为什么POJO中变量不能用is开头

怼烎@ 2023-10-01 12:49 26阅读 0赞

### 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;
        }
     
        @Override
        public 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：

![img][]

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 merged
            for (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 difference
            for (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;
        }

### 四、结论 ###

通过上述论证，我们不难发现，类中的属性用is开头会导致对象序列化后产生意想不到的结果。所以在平时编码的过程中，我们不能在类变量中用is开头如: “isSuccess”，这样容易造成程序错误。

如果情况特殊，也不必墨守成规，只是需要预先判断可能产生的影响、影响范围、能否监控等问题。更特殊的情况，你可以将变量名定义为isIsSuccess，或者将isSuccess定义为public。

[img]: https://img-blog.csdnimg.cn/fadca7fd38104b2dab5be460fec24d66.png?x-oss-process=image/watermark,type_d3F5LXplbmhlaQ,shadow_50,text_Q1NETiBASU9UX3BsYXllcg==,size_20,color_FFFFFF,t_70,g_se,x_16