项目场景：

Spring Boot项目使用RestTemplate调用接口，使用嵌套实体对象接收的返回结果

实体对象TokenUser，其中TokenUserInfo是嵌套的对象：

@Data
public class TokenUser {private Integer code;private TokenUserInfo data;private Boolean success;
}

@Data
public class TokenUserInfo {/*** 用户id*/private String userId;/*** 用户名*/private String userName;}

 使用RestTemplate调用接口

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON_UTF8);HttpEntity<String> formEntity= new HttpEntity<>(requestJson, headers);TokenUser tokenUser = null;
try {tokenUser = restTemplate.postForObject(url, formEntity, TokenUser.class);
} catch (Exception e) {logger.error("调用接口出错: ", e);
}

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问题描述

Spring Boot项目使用RestTemplate调用接口，使用嵌套实体对象接收的返回结果

返回的嵌套对象为null不会有问题

{"data": null,"success": true,"code": 200
}

但是返回的嵌套对象是''空字符串就会报错

{"data": "","success": true,"code": 200
}

报错详情：

Error while extracting response for type [class com.test.dto.TokenUser] and content type [application/json;charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of `com.test.dto.TokenUserInfo` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value (''); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of `com.test.dto.TokenUserInfo` (although at least one Creator exists): no String-argument constructor/factory method to deserialize from String value ('')at [Source: (PushbackInputStream); line: 1, column: 55] (through reference chain: com.test.dto.TokenUser["data"])

---

原因分析：

        简单来说就是 json转化成对象失败了，失败的原因是TokenUserInfo对象中虽然有一个构造器，却是全参构造器或是无参构造器，就是缺少一个只有一个String类型参数的构造器，导致在解析 ' ' 空字符串时失败了。

---

解决方案：

第一种：实体类加上String类型参数的构造器

同时加上@NoArgsConstructor, @AllArgsConstructor 注解，提供无参数的构造函数和全参数的构造函数

@Data
@NoArgsConstructor
@AllArgsConstructor
public class TokenUserInfo {/*** 防止返回值是空字符串，用string来接收*/public TokenUserInfo(String userId){this.userId = userId;}/*** 用户id*/private String userId;/*** 用户名*/private String userName;}

第二种：用String接收返回结果，再转json对象

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON_UTF8);HttpEntity<String> formEntity= new HttpEntity<>(requestJson, headers);TokenUser tokenUser = null;
try {String jsonResult = restTemplate.postForObject(url, formEntity, String.class);JSONObject userJson = JSON.parseObject(jsonResult);tokenUser = JSONObject.toJavaObject(userJson, TokenUser.class);
} catch (Exception e) {logger.error("调用接口出错: ", e);
}

我这里用的是alibaba的包

<dependency><groupId>com.alibaba</groupId><artifactId>fastjson</artifactId><version>1.2.44</version>
</dependency>