INVOKE = builder -> builder
 			.withMode(ExecutableMode.INVOKE);
 
+	private static final String KOTLIN_COMPANION_SUFFIX = "$Companion";
+
 	/**
 	 * Register the necessary reflection hints to bind the specified types.
 	 * @param hints the hints instance to use
@@ -103,6 +107,14 @@ private void registerMembers(ReflectionHints hints, Class<?> type, Builder build
 					registerReflectionHints(hints, methodParameter.getGenericParameterType());
 				}
 			}
+			String companionClassName = type.getCanonicalName() + KOTLIN_COMPANION_SUFFIX;
+			if (KotlinDetector.isKotlinType(type) && ClassUtils.isPresent(companionClassName, null)) {
+				Class<?> companionClass = ClassUtils.resolveClassName(companionClassName, null);
+				Method serializerMethod = ClassUtils.getMethodIfAvailable(companionClass, "serializer");
+				if (serializerMethod != null) {
+					hints.registerMethod(serializerMethod);
+				}
+			}
 		}
 		catch (IntrospectionException ex) {
 			if (logger.isDebugEnabled()) {

spring-core/src/test/kotlin/org/springframework/aot/hint/support/KotlinBindingReflectionHintsRegistrarTests.kt

@@ -0,0 +1,67 @@
+/*
+ * Copyright 2002-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.aot.hint.support
+
+import org.assertj.core.api.Assertions.assertThat
+import org.assertj.core.api.ThrowingConsumer
+import org.junit.jupiter.api.Test
+import org.springframework.aot.hint.*
+
+/**
+ * Tests for Kotlin support in [BindingReflectionHintsRegistrar].
+ *
+ * @author Sebastien Deleuze
+ */
+class KotlinBindingReflectionHintsRegistrarTests {
+
+	private val bindingRegistrar = BindingReflectionHintsRegistrar()
+
+	private val hints = RuntimeHints()
+
+	@Test
+	fun `Register type for Kotlinx serialization`() {
+		bindingRegistrar.registerReflectionHints(hints.reflection(), SampleSerializableClass::class.java)
+		assertThat(hints.reflection().typeHints()).satisfiesExactlyInAnyOrder(
+			ThrowingConsumer { typeHint: TypeHint ->
+				assertThat(typeHint.type).isEqualTo(TypeReference.of(String::class.java))
+				assertThat(typeHint.memberCategories).isEmpty()
+				assertThat(typeHint.constructors()).isEmpty()
+				assertThat(typeHint.fields()).isEmpty()
+				assertThat(typeHint.methods()).isEmpty()
+			},
+			ThrowingConsumer { typeHint: TypeHint ->
+				assertThat(typeHint.type).isEqualTo(TypeReference.of(SampleSerializableClass::class.java))
+				assertThat(typeHint.methods()).singleElement()
+					.satisfies(ThrowingConsumer { methodHint: ExecutableHint ->
+						assertThat(methodHint.name).isEqualTo("getName")
+						assertThat(methodHint.modes)
+							.containsOnly(ExecutableMode.INVOKE)
+					})
+			},
+			ThrowingConsumer { typeHint: TypeHint ->
+				assertThat(typeHint.type).isEqualTo(TypeReference.of(SampleSerializableClass::class.qualifiedName + "\$Companion"))
+				assertThat(typeHint.methods()).singleElement()
+					.satisfies(ThrowingConsumer { methodHint: ExecutableHint ->
+						assertThat(methodHint.name).isEqualTo("serializer")
+						assertThat(methodHint.modes).containsOnly(ExecutableMode.INVOKE)
+					})
+			})
+	}
+}
+
+@kotlinx.serialization.Serializable
+class SampleSerializableClass(val name: String)