Spring中/register接口POST请求出现NullPointerException：service为null

问题描述 #

调用/register端点发送POST请求时，抛出NullPointerException，报错提示service对象为null。相关代码如下：

SecurityController代码 #

@AllArgsConstructor
@RestController
@Validated
public class SecurityController {

    private final UserService service;

    @PostMapping("/register")
    private ResponseEntity<UserDTO> registerUser (@RequestBody @Valid RegisterDTO registerDTO) {
        return ResponseEntity.created(URI.create("/user")).body(service.createUser(registerDTO));
    }
}

UserService代码 #

@RequiredArgsConstructor
@Service
public class UserService implements UserServiceApi {
    private final UserRepository userRepository;
    private final RoleRepository roleRepository;
    private final PasswordEncoder encoder;

    @Override
    public UserDTO createUser(RegisterDTO user) {
        if (user.equals(userRepository.findByUsername(user.getUsername()))) {
            throw new RuntimeException("This nickname is already taken");
        }
        if (user.equals(userRepository.findByEmail(user.getEmail()))) {
            throw new RuntimeException("This email is already taken");
        }
        // Encoding password
        user.setPassword(encoder.encode(user.getPassword()));
        // On creating new Account it's going to have USER role
        Role role = roleRepository.findByName("USER");

        String username = user.getUsername();
        String password = user.getPassword();
        String email = user.getEmail();

        User dto = buildUser(username, password, email, role);

        userRepository.save(dto);
        return UserDTO.builder()
                .username(username)
                .password(password)
                .email(email)
                .build();
    }
}

UserController代码 #

@RestController(value = "/user")
@AllArgsConstructor
@Validated
public class UserController {
    private final UserService service;

    @GetMapping("/getusers")
    public ResponseEntity<List<User>> getAllUser() {
        return ResponseEntity.ok(service.getUsers());
    }
}

问题原因分析 #

1. SecurityController请求方法权限错误：registerUser方法被定义为private，Spring MVC无法识别并调用私有方法处理HTTP请求，导致SecurityController实例未被Spring正确初始化，依赖注入的UserService无法赋值，最终为null。
2. UserController路径配置错误：@RestController(value = "/user")中的value属性是用来指定Bean名称的，并非请求路径前缀，这个错误会导致/user/getusers路径无法正确映射，但不是当前NPE的直接原因，需一并修正。

解决方案 #

1. 修正SecurityController方法权限 #

将registerUser方法的访问权限改为public，确保Spring MVC能正常调用：

@PostMapping("/register")
public ResponseEntity<UserDTO> registerUser (@RequestBody @Valid RegisterDTO registerDTO) {
    return ResponseEntity.created(URI.create("/user")).body(service.createUser(registerDTO));
}

2. 修正UserController路径配置 #

使用@RequestMapping指定请求路径前缀，替换错误的@RestController(value = "/user")：

@RestController
@RequestMapping("/user")
@AllArgsConstructor
@Validated
public class UserController {
    private final UserService service;

    @GetMapping("/getusers")
    public ResponseEntity<List<User>> getAllUser() {
        return ResponseEntity.ok(service.getUsers());
    }
}

3. 额外检查项 #

• 确认项目Lombok依赖配置正确，@AllArgsConstructor、@RequiredArgsConstructor等注解能正常生成构造方法，保证依赖注入生效。
• 确认UserService所在包被Spring组件扫描覆盖（@SpringBootApplication默认扫描启动类所在包及其子包）。

内容的提问来源于stack exchange，提问作者huga721