各位大神好，请教个问题

1.环境

spring 4.3.7
hibernate-validator-5.4.1

2.配置如下

    <bean id="messageSource"
        class="org.springframework.context.support.ReloadableResourceBundleMessageSource" name="messageSource">
        <property name="basenames">
            <list>
                <value>classpath:messages/messages</value>
                <value>classpath:messages/ValidationMessages</value>
            </list>
        </property>
        <property name="useCodeAsDefaultMessage" value="false" />
        <property name="defaultEncoding" value="UTF-8" />
        <property name="cacheSeconds" value="60" />
    </bean>

    <bean id="validator" class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean">
        <property name="providerClass" value="org.hibernate.validator.HibernateValidator"/>
        <property name="validationMessageSource" ref="messageSource"/>
    </bean>
    
    
    <mvc:annotation-driven validator="validator" />

3.bean和Controller

public class UserRequest {
    
    @NotBlank(message = "{login.valid.username.notnull}")
    private String username;
    
    @NotBlank(message = "{login.valid.password.notnull}")
    private String password;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

}

    @RequestMapping(value = "/login", method = RequestMethod.POST)
    @ResponseBody
    public Object login(@Valid @RequestBody UserRequest ur, BindingResult result, HttpServletRequest request) {
        
        log.debug("login");
        
        if(result.hasErrors()) {  
            return result.getAllErrors().get(0);  
        } 
        
        ……
    }

问题:
为什么一定要在代码中用result.hasErrors()显示判断？
难道不能验证UserRequest中的字段不符合定义就直接抛异常，这样不更合理些吗？