一、Validation参数校验准备工作

参考学习资料： https://juejin.cn/post/6856541106626363399

1.1需要的依赖:

```xml

<parent>
    <groupId>org.springframework.bootgroupId>
    <artifactId>spring-boot-starter-parentartifactId>
    <version>2.6.10version>
    <relativePath/> 
parent>

        

        
        
        
        
        

<dependency>
<groupId>org.springframework.bootgroupId>
<artifactId>spring-boot-starter-validationartifactId>
dependency>

```
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1.2 简单的约束注解

二、基础使用

参考代码: tom-technology-demo中的technology-validation模块

包含了:单个参数、对象参数、分组校验、嵌套单个校验、嵌套集合校验

三、校验原理

3.1 RequestBody参数校验实现原理

在spring-mvc中，RequestResponseBodyMethodProcessor是用于解析@RequestBody标注的参数以及处理@ResponseBody标注方法的返回值的。显然，执行参数校验的逻辑肯定就在解析参数的方法resolveArgument()中

public class RequestResponseBodyMethodProcessor extends AbstractMessageConverterMethodProcessor {  
    @Override  
    public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,  
                                  NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {  
  
        parameter = parameter.nestedIfOptional();  
  
        Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());  
        String name = Conventions.getVariableNameForParameter(parameter);  
  
        if (binderFactory != null) {  
            WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);  
            if (arg != null) {  
  				//validate参数校验
                validateIfApplicable(binder, parameter);  
                if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {  
                    throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());  
                }  
            }  
            if (mavContainer != null) {  
                mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());  
            }  
        }  
        return adaptArgumentIfNecessary(arg, parameter);  
    }  
}  
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可以看到，resolveArgument()调用了validateIfApplicable()进行参数校验。

protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) {  
  
    Annotation[] annotations = parameter.getParameterAnnotations();  
    for (Annotation ann : annotations) {  
  
        Validated validatedAnn = AnnotationUtils.getAnnotation(ann, Validated.class);  
  
        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;  
        }  
    }  
}
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这种场景下@Validated、@Valid两个注解可以混用。接下来继续看WebDataBinder.validate()实现。

@Override  
public void validate(Object target, Errors errors, Object... validationHints) {  
    if (this.targetValidator != null) {  
        processConstraintViolations(  
  
            this.targetValidator.validate(target, asValidationGroups(validationHints)), errors);  
    }  
}  
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底层最终还是调用了Hibernate Validator进行真正的校验处理

3.2 方法级别的参数校验实现原理

将参数一个个平铺到方法参数中，然后在每个参数前面声明约束注解的校验方式，就是方法级别的参数校验。实际上，这种方式可用于任何Spring Bean的方法上，比如Controller/Service等。其底层实现原理就是**AOP，具体来说是通过MethodValidationPostProcessor动态注册AOP切面，然后使用MethodValidationInterceptor对切点方法织入增强**

public class MethodValidationPostProcessor extends AbstractBeanFactoryAwareAdvisingPostProcessorimplements InitializingBean {  
    @Override  
    public void afterPropertiesSet() {  
  
        Pointcut pointcut = new AnnotationMatchingPointcut(this.validatedAnnotationType, true);  
  
        this.advisor = new DefaultPointcutAdvisor(pointcut, createMethodValidationAdvice(this.validator));  
    }  
  
    protected Advice createMethodValidationAdvice(@Nullable Validator validator) {  
        return (validator != null ? new MethodValidationInterceptor(validator) : new MethodValidationInterceptor());  
    }  
}  
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接着看一下MethodValidationInterceptor：

public class MethodValidationInterceptor implements MethodInterceptor {  
    @Override  
    public Object invoke(MethodInvocation invocation) throws Throwable {  
  
        if (isFactoryBeanMetadataMethod(invocation.getMethod())) {  
            return invocation.proceed();  
        }  
  
        Class<?>[] groups = determineValidationGroups(invocation);  
        ExecutableValidator execVal = this.validator.forExecutables();  
        Method methodToValidate = invocation.getMethod();  
        Set<ConstraintViolation<Object>> result;  
        try {  
  
            result = execVal.validateParameters(  
                invocation.getThis(), methodToValidate, invocation.getArguments(), groups);  
        }  
        catch (IllegalArgumentException ex) {  
            ...  
        }  
  
        if (!result.isEmpty()) {  
            throw new ConstraintViolationException(result);  
        }  
  
        Object returnValue = invocation.proceed();  
  
        result = execVal.validateReturnValue(invocation.getThis(), methodToValidate, returnValue, groups);  
  
        if (!result.isEmpty()) {  
            throw new ConstraintViolationException(result);  
        }  
        return returnValue;  
    }  
}  
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不管是**requestBody参数校验还是方法级别的校验，最终都是调用Hibernate Validator执行校验，Spring Validation只是做了一层封装**。