Apache InterfaceAudience

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 }