Add Private annotation.

This was originally added as part of #4435, which was reverted.

core/src/main/scala/org/apache/spark/annotation/Private.java

@@ -0,0 +1,41 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.apache.spark.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * A class that is considered private to the internals of Spark -- there is a high-likelihood
+ * they will be changed in future versions of Spark.
+ *
+ * This should be used only when the standard Scala / Java means of protecting classes are
+ * insufficient.  In particular, Java has no equivalent of private[spark], so we use this annotation
+ * in its place.
+ *
+ * NOTE: If there exists a Scaladoc comment that immediately precedes this annotation, the first
+ * line of the comment must be ":: Private ::" with no trailing blank line. This is because
+ * of the known issue that Scaladoc displays only either the annotation or the comment, whichever
+ * comes first.
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target({ElementType.TYPE, ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER,
+        ElementType.CONSTRUCTOR, ElementType.LOCAL_VARIABLE, ElementType.PACKAGE})
+public @interface Private {}