Spring Boot接口读取JSON文件触发MismatchedInputException异常求助

代码实现 #

UserController.java #

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;

import java.io.IOException;
import java.util.List;

@RestController
@RequestMapping("/api/users")
public class UserController {

    @Autowired
    private UserService userService;

    @GetMapping
    public List<User> getAllUsers() throws IOException {
        return userService.findAll();
    }
    
}

UserService.java #

import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.stereotype.Service;

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

@Service
public class UserService {
    
    private static final String FILE_PATH = "C:\\Users\\test\\Users.json"; 
    private final ObjectMapper objectMapper = new ObjectMapper();

    private List<User> getUsers() throws IOException {
        File file = new File(FILE_PATH);
        if (file.exists()) {
            return objectMapper.readValue(file, objectMapper.getTypeFactory().constructCollectionType(List.class, User.class));
        }
        return new ArrayList<>();
    }

    public List<User> findAll() throws IOException {
        return getUsers();
    }

}

JSON文件内容 #

[
{"user_id": 9, 
"username": "user9", 
"password": "xxxx", 
"name": "abcd", 
"college": "test", 
"is_edited": false, 
"is_deleted": false, 
"user_type_id": 1
},
{
"user_id": 10, 
"username": "user10", 
"password": "xxxx", 
"name": "abcd", 
"college": "test", 
"is_edited": false, 
"is_deleted": false, 
"user_type_id": 2
}
]

异常信息 #

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<com.example.demo.User>` from Object value (token `JsonToken.START_OBJECT`)
 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 1, column: 1]
    at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59) ~[jackson-databind-2.17.2.jar:2.17.2]
    at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1767) ~[jackson-databind-2.17.2.jar:2.17.2]
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1541) ~[jackson-databind-2.17.2.jar:2.17.2]
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1488) ~[jackson-databind-2.17.2.jar:2.17.2]
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.handleNonArray(CollectionDeserializer.java:402) ~[jackson-databind-2.17.2.jar:2.17.2]

解决方案 #

异常核心原因：Jackson尝试将JSON对象反序列化为List<User>，但预期的是JSON数组。按以下步骤排查修复：

1. 修正JSON语法错误
   你提供的JSON示例中password字段值xxxx未加双引号，这会导致JSON解析异常，Jackson可能将错误的JSON解析为异常结构。确保所有字符串类型字段值都用双引号包裹，如"password": "xxxx"。同时检查文件是否存在括号不匹配、逗号遗漏等其他语法问题。

2. 验证文件路径准确性
   确认FILE_PATH指向的是目标JSON文件，避免读取到其他内容不符的文件。可以在getUsers()方法中添加日志输出文件绝对路径，或临时打印文件内容，确认读取的文件是否正确。

3. 优化反序列化代码
   改用TypeReference简化集合类型指定，代码可读性更强，避免潜在的类型构造错误：

   import com.fasterxml.jackson.core.type.TypeReference;
   // ...
   private List<User> getUsers() throws IOException {
       File file = new File(FILE_PATH);
       if (file.exists()) {
           return objectMapper.readValue(file, new TypeReference<List<User>>() {});
       }
       return new ArrayList<>();
   }
   

4. 添加错误排查日志
   捕获JSON解析异常并输出详细信息，方便定位问题：

   import com.fasterxml.jackson.core.JsonProcessingException;
   import java.nio.file.Files;
   // ...
   private List<User> getUsers() throws IOException {
       File file = new File(FILE_PATH);
       if (file.exists()) {
           try {
               return objectMapper.readValue(file, new TypeReference<List<User>>() {});
           } catch (JsonProcessingException e) {
               System.err.println("JSON解析错误: " + e.getMessage());
               System.err.println("文件内容: " + Files.readString(file.toPath()));
               throw e;
           }
       }
       return new ArrayList<>();
   }
   

内容的提问来源于stack exchange，提问作者lse23