Fix NoClassDefFoundError for kotlin.reflect on pure-Java projects

springdoc-openapi-starter-common/src/main/java/org/springdoc/core/configuration/SpringDocKotlinConfiguration.kt

@@ -69,12 +69,6 @@ class SpringDocKotlinConfiguration() {
 			.addDeprecatedType(Deprecated::class.java)
 	}
 
-	@Bean
-	@Lazy(false)
-	fun kotlinRequiredPropertyCustomizer(objectMapperProvider: ObjectMapperProvider): KotlinRequiredPropertyCustomizer {
-		return KotlinRequiredPropertyCustomizer(objectMapperProvider)
-	}
-
 	@ConditionalOnClass(name = ["kotlin.reflect.full.KClasses"])
 	class KotlinReflectDependingConfiguration {
 
@@ -98,6 +92,12 @@ class SpringDocKotlinConfiguration() {
 		fun kotlinModelConverter(objectMapperProvider: ObjectMapperProvider): KotlinInlineClassUnwrappingConverter {
 			return KotlinInlineClassUnwrappingConverter(objectMapperProvider)
 		}
+		@Bean
+		@Lazy(false)
+		@ConditionalOnMissingBean
+		fun kotlinRequiredPropertyCustomizer(objectMapperProvider: ObjectMapperProvider): KotlinRequiredPropertyCustomizer {
+			return KotlinRequiredPropertyCustomizer(objectMapperProvider)
+		}
 
 		@Bean
 		@ConditionalOnProperty(

springdoc-openapi-starter-common/src/test/java/org/springdoc/core/configuration/SpringDocKotlinConfigurationTest.java

@@ -0,0 +1,43 @@
+package org.springdoc.core.configuration;
+
+import org.junit.jupiter.api.Test;
+import org.springdoc.core.customizers.KotlinRequiredPropertyCustomizer;
+import org.springdoc.core.properties.SpringDocConfigProperties;
+
+import org.springframework.boot.autoconfigure.AutoConfigurations;
+import org.springframework.boot.test.context.FilteredClassLoader;
+import org.springframework.boot.test.context.runner.WebApplicationContextRunner;
+import org.springframework.boot.webmvc.autoconfigure.WebMvcAutoConfiguration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Regression test for GitHub issue #54.
+ *
+ * <p>The {@code kotlinRequiredPropertyCustomizer} bean must only be created when
+ * {@code kotlin-reflect} is on the classpath. Previously it was declared directly in
+ * {@link SpringDocKotlinConfiguration} (gated only by kotlin-stdlib presence), which forced a
+ * {@code NoClassDefFoundError: kotlin/reflect/full/KClasses} for pure-Java projects that have
+ * kotlin-stdlib but not kotlin-reflect on the classpath.</p>
+ */
+class SpringDocKotlinConfigurationTest {
+
+	private final WebApplicationContextRunner runner = new WebApplicationContextRunner()
+			.withPropertyValues("springdoc.api-docs.enabled=true")
+			.withConfiguration(AutoConfigurations.of(
+					WebMvcAutoConfiguration.class,
+					SpringDocConfiguration.class,
+					SpringDocConfigProperties.class,
+					SpringDocKotlinConfiguration.class));
+
+	@Test
+	void kotlinRequiredPropertyCustomizerRegisteredWhenKotlinReflectPresent() {
+		runner.run(context -> assertThat(context).hasBean("kotlinRequiredPropertyCustomizer"));
+	}
+
+	@Test
+	void kotlinRequiredPropertyCustomizerNotRegisteredWhenKotlinReflectAbsent() {
+		runner.withClassLoader(new FilteredClassLoader("kotlin.reflect.full.KClasses"))
+				.run(context -> assertThat(context).doesNotHaveBean(KotlinRequiredPropertyCustomizer.class));
+	}
+}