DATACMNS-1157 - Enforce nullable/non-null API constraints on repository query methods.

We now validate query method invocations to check method parameters whether they accept null values and reject execution if null values are not supported. We derive nullability support from repository interfaces declared using Kotlin and Spring's NonNullApi/Nullable annotations.

We also check whether a query method can return null. If a query method returns null which isn't supposed to do so, then we throw EmptyResultDataAccessException to prevent null return values.

interface UserRepository extends Repository<User, String> {

  List<User> findByLastname(@Nullable String firstname);

  @Nullable
  User findByFirstnameAndLastname(String firstname, String lastname);
}

interface UserRepository : Repository<User, String> {

  fun findByLastname(username: String?): List<User>

  fun findByFirstnameAndLastname(firstname: String, lastname: String): User?
}

Original pull request: #241.

src/test/kotlin/org/springframework/data/repository/core/support/KotlinUserRepository.kt

@@ -0,0 +1,31 @@
+/*
+ * Copyright 2017 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
+ *
+ *      http://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.repository.core.support
+
+import org.springframework.data.repository.Repository
+import org.springframework.data.repository.sample.User
+
+/**
+ * @author Mark Paluch
+ */
+interface KotlinUserRepository : Repository<User, String> {
+
+	fun findByUsername(username: String): User?
+
+	fun findById(username: String): User
+
+	fun findByOptionalId(username: String?): User?
+}

src/test/kotlin/org/springframework/data/util/DummyInterface.kt

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2017 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
+ *
+ *      http://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.util
+
+import org.springframework.data.repository.sample.User
+
+/**
+ * @author Mark Paluch
+ */
+interface DummyInterface {
+
+	fun noReturnValue()
+
+	fun nullableReturnValue(): User?
+
+	fun mandatoryReturnValue(): User
+
+	fun nullableParameter(user: User?)
+
+	fun primitive(number: Int)
+}