DATACMNS-1322 - Added @Immutable to allow users to express a persistent entity to be considered immutable.

This is needed for downstream projects that attempt to merge persistent entity instances and previously didn't have a chance to detect that an object had to be set as-is instead of being merged recursively.

src/main/java/org/springframework/data/annotation/Immutable.java

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2018 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.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Annotation for persistent entities to indicate the class is designed in immutable way.
+ * 
+ * @author Oliver Gierke
+ * @since 2.1
+ */
+@Target(ElementType.TYPE)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface Immutable {
+}