mongodb的ObjectId默认序列化为bean对象，如果需要转换为json字符对象，配置如下

@Configuration
public class JacksonConfig implements InitializingBean {

    @Resource
    private ObjectMapper objectMapper;

    @Override
    public void afterPropertiesSet() {
        SimpleModule simpleModule = new SimpleModule();
        //指定mongodb ObjectId 序列化方式
        simpleModule.addSerializer(ObjectId.class, ToStringSerializer.instance);
        objectMapper.registerModule(simpleModule);
    }
}