参数校验通常是OpenApi必做的操作，其会对不合法的输入做统一的校验以防止恶意的请求。本文则对参数校验这方面作下简单的分析
spring.factories
读者应该对此文件加以深刻的印象，很多springboot整合第三方插件的方式均是从此配置文件去读取的，本文关注下检验方面的东西。在相应的文件搜索validation关键字，最终定位至ValidationAutoConfiguration类，笔者这就针对此类作主要的分析
ValidationAutoConfiguration
优先看下其头上的注解
@Configuration@ConditionalOnClass(ExecutableValidator.class)@ConditionalOnResource(resources = "classpath:META-INF/services/javax.validation.spi.ValidationProvider")@Import(PrimaryDefaultValidatorPostProcessor.class)
使此类成功被注册的条件有两个，第一是当前环境下存在ExecutableValidator类，第二是当前类环境存在META-INF/services/javax.validation.spi.ValidationProvider文件。
通过查看maven依赖得知，其实springboot在引入starter-web板块便引入了hibernate-validator包，此包便满足了上述的两个要求。 笔者发现其也引入了PrimaryDefaultValidatorPostProcessor类，主要是判断当前的bean工厂是否已经包含了LocalValidatorFactoryBean和Validator对象，不影响大局。即使没有配置，下述的代码也是会注册的@Bean @Role(BeanDefinition.ROLE_INFRASTRUCTURE) @ConditionalOnMissingBean(Validator.class) public static LocalValidatorFactoryBean defaultValidator() { LocalValidatorFactoryBean factoryBean = new LocalValidatorFactoryBean(); MessageInterpolatorFactory interpolatorFactory = new MessageInterpolatorFactory(); factoryBean.setMessageInterpolator(interpolatorFactory.getObject()); return factoryBean; } @Bean @ConditionalOnMissingBean public static MethodValidationPostProcessor methodValidationPostProcessor( Environment environment, @Lazy Validator validator) { MethodValidationPostProcessor processor = new MethodValidationPostProcessor(); boolean proxyTargetClass = environment .getProperty("spring.aop.proxy-target-class", Boolean.class, true); processor.setProxyTargetClass(proxyTargetClass); processor.setValidator(validator); return processor; } 通过查阅代码得知，使用注解式的校验方式是通过添加@Validated注解来实现的，但是其作用于参数上还是类上是有不同的操作逻辑的。笔者将之区分开，方便后续查阅。先附上@Validated注解源码
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.PARAMETER})@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface Validated { /** * Specify one or more validation groups to apply to the validation step * kicked off by this annotation. * JSR-303 defines validation groups as custom annotations which an application declares * for the sole purpose of using them as type-safe group arguments, as implemented in * {@link org.springframework.validation.beanvalidation.SpringValidatorAdapter}. *
Other {@link org.springframework.validation.SmartValidator} implementations may * support class arguments in other ways as well. */ Class
[] value() default {};} 类级别的校验
即@Validated作用于类上，其相关的处理逻辑便是由MethodValidationPostProcessor来实现的，笔者稍微看下关键源码方法afterPropertiesSet()
@Override public void afterPropertiesSet() { // 查找对应的类以及祖先类上是否含有@Validated注解 Pointcut pointcut = new AnnotationMatchingPointcut(this.validatedAnnotationType, true); // 创建MethodValidationInterceptor处理类来处理具体的逻辑 this.advisor = new DefaultPointcutAdvisor(pointcut, createMethodValidationAdvice(this.validator)); } 上述的配置表明只要某个类上使用了@Validated注解，其相应的方法就会被校验相关的参数。笔者紧接着看下MethodValidationInterceptor#invoke()方法
@Override @SuppressWarnings("unchecked") public Object invoke(MethodInvocation invocation) throws Throwable { // 读取相应方法上的@Validated的value属性，为空也是没问题的 Class [] groups = determineValidationGroups(invocation); // Standard Bean Validation 1.1 API ExecutableValidator execVal = this.validator.forExecutables(); Method methodToValidate = invocation.getMethod(); Set 只要类上使用了@Validated注解，则其下的所有方法都会被校验。
检验规则如下：参数和返回值都会被校验，只要某一个没有通过，则会抛出ConstraintViolationException异常以示警告。 具体的参数校验属于hibernate-validator的范畴了，感兴趣的读者可自行分析~参数级别的校验(推荐)
即@Validated注解作用于方法的参数上，其有关的校验则是被springmvc的参数校验器处理的。笔者在ModelAttributeMethodProcessor#resolveArgument()方法中查找到了相应的蛛丝马迹，列出关键的代码
@Override @Nullable public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception { .... Object attribute = null; BindingResult bindingResult = null; if (mavContainer.containsAttribute(name)) { attribute = mavContainer.getModel().get(name); } else { // Create attribute instance try { attribute = createAttribute(name, parameter, binderFactory, webRequest); } catch (BindException ex) { ..... } } if (bindingResult == null) { WebDataBinder binder = binderFactory.createBinder(webRequest, attribute, name); if (binder.getTarget() != null) { if (!mavContainer.isBindingDisabled(name)) { bindRequestParameters(binder, webRequest); } // 就是这里 validateIfApplicable(binder, parameter); if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) { throw new BindException(binder.getBindingResult()); } } // Value type adaptation, also covering java.util.Optional if (!parameter.getParameterType().isInstance(attribute)) { attribute = binder.convertIfNecessary(binder.getTarget(), parameter.getParameterType(), parameter); } bindingResult = binder.getBindingResult(); } .... return attribute; } 我们继续看下其下的validateIfApplicable()方法
protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) { // 对参数上含有@Validated注解的进行校验器解析 Annotation[] annotations = parameter.getParameterAnnotations(); for (Annotation ann : annotations) { Validated validatedAnn = AnnotationUtils.getAnnotation(ann, Validated.class); // 兼容@Valid注解方式 if (validatedAnn != null || ann.annotationType().getSimpleName().startsWith("Valid")) { Object hints = (validatedAnn != null ? validatedAnn.value() : AnnotationUtils.getValue(ann)); Object[] validationHints = (hints instanceof Object[] ? (Object[]) hints : new Object[] {hints}); binder.validate(validationHints); break; } } } 上述的代码已经很简明概要了，笔者就不展开了。当然如果用户想要在出现异常的时候进行友好的返回，建议参考的异常机制文章便可迎刃而解
小结
参数的校验一般都是结合spring-context板块内的@Validated注解搭配hibernate的校验器便完成了相应的检测功能。
经过笔者的测试，发现在类上使用@Validated注释基本没啥用，还是会依赖在参数上添加@Validated注解方可生效。 如果大家对此有什么补充欢迎留言，在此希望此篇对大家有所帮助