一、问题描述

  最近项目使用SpringCloud开发时，Json转换把属性值为null的属性过虑掉了。后续经过两小时的代码跟踪与调试才找出“真凶”。以下是本人根据当时的情况写的用例，如发现有不正确的地方，欢迎积极指正。

实体Bean定义如下：

public class Bean{
    private Integer id;
    private String name;
    private Date createDate;

    public Integer getId(){
        return id;
    }
    public void setId(Integer id){
        this.id = id;
    }
    public String getName(){
        return name;
    }
    public void setName(String name){
        this.name = name;
    }
    public Date getCreateDate(){
        return createDate;
    }
    public void setCreateDate(Date createDate){
        this.createDate = createDate;
    }
}

代码片段如：

@RestController
@RequestMapping(value = "/api/")
public class ConfigApi{
    private static final Logger logger = LoggerFactory.getLogger(ConfigApi.class);
    @Autowired
    private ObjectMapper objectMapper;
    
    @RequestMapping(value = "json", method = RequestMethod.GET)
    public @ResponseBody Bean rulePrefix(){
        try{
            Bean bean = new Bean();
            bean.setId(100);
            bean.setName(null);
            bean.setCreateDate(new Date());
            System.out.println(objectMapper.writeValueAsString(bean));
            return bean;
        }catch(Exception e){
            logger.error(e.getMessage(),e);
        }
    }
}      

期望转换后的json字符串应该为：

{"id":100,"name":null,"createDate":1562858066864}

实际转换的json为：

{"id":100,"createDate":1562858066864}

二、原因分析

  出现以上原因是由于定义ObjectMapper的时执行objectMapper.findAndRegisterModules();代码，这会把com.fasterxml.jackson.module.jaxb.JaxbAnnotationModule注入，在以下jar包中。

<dependency>
    <groupId>com.fasterxml.jackson.module</groupId>
    <artifactId>jackson-module-jaxb-annotations</artifactId>
    <version>2.8.7</version>
</dependency>

依赖包如下：

<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-eureka-server</artifactId>
    <version>1.2.6.RELEASE</version>
</dependency>

ObjectMapper完整配置如下：

@Configuration
public class JacksonConfig{
    @Bean
    @Primary
    public ObjectMapper jsonMapper() {
        ObjectMapper objectMapper = JsonMapper.getInstance().getMapper();
        //注册所有Module接口的实现类
        objectMapper.findAndRegisterModules();
        //执行上面代码后，手动设置序列化方式都无效
        objectMapper.setSerializationInclusion(JsonInclude.Include.ALWAYS);
        return objectMapper;
    }

    @Bean
    @Primary
    public MappingJackson2HttpMessageConverter jsonConverter(){
        MappingJackson2HttpMessageConverter jsonConverter = new MappingJackson2HttpMessageConverter();
        jsonConverter.setObjectMapper(jsonMapper());
        return jsonConverter;
    }
}