hadoop 注解之 interfaceAudience interfaceStability

在阅读hadoop相关源码的过程中，经常见到类似

@InterfaceAudience.Public
@InterfaceStability.Stable

之类的标识。特意查阅了一下相关资料，在此做一个小结

InterfaceAudience

InterfaceAudience 类包含有三个注解类型，用来说明被他们注解的类型的潜在使用范围，即audience

InterfaceAudience的源码如下

/*
* 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.hadoop.classification;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
* 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
@Retention(RetentionPolicy.RUNTIME)
public @interface Public {};

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

/**
* Intended for use only within Hadoop itself.
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface Private {};

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

从源码中很容易看出来：

1.如果标注的是Public，说明被注解的类型对多有工程和应用可用。

2.如果标注的是LimitedPrivate,说明被注解的类型只能用于某些特定的工程或应用，如Common,HDFS,MapReduce,ZooKeeper,HBase等。

3.如果标注的是Private，说明被注解的类型只能用于Hadoop。

2.InterfaceStability

InterfaceStability 类包含三个注解，用于说明被他们注解的类型的稳定性。

InterfaceStability源码如下

/*
* 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.hadoop.classification;

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
* Annotation to inform users of how much to rely on a particular package,
* class or method not changing over time.
* </ul>
*/
@InterfaceAudience.Public
@InterfaceStability.Evolving
public class InterfaceStability {
/**
* Can evolve while retaining compatibility for minor release boundaries.;
* can break compatibility only at major release (ie. at m.0).
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface Stable {};

/**
* Evolving, but can break compatibility at minor release (i.e. m.x)
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface Evolving {};

/**
* No guarantee is provided as to reliability or stability across any
* level of release granularity.
*/
@Documented
@Retention(RetentionPolicy.RUNTIME)
public @interface Unstable {};
}

从上述源码也很容易看出：

1.如果标注的是Stable，说明主版本是稳定的，不同主版本之间可能不兼容。

2.如果标注的是Evolving，说明是不停在变化的，不同小版本之间也可能不兼容。

3.如果标注的是Unstable，说明稳定性没有任何保证。