DATACMNS-1015 - Introduced IdentifierAccessor.getRequiredIdentifier().

The new method allows to look up identifiers and immediately fail in the case of absence. Introduced the method as default method but also a new base type TargetAwareIdentifierAccessor to throw an exception with more context, i.e. the actual target bean we try to look up the identifier on.

src/test/java/org/springframework/data/mapping/TargetAwareIdentifierAccessorUnitTests.java

@@ -0,0 +1,49 @@
+/*
+ * 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.mapping;
+
+import static org.assertj.core.api.Assertions.*;
+
+import java.util.Optional;
+
+import org.junit.Test;
+
+/**
+ * Unit tests for {@link TargetAwareIdentifierAccessor}.
+ * 
+ * @author Oliver Gierke
+ * @soundtrack Anika Nilles - Greenfield (Pikalar)
+ */
+public class TargetAwareIdentifierAccessorUnitTests {
+
+	@Test // DATACMNS-1015
+	public void throwsExceptionContainingInformationAboutTargetIfIdentifierAbsent() {
+
+		Object sample = new Object();
+
+		IdentifierAccessor accessor = new TargetAwareIdentifierAccessor(() -> sample) {
+
+			@Override
+			public Optional<Object> getIdentifier() {
+				return Optional.empty();
+			}
+		};
+
+		assertThatExceptionOfType(IllegalStateException.class)//
+				.isThrownBy(() -> accessor.getRequiredIdentifier())//
+				.withMessageContaining(sample.toString());
+	}
+}