Add getRequiredKeySpace to KeyValuePersistentEntity.

Improve nullability handling for entities that are required to provide a keyspace.

Closes #640

Author: mp911de

src/main/java/org/springframework/data/keyvalue/core/mapping/KeyValuePersistentEntity.java

@@ -16,10 +16,15 @@
 package org.springframework.data.keyvalue.core.mapping;
 
 import org.jspecify.annotations.Nullable;
+
+import org.springframework.data.mapping.PersistentEntity;
 import org.springframework.data.mapping.model.MutablePersistentEntity;
 
 /**
+ * KeyValue-specific extension of {@link PersistentEntity} declaring a {@literal keySpace}.
+ *
  * @author Christoph Strobl
+ * @author Mark Paluch
  * @param <T>
  */
 public interface KeyValuePersistentEntity<T, P extends KeyValuePersistentProperty<P>>
@@ -32,4 +37,24 @@ public interface KeyValuePersistentEntity<T, P extends KeyValuePersistentPropert
 	 */
 	@Nullable
 	String getKeySpace();
+
+	/**
+	 * Returns the required {@literal keySpace} or throws an {@link IllegalStateException} if the {@literal keySpace} is
+	 * not defined.
+	 *
+	 * @return the {@literal keySpace} property of this {@link PersistentEntity}.
+	 * @throws IllegalStateException if the {@literal keySpace} is not defined.
+	 * @since 4.0
+	 */
+	default String getRequiredKeySpace() {
+
+		String keySpace = getKeySpace();
+
+		if (keySpace != null) {
+			return keySpace;
+		}
+
+		throw new IllegalStateException(String.format("Required keySpace not defined for %s", getType()));
+	}
+
 }

src/test/java/org/springframework/data/keyvalue/core/mapping/BasicKeyValuePersistentEntityUnitTests.java

@@ -35,12 +35,12 @@
  */
 class BasicKeyValuePersistentEntityUnitTests {
 
-	private KeyValueMappingContext<? extends KeyValuePersistentEntity<?, ?>, ? extends KeyValuePersistentProperty<?>> mappingContext = new KeyValueMappingContext<>();
+	KeyValueMappingContext<? extends KeyValuePersistentEntity<?, ?>, ? extends KeyValuePersistentProperty<?>> mappingContext = new KeyValueMappingContext<>();
 
 	@Test // DATAKV-268
 	void shouldDeriveKeyspaceFromClassName() {
 
-		assertThat(mappingContext.getPersistentEntity(KeyspaceEntity.class).getKeySpace())
+		assertThat(mappingContext.getPersistentEntity(KeyspaceEntity.class).getRequiredKeySpace())
 				.isEqualTo(KeyspaceEntity.class.getName());
 	}
 
@@ -54,7 +54,7 @@ void shouldEvaluateKeyspaceExpression() {
 		persistentEntity.setEvaluationContextProvider(
 				new ExtensionAwareEvaluationContextProvider(Collections.singletonList(new SampleExtension())));
 
-		assertThat(persistentEntity.getKeySpace()).isEqualTo("some_foo");
+		assertThat(persistentEntity.getRequiredKeySpace()).isEqualTo("some_foo");
 	}
 
 	@Test // DATAKV-268
@@ -64,7 +64,7 @@ void shouldEvaluateEntityWithoutKeyspace() {
 		persistentEntity.setEvaluationContextProvider(
 				new ExtensionAwareEvaluationContextProvider(Collections.singletonList(new SampleExtension())));
 
-		assertThat(persistentEntity.getKeySpace()).isEqualTo(NoKeyspaceEntity.class.getName());
+		assertThat(persistentEntity.getRequiredKeySpace()).isEqualTo(NoKeyspaceEntity.class.getName());
 	}
 
 	@Test // GH-461