想自己写框架？不会写Java注解可不行

{"type":"doc","content":[{"type":"heading","attrs":{"align":null,"level":2}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/d0/d0a2a47c6d1a9d3df79c270273fb6db0.png","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"用注解一时爽，一直用一直爽"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Java后端开发进入spring全家桶时代后，开发一个微服务提供简单的增删改查接口跟玩泥巴似的非常简单，一顿操作猛如虎，回头一看代码加了一堆注解：@Controller @Autowired @Value，面向注解编程变成了大家不可缺少的操作。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"想象一下如果没有注解Java程序员可以要哭瞎:sob:"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"既然注解（annotation）这么重要，用的这么爽，那注解的实现原理你知道么？我猜你只会用注解不会自己写注解（手动滑稽）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"好了，下面的内容带大家从零开始写一个注解，揭开注解神秘的面纱。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"原来注解不神秘"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注解用大白话来说就是一个标记或者说是特殊的注释，如果没有解析这些标记的操作那它啥也不是。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注解的格式如同类或者方法一样有自己特殊的语法，这个语法下文会详细介绍。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"那如何去解析注解呢？这就要用到Java强大的反射功能了。反射大家应该都用过，可以通过类对象获取到这个类的各种信息比如成员变量、方法等，那注解标记能不能通过反射获取呢？当然可以了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所以注解的原理其实很简单，本质上是通过反射功能动态获取注解标记，然后按照不同的注解执行不同的操作，比如@Autowired可以注入一个对象给变量赋值。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"看到这里是不是很躁动啊，来吧自己也撸一个注解。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"造火箭啦，自己动手写一个注解"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"便于大家理解，这里先引入一个场景：在线教育火了，经理让我写一个模块实现学生信息管理功能，考虑到分布式并发问题，经理让我务必加上分布式锁。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"经理问我几天能搞定？我说至少3天。如是脑补了以下代码："}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/02/025609a5e1186b19c97d203d7096c055.png","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"未使用注解前"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"经理走后我在思考，我能不能只花一天时间写完，剩下两天时间用来写博客划水呢？突然灵感来了，我可以把重复的代码逻辑抽出来用注解实现不就节省代码了，哈哈，赶紧写。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用注解之后整个方法清爽了很多，HR小姐姐都夸我写的好呢。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/68/6813e9fa65582d0bf937db7c9e72e4f6.png","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"使用注解后"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"代码已经写完上库了，现在我在划水写博客呢。是不是很简洁很优雅很牛逼，怎么做到的呢，主要分为三步： 1打开冰箱门，2把大象放进去，3把冰箱门关好 。好了，扯远了，大家接着往下看。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"第一步定义一个注解"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/ce/cea249f4c2841d90665b873332a62c0f.png","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注解的三大组成部分"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一个注解可以简单拆解为三个部分："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第一部分：注解体"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注解的定义有点类似于接口（interface），只不过前面一个加了一个@符号，这个千万不能省。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第二部分：注解变量"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注解变量的语法有点类似于接口里面定义的方法，变量名后面带一对括号，不同的是注解变量后面可以有默认值。另外返回值只能是Java基本类型、String类型或者枚举类，不可以是对象类型。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"第三部分：元注解"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"元注解（meta-annotation）说白了就是给注解加注解的注解，是不是有点晕了，这种注解是JDK提前内置好的，可以直接拿来用的。不太懂也没有关系反正数量也不多，总共就4个，我们背下来吧：@Target @Retention @Documented @Inherited"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Target注解"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用来描述注解的使用范围，即被修饰的注解可以用在什么地方 。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"注解可以用于修饰 packages、types（类、接口、枚举、注解类）、类成员（方法、构造方法、成员变量、枚举值）、方法参数和本地变量（如循环变量、catch参数），在定义注解类时使用了@Target 能够更加清晰的知道它能够被用来修饰哪些对象，具体的取值范围定义在ElementType.java 枚举类中。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"比如上面我们写的Redis锁的注解就只能用于方法上了。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Retention注解"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用来描述注解保留的时间范围，即注解的生命周期。在 RetentionPolicy 枚举类中定义了三个周期："}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"public enum RetentionPolicy {\n SOURCE, // 源文件保留\n CLASS, // 编译期保留，默认值\n RUNTIME // 运行期保留，可通过反射去获取注解信息\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"像我们熟知的@Override注解就只能保留在源文件中，代码编译后注解就消失了。比如上面我们写的Redis锁的注解就保留到了运行期，运行的时候可以通过反射获取信息。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Documented注解"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"用来描述在使用 javadoc 工具为类生成帮助文档时是否要保留其注解信息，很简单不多解释了。"}]},{"type":"bulletedlist","content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Inherited注解"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"被Inherited注解修饰的注解具有继承性，如果父类使用了被@Inherited修饰的注解，则其子类将自动继承该注解。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"好了，这一步我们已经将注解定义好了，但是这个注解如何工作呢？接着看。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"第二步实现注解的业务逻辑"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在第一步中我们发现定义的注解（@EnableRedisLock）中没有业务逻辑，只有一些变量，别忘了我们的注解是要使能Redis分布式锁的功能，那这个注解到底是怎么实现加锁和释放锁的功能呢？这个就需要我们借助反射的强大功能了。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/89/89b9e3b4e03df12ea551b5ee2f743e70.png","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":" 注解的操作"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"这里借助了切面的功能，将EnableRedisLock注解作为一个切点，只要方法上标注了这个注解就会自动执行这里的代码逻辑。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"通过反射机制拿到注解对象后就可以执行加锁解锁的常用逻辑啦。Redis实现分布式锁相信大家已经很熟悉了，这里就不在啰嗦了。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"第三步在业务代码中尽情的使用注解"}]},{"type":"codeblock","attrs":{"lang":null},"content":[{"type":"text","text":"@EnableRedisLock(lockKey = \"student\", expireTime = 10, timeUnit = TimeUnit.SECONDS, retryTimes = 5)\npublic void method1(Student student) {\n // 这里写业务逻辑\n}\n"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在需要加锁的方法上直接加上注解就可以啦，怎么样是不是很简单呀，赶紧在你的项目中运用起来吧。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"好了，自己写一个注解的内容就介绍到这里了，学会了吗？"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"- END -"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"看完三件事❤️"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果你觉得这篇内容对你还蛮有帮助，我想邀请你帮我三个小忙："}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"点赞，转发，有你们的 『点赞和评论』，才是我创造的动力。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"关注公众号 『 Java斗帝 』，不定期分享原创知识。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"同时公众号内回复“666”即可免费领取1000道互联网面试题"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}