Fix check for primitive return types

README.md

@@ -67,6 +67,10 @@ The individual methods may have arbitrary classes as return types, in particular
 
 **Note**: Primitives are not supported as return types as they cannot be null and therefore cannot express a missing configuration value.
 
+> [!CAUTION]
+> Primitives are not supported as return types as they cannot be null and therefore cannot express a missing configuration value.
+> If you use Baigan with Kotlin, it means you need to use nullable primitive types, e.g. `Int?` instead of `Int`.
+
 The above example code enables the application to inject _ExpressFeature_ spring bean into any other Spring bean:
 
 ```Java

src/main/java/org/zalando/baigan/proxy/BaiganConfigClasses.java

@@ -9,11 +9,6 @@ public class BaiganConfigClasses {
     public BaiganConfigClasses() {}
 
     public void setConfigTypesByKey(Map<String, Type> configTypesByKey) {
-        configTypesByKey.forEach((key, value) -> {
-            if (value.getClass().isPrimitive()) {
-                throw new IllegalArgumentException("Config " + key + " has an illegal return type " + value + ". Primitives are not supported as return type.");
-            }
-        });
         this.configTypesByKey = configTypesByKey;
     }
 

src/main/java/org/zalando/baigan/proxy/ConfigurationBeanDefinitionRegistrar.java

@@ -115,12 +115,22 @@ private void createAndRegisterBeanDefinitions(final Set<String> packages,
         Map<String, Type> configTypesByKey = baiganConfigClasses.stream().flatMap(clazz ->
                 Arrays.stream(clazz.getMethods()).map(method -> new ConfigType(createKey(clazz, method), method.getGenericReturnType()))
         ).collect(toMap(c -> c.key, c -> c.type));
+        assertNoPrimitiveTypes(configTypesByKey);
         GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
         beanDefinition.setBeanClass(BaiganConfigClasses.class);
         beanDefinition.getPropertyValues().add("configTypesByKey", configTypesByKey);
         registry.registerBeanDefinition("baiganConfigClasses", beanDefinition);
     }
 
+    private void assertNoPrimitiveTypes(final Map<String, Type> configTypesByKey) {
+        configTypesByKey.forEach((key, value) -> {
+            if (value instanceof Class && ((Class<?>)value).isPrimitive()) {
+                throw new IllegalArgumentException("Config " + key + " has an illegal return type " + value
+                        + ". Primitives are not supported as return type.");
+            }
+        });
+    }
+
     private Class<?> registerAsBean(final BeanDefinitionRegistry registry, final GenericBeanDefinition genericDefinition) {
         try {
             final Class<?> interfaceToImplement = genericDefinition.resolveBeanClass(

src/test/java/org/zalando/baigan/context/ConfigurationBeanDefinitionRegistrarTest.java

@@ -119,4 +119,17 @@ public void whenNothingAnnotatedWithConfigurationServiceScan_shouldThrowExceptio
         when(metaData.getAnnotationAttributes(ConfigurationServiceScan.class.getName())).thenReturn(ImmutableMap.of());
         assertThrows(IllegalArgumentException.class, () -> registrar.registerBeanDefinitions(metaData, registry));
     }
+
+    @Test
+    public void whenConfigurationContainsPrimitiveTypes_shouldThrowException() {
+        when(metaData.getAnnotationAttributes(ConfigurationServiceScan.class.getName())).thenReturn(
+                ImmutableMap.of(
+                        "value", new String[]{"org.zalando.baigan.context.packaged"},
+                        "basePackages", new String[]{},
+                        "basePackageClasses", new Class[]{}
+                )
+        );
+
+        assertThrows(IllegalArgumentException.class, () -> registrar.registerBeanDefinitions(metaData, registry));
+    }
 }

src/test/java/org/zalando/baigan/context/packaged/PrimitiveTypeConfig.java

@@ -0,0 +1,8 @@
+package org.zalando.baigan.context.packaged;
+
+import org.zalando.baigan.annotation.BaiganConfig;
+
+@BaiganConfig
+public interface PrimitiveTypeConfig {
+    boolean enabled();
+}