InterfaceAudience 类包含三个注解类型，用来被说明被他们注解的类型的潜在的使用范围（audience）。
@InterfaceAudience.Public： 对所有工程和应用可用
@InterfaceAudience.LimitedPrivate： 仅限于某些本项目的衍生外围项目
@InterfaceAudience.Private： 仅限于本项目自身
与 Java 自身的public private不同的是，java自身的权限指的是代码层面的，用户不遵循就会报错（比如外部调用了private）。
但是通常情况下Java的权限并不能代表用户所期望的权限，有时可能是因为package划分的原因，一些不希望对外的类也需要public。所以InterfaceAudience实际提供的则是逻辑层面的注释，以一种规范告诉使用者原开发者的意图，但不具有强制性。但作为一种良好的项目风格也是一个不错的选择。
1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one
3 * or more contributor license agreements. See the NOTICE file
4 * distributed with this work for additional information
5 * regarding copyright ownership. The ASF licenses this file
6 * to you under the Apache License, Version 2.0 (the
7 * "License"); you may not use this file except in compliance
8 * with the License. You may obtain a copy of the License at
9 *
10 * http://www.apache.org/licenses/LICENSE-2.0
11 *
12 * Unless required by applicable law or agreed to in writing, software
13 * distributed under the License is distributed on an "AS IS" BASIS,
14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15 * See the License for the specific language governing permissions and
16 * limitations under the License.
17 */
18 package org.apache.hadoop.classification;
19
20 import java.lang.annotation.Documented;
21
22 /**
23 * Annotation to inform users of a package, class or method's intended audience.
24 */
25 @InterfaceAudience.Public
26 @InterfaceStability.Evolving
27 public class InterfaceAudience {
28 /**
29 * Intended for use by any project or application.
30 */
31 @Documented public @interface Public {};
32
33 /**
34 * Intended only for the project(s) specified in the annotation.
35 * For example, "Common", "HDFS", "MapReduce", "ZooKeeper", "HBase".
36 */
37 @Documented public @interface LimitedPrivate {
38 String[] value();
39 };
40
41 /**
42 * Intended for use only within Hadoop itself.
43 */
44 @Documented public @interface Private {};
45
46 private InterfaceAudience() {} // Audience can't exist on its own
47 }