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。

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  * Licensed to the Apache Software Foundation (ASF) under one
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 package org.apache.hadoop.classification;   

 import java.lang.annotation.Documented;   

 /**
  * Annotation to inform users of a package, class or method's intended audience.
  */
 @InterfaceAudience.Public
 @InterfaceStability.Evolving
 public class InterfaceAudience {
   /**
    * Intended for use by any project or application.
    */
   @Documented public @interface Public {};   

   /**
    * Intended only for the project(s) specified in the annotation.
    * For example, "Common", "HDFS", "MapReduce", "ZooKeeper", "HBase".
    */
   @Documented public @interface LimitedPrivate {
     String[] value();
   };   

   /**
    * Intended for use only within Hadoop itself.
    */
   @Documented public @interface Private {};   

   private InterfaceAudience() {} // Audience can't exist on its own
 }