为什么POJO中变量不能用is开头

一、前言

在阿里编码规约中，有一个约定如下

【强制】POJO 类中的任何布尔类型的变量，都不要加 is 前缀，否则部分框架解析会引起序列化错误。

但为什么类中的field不能用is开头呢？本文将从问题演示、框架源码（本文使用的Jackson）两个方面来进行阐述。

二、问题演示

本文使用的Jackson，需要引入相关依赖，maven依赖如下，gradle配置请自行寻找。

<dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>2.11.0</version>
</dependency>

首先写一个简单的POJO，在这个POJO中，有三个属性，分别是isSuccess/code/msg，并自动生成了对应的get方法，这里要注意，isSuccess属性对应的get方法名也是isSuccess()。代码如下

public class Pojo {private boolean isSuccess;private int code;private String msg;private Pojo(boolean success, int code, String msg) {this.isSuccess = success;this.code = code;this.msg = msg;}public static Pojo buildSuccess() {return new Pojo(true, 2000, "success");}public boolean isSuccess() {return isSuccess;}public int getCode() {return code;}public String getMsg() {return msg;}@Overridepublic String toString() {return "Pojo{" +"isSuccess=" + isSuccess +", code=" + code +", msg='" + msg + '\'' +'}';}
}

然后再写一个类，在这个类中测试对POJO的序列化。在主函数中，会先创建一个POJO，然后将其打印，接着会使用Jackson中的ObjectMapper将对象序列化成Json字符串并打印。

public class Main {public static void main(String[] args) {ObjectMapper mapper = new ObjectMapper();Pojo pojo = Pojo.buildSuccess();System.out.println("raw pojo1: " + pojo.toString());try {System.out.println("serialize pojo1: " + mapper.writeValueAsString(pojo));} catch (IOException e) {System.out.println(e);}}
}

执行main()方法后，得到运行结果如下图：

可以发现，使用ObjectMapper对象将POJO序列化成String后，输出的变量名是success，而原有POJO中的变量isSuccess丢失。将该Json串反序列化成对象，会出现问题。

三、源码解析

// TODO 待源码大致读懂后再补充

源码看不太懂，只讲解一下核心的流程。由于Jackson扩展性很好，对各种复杂情况都进行了处理，这里只讲解最普通的分支流程。

主要方法是POJOPropertiesCollector#collectAll()

    protected void collectAll(){LinkedHashMap<String, POJOPropertyBuilder> props = new LinkedHashMap<String, POJOPropertyBuilder>();// First: gather basic data_addFields(props); // note: populates _fieldRenameMappings_addMethods(props);// 25-Jan-2016, tatu: Avoid introspecting (constructor-)creators for non-static//    inner classes, see [databind#1502]if (!_classDef.isNonStaticInnerClass()) {_addCreators(props);}// Remove ignored properties, first; this MUST precede annotation merging// since logic relies on knowing exactly which accessor has which annotation_removeUnwantedProperties(props);// and then remove unneeded accessors (wrt read-only, read-write)_removeUnwantedAccessor(props);// Rename remaining properties_renameProperties(props);// and now add injectables, but taking care to avoid overlapping ones// via creator and regular properties_addInjectables(props);// then merge annotations, to simplify further processing// 26-Sep-2017, tatu: Before 2.9.2 was done earlier but that prevented some of//   annotations from getting properly mergedfor (POJOPropertyBuilder property : props.values()) {property.mergeAnnotations(_forSerialization);}// And use custom naming strategy, if applicable...PropertyNamingStrategy naming = _findNamingStrategy();if (naming != null) {_renameUsing(props, naming);}// Sort by visibility (explicit over implicit); drop all but first of member// type (getter, setter etc) if there is visibility differencefor (POJOPropertyBuilder property : props.values()) {property.trimByVisibility();}// and, if required, apply wrapper name: note, MUST be done after// annotations are merged.if (_config.isEnabled(MapperFeature.USE_WRAPPER_NAME_AS_PROPERTY_NAME)) {_renameWithWrappers(props);}// well, almost last: there's still ordering..._sortProperties(props);_properties = props;_collected = true;}