Spring Boot 自定义校验器依赖注入失效（NPE）问题及解决方案

// UniqueEmail.java
@Constraint(validatedBy = UniqueEmailValidator.class)
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.FIELD })
public @interface UniqueEmail {
    public String message() default "There is already user with this email!";
    public Class<?>[] groups() default {};
    public Class<? extends Payload>[] payload() default{};
}

// UniqueEmailValidator.java
@Component // 尝试将其注册为Spring Bean
public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail, String> {

    @Autowired
    private UserService userService; // 依赖注入 UserService

    @Override
    public boolean isValid(String value, ConstraintValidatorContext context) {
        // 校验逻辑：检查邮箱是否已存在
        return value != null && !userService.findUserByUserEmail(value);
    }
}

// User.class (或 UserRequestDTO)
@NotNull
@NotEmpty
@UniqueEmail // 应用自定义校验
private String email;

// UserController.java
@RestController
public class UserController {
   @Autowired
   private UserService userService;

   @PostMapping(path = "api/addUser", consumes = "application/json")
   public GenericResponseMessage addUser(@Valid @RequestBody UserRequestDTO userRequestDTO){
      userService.createUser(userRequestDTO);
      return new GenericResponseMessage("ok");
   }
}

// UniqueEmail.java
import javax.validation.Constraint;
import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
import javax.validation.Payload;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component; // 可以保留，但核心是下面的LocalValidatorFactoryBean

import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

@Documented
@Target(FIELD)
@Retention(RUNTIME)
@Constraint(validatedBy = UniqueEmail.Validator.class) // 关联内部类
public @interface UniqueEmail {

    String message() default "There is already a user with this email!";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

    // 将 ConstraintValidator 定义为注解的静态内部类
    @Component // 确保Spring能够扫描到并管理这个内部类
    class Validator implements ConstraintValidator<UniqueEmail, String> {

        @Autowired
        private UserService userService; // 依赖注入 UserService

        @Override
        public void initialize(UniqueEmail constraintAnnotation) {
            // 可选：初始化逻辑
        }

        @Override
        public boolean isValid(String email, ConstraintValidatorContext context) {
            // 确保 email 不为 null，并且 UserService 已被正确注入
            return email != null && !userService.existsUserByEmail(email); // 使用优化后的 existsByEmail 方法
        }
    }
}

// ValidationConfiguration.java
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean;

@Configuration
public class ValidationConfiguration {

    @Bean
    @Primary // 如果存在多个 Validator Bean，@Primary 确保此 Bean 被优先使用
    public LocalValidatorFactoryBean localValidatorFactoryBean() {
        return new LocalValidatorFactoryBean();
    }
}

// UserRepository.java
import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<User,Long> {
   // 推荐使用 existsBy 替代 findBy 来检查是否存在
   boolean existsByEmail(String email);
   // 如果需要，也可以为其他字段添加 existsBy
   // boolean existsByUsername(String username);
}

// UserService.java
import org.springframework.stereotype.Service;

@Service
public class UserService {

    private final UserRepository userRepository;
    // ... 其他字段和构造函数

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
        // ...
    }

    // 优化后的方法，使用 existsByEmail
    public boolean existsUserByEmail(String email){
        return userRepository.existsByEmail(email);
    }

    // 原来的 findUserByUserEmail 方法可以移除或修改为返回 Optional<User>
    // public boolean findUserByUserEmail(String email){
    //     // 假设 userRepository.findByEmail(email) 返回 Optional<User>
    //     return userRepository.findByEmail(email).isPresent();
    // }
}

// UniqueEmail.java (内部 Validator 类)
// ...
    class Validator implements ConstraintValidator<UniqueEmail, String> {
        @Autowired
        private UserService userService;

        // ...

        @Override
        public boolean isValid(String email, ConstraintValidatorContext context) {
            return email != null && !userService.existsUserByEmail(email); // 调用优化后的方法
        }
    }
// ...