Throw exception if Kotlin projection requires non-null value but null result present

Closes: #3242
Original Pull Request: #3244

Signed-off-by: Yanming Zhou <zhouyanming@gmail.com>

src/main/java/org/springframework/data/projection/ProjectingMethodInterceptor.java

@@ -16,6 +16,7 @@
 package org.springframework.data.projection;
 
 import java.lang.reflect.Array;
+import java.lang.reflect.Method;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -23,10 +24,13 @@ import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 
+import kotlin.reflect.KFunction;
 import org.aopalliance.intercept.MethodInterceptor;
 import org.aopalliance.intercept.MethodInvocation;
 import org.springframework.core.CollectionFactory;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.convert.ConversionService;
+import org.springframework.data.util.KotlinReflectionUtils;
 import org.springframework.data.util.NullableWrapper;
 import org.springframework.data.util.NullableWrapperConverters;
 import org.springframework.data.util.TypeInformation;
@@ -44,6 +48,7 @@ import org.springframework.util.ObjectUtils;
  * @author Mark Paluch
  * @author Christoph Strobl
  * @author Johannes Englmeier
+ * @author Yanming Zhou
  * @since 1.10
  */
 class ProjectingMethodInterceptor implements MethodInterceptor {
@@ -64,11 +69,13 @@ class ProjectingMethodInterceptor implements MethodInterceptor {
 	@Override
 	public Object invoke(@SuppressWarnings("null") @NonNull MethodInvocation invocation) throws Throwable {
 
-		TypeInformation<?> type = TypeInformation.fromReturnTypeOf(invocation.getMethod());
+		Method method = invocation.getMethod();
+		TypeInformation<?> type = TypeInformation.fromReturnTypeOf(method);
 		TypeInformation<?> resultType = type;
 		TypeInformation<?> typeToReturn = type;
 
 		Object result = delegate.invoke(invocation);
+
 		boolean applyWrapper = false;
 
 		if (NullableWrapperConverters.supports(type.getType())
@@ -83,6 +90,14 @@ class ProjectingMethodInterceptor implements MethodInterceptor {
 			return conversionService.convert(new NullableWrapper(result), typeToReturn.getType());
 		}
 
+		if (result == null) {
+			KFunction<?> function = KotlinDetector.isKotlinType(method.getDeclaringClass()) ?
+					KotlinReflectionUtils.findKotlinFunction(method) : null;
+			if (function != null && !function.getReturnType().isMarkedNullable()) {
+				throw new IllegalArgumentException("Kotlin function '%s' requires non-null return value".formatted(method.toString()));
+			}
+		}
+
 		return result;
 	}
 

src/test/java/org/springframework/data/projection/ProjectingMethodInterceptorUnitTests.java

@@ -43,6 +43,7 @@ import org.springframework.core.convert.support.DefaultConversionService;
  * @author Saulo Medeiros de Araujo
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Yanming Zhou
  */
 @ExtendWith(MockitoExtension.class)
 class ProjectingMethodInterceptorUnitTests {
@@ -204,6 +205,30 @@ class ProjectingMethodInterceptorUnitTests {
 		assertThat(collection).containsOnly(HelperEnum.Helpful);
 	}
 
+	@Test
+	void throwExceptionIfKotlinProjectionRequiresNonNullWithNullResult() throws Throwable {
+
+		MethodInterceptor methodInterceptor = new ProjectingMethodInterceptor(new ProxyProjectionFactory(), interceptor,
+				conversionService);
+
+		when(invocation.getMethod()).thenReturn(Person.class.getMethod("getName"));
+		when(interceptor.invoke(invocation)).thenReturn(null);
+
+		assertThatIllegalArgumentException().isThrownBy(() -> methodInterceptor.invoke(invocation));
+	}
+
+	@Test
+	void returnsNullIfKotlinProjectionDoesNotRequiresNonNullWithNullResult() throws Throwable {
+
+		MethodInterceptor methodInterceptor = new ProjectingMethodInterceptor(new ProxyProjectionFactory(), interceptor,
+				conversionService);
+
+		when(invocation.getMethod()).thenReturn(Person.class.getMethod("getAge"));
+		when(interceptor.invoke(invocation)).thenReturn(null);
+
+		assertThat(methodInterceptor.invoke(invocation)).isNull();
+	}
+
 	/**
 	 * Mocks the {@link Helper} method of the given name to return the given value.
 	 *

src/test/kotlin/org/springframework/data/projection/Person.kt

@@ -0,0 +1,25 @@
+/*
+ * Copyright 2024-2025 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.data.projection
+
+/**
+ * @author Yanming Zhou
+ */
+interface Person {
+	val name: String
+	val age: Int?
+}