RestTemplate exchange获取List<LdapUserDto>反序列化失败求助

问题详情 #

调用POST接口时，使用RestTemplate尝试接收List<LdapUserDto>类型响应，触发JSON反序列化错误，提示无法从Object类型转为ArrayList。

请求代码 #

String url = "http://localhost:8080/admin/user/find/all/email"+"?email={email}";
ResponseEntity<List<LdapUserDto>> responseEntity = restTemplate.exchange(
                                                              url,
                                                              HttpMethod.POST,
                                                              null,  
                                                              new ParameterizedTypeReference<List<LdapUserDto>>() {},
                                                              email);

DTO定义 #

@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class LdapUserDto {
    @JsonProperty("id")
    private String id;

    @JsonProperty("email")
    private String email;

    @JsonProperty("fullName")
    private String fullName;
}

预期响应格式 #

[
    {
        "id": "7264a0e5",
        "email": "Admin@telecom.mu",
        "fullName": "Administrator"
    },
    {
        "id": "0eccd314",
        "email": "ABC.John@telecom.mu",
        "fullName": "ABC John"
    }
]

错误日志 #

2022-08-16 01:06:48.407 DEBUG [admin,917e5c4b9e42604c,917e5c4b9e42604c] 15716 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : Failed to complete request: org.springframework.web.client.RestClientException: Error while extracting response for type [java.util.List<com.cloudportal.admin.dto.response.user.LdapUserDto>] and content type [application/json;charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize value of type `java.util.ArrayList<com.cloudportal.admin.dto.response.user.LdapUserDto>` from Object value (token `JsonToken.START_OBJECT`); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<com.cloudportal.admin.dto.response.user.LdapUserDto>` from Object value (token `JsonToken.START_OBJECT`)
 at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 1]
2022-08-16 01:06:48.415 ERROR [admin,917e5c4b9e42604c,917e5c4b9e42604c] 15716 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.web.client.RestClientException: Error while extracting response for type [java.util.List<com.cloudportal.admin.dto.response.user.LdapUserDto>] and content type [application/json;charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize value of type `java.util.ArrayList<com.cloudportal.admin.dto.response.user.LdapUserDto>` from Object value (token `JsonToken.START_OBJECT`); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<com.cloudportal.admin.dto.response.user.LdapUserDto>` from Object value (token `JsonToken.START_OBJECT`)
 at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 1]] with root cause

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<com.cloudportal.admin.dto.response.user.LdapUserDto>` from Object value (token `JsonToken.START_OBJECT`)
 at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 1]

解决思路 #

错误本质是接口实际返回的JSON是对象（{}），而非你预期的数组（[]），导致Jackson无法完成反序列化。可按以下方向排查修复：

1. 确认接口真实返回格式 #

先用Postman/curl调用目标接口，查看实际返回的JSON结构：

• 如果返回的是包裹数组的对象（例：{"users": [{},{}]}），需新增响应包装类适配：

  @Getter
  @Setter
  public class LdapUserListResponse {
      @JsonProperty("users") // 对应实际返回的数组字段名
      private List<LdapUserDto> users;
  }
  

  再修改RestTemplate调用代码：

  ResponseEntity<LdapUserListResponse> responseEntity = restTemplate.exchange(
                                                               url,
                                                               HttpMethod.POST,
                                                               null,  
                                                               new ParameterizedTypeReference<LdapUserListResponse>() {},
                                                               email);
  List<LdapUserDto> userList = responseEntity.getBody().getUsers();
  

2. 调整请求参数传递方式 #

你当前用POST请求但将参数放在URL查询串中，部分接口要求POST参数放在请求体：

HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.APPLICATION_JSON);
HttpEntity<String> requestEntity = new HttpEntity<>(email, headers);

ResponseEntity<List<LdapUserDto>> responseEntity = restTemplate.exchange(
                                                           "http://localhost:8080/admin/user/find/all/email",
                                                           HttpMethod.POST,
                                                           requestEntity,  
                                                           new ParameterizedTypeReference<List<LdapUserDto>>() {});

3. 配置RestTemplate兼容格式 #

若接口可能返回单个对象或数组，可配置Jackson支持单值转数组：

@Bean
public RestTemplate restTemplate() {
    RestTemplate restTemplate = new RestTemplate();
    MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
    ObjectMapper mapper = new ObjectMapper();
    // 允许将单个对象反序列化为数组
    mapper.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);
    converter.setObjectMapper(mapper);
    restTemplate.getMessageConverters().add(converter);
    return restTemplate;
}

内容的提问来源于stack exchange，提问作者SjAnupa