Fix check for primitive return types

README.md

@@ -66,6 +66,10 @@ 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

@@ -32,13 +32,15 @@
 import org.zalando.baigan.annotation.BaiganConfig;
 import org.zalando.baigan.annotation.ConfigurationServiceScan;
 
+import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 import static java.util.stream.Collectors.toMap;
 import static org.zalando.baigan.proxy.ProxyUtils.createKey;
@@ -112,15 +114,25 @@ 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));
+        Map<String, Type> configTypesByKey = baiganConfigClasses.stream()
+                .flatMap(clazz -> Stream.of(clazz.getMethods()).map(method -> mapToConfigType(clazz, method)))
+                .collect(toMap(c -> c.key, c -> c.type));
         GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
         beanDefinition.setBeanClass(BaiganConfigClasses.class);
         beanDefinition.getPropertyValues().add("configTypesByKey", configTypesByKey);
         registry.registerBeanDefinition("baiganConfigClasses", beanDefinition);
     }
 
+    private ConfigType mapToConfigType(final Class<?> clazz, final Method method) {
+        final String key = createKey(clazz, method);
+        final Class<?> returnType = method.getReturnType();
+        if (returnType.isPrimitive()) {
+            throw new IllegalArgumentException("Config " + key + " has an illegal return type " + returnType +
+                    ". Primitives are not supported as return type!");
+        }
+        return new ConfigType(key, method.getGenericReturnType());
+    }
+
     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();
+}