Spring Validation實現原理分析

最近要作動態數據的提交處理，即須要分析提交數據字段定義信息後才能明確對應的具體字段類型，進而作數據類型轉換和字段有效性校驗，而後作業務處理後提交數據庫，本身開發一套校驗邏輯的話週期太長，所以分析了Spring Validation的實現原理，複用了其底層花樣繁多的Validator，在此將分析Spring Validation原理的過程記錄下，不深刻細節

如何使用Spring Validation

• Spring Bean初始化時校驗Bean是否符合JSR-303規範
  一、手動添加BeanValidationPostProcessor Bean
  二、在model類中定義校驗規則，如@Max、@Min、@NotEmpty
  三、聲明Bean，綜合代碼以下：

@Bean
public BeanPostProcessor beanValidationPostProcessor() {
    return new BeanValidationPostProcessor();
}

@Bean
public UserModel getUserModel() {
    UserModel userModel = new UserModel();
    userModel.setUsername(null);
    userModel.setPassword("123");
    return userModel;
}

@Data
class UserModel {
    @NotNull(message = "username can not be null")
    @Pattern(regexp = "[a-zA-Z0-9_]{5,10}", message = "username is illegal")
    private String username;
    @Size(min = 5, max = 10, message = "password's length is illegal")
    private String password;
}
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四、BeanValidationPostProcessor Bean內部有個boolean類型的屬性afterInitialization，默認是false，若是是false，在postProcessBeforeInitialization過程當中對bean進行驗證，不然在postProcessAfterInitialization過程對bean進行驗證
五、此種校驗使用了spring的BeanPostProcessor邏輯，可參考Spring Boot系列之一：如何快速熟悉Spring技術棧
六、校驗底層調用了doValidate方法，進一步調用validator.validate，默認validator爲HibernateValidator，validation-api包爲JAVA規範，Spring默認的規範實現爲hibernate-validator包，此hibernate非ORM框架Hibernate

protected void doValidate(Object bean) {
	Assert.state(this.validator != null, "No Validator set");
	Set<ConstraintViolation<Object>> result = this.validator.validate(bean);
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七、HibernateValidator默認調用ValidatorFactoryImpl來生成validator，後面展開將ValidatorFactoryImpl

• 支持方法級別的JSR-303規範
  一、手動添加MethodValidationPostProcessor Bean
  二、類上加上@Validated註解（也支持自定義註解，建立MethodValidationPostProcessor Bean時傳入）
  三、在方法的參數中加上驗證註解，好比@Max、@Min、@NotEmpty、@NotNull等，如

@Component
@Validated
public class BeanForMethodValidation {
    public void validate(@NotEmpty String name, @Min(10) int age) {
        System.out.println("validate, name: " + name + ", age: " + age);
    }
}  
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四、MethodValidationPostProcessor內部使用aop完成對方法的調用

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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五、底層一樣默認調用ValidatorFactoryImpl來生成validator，由validator完成校驗

• 直接編碼調用校驗邏輯，如

public class Person {
@NotNull(message = "性別不能爲空")
private Gender gender;
@Min(10)
private Integer age;
...
}
ValidatorFactory validatorFactory = Validation.buildDefaultValidatorFactory();
Validator validator = validatorFactory.getValidator();
Person person = new Person();
person.setGender(Gender.Man);
validator.validate(person);
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同上，默認調用ValidatorFactoryImpl來生成validator，由validator完成具體校驗

• 在Spring controller方法參數中使用valid或validated註解標註待校驗參數
  一、先熟悉下Spring的請求調用流程
  二、能夠看到在各類resolver處理請求參數的過程當中作了參數校驗
  三、底層統一調用了DataBinder的validate方法
  四、DataBinder的做用：Binder that allows for setting property values onto a target object, including support for validation and binding result analysis，也就是binder處理了request提交的字符串形式的參數，將其轉換成服務端真正須要的類型，binder提供了對validation的支持，能夠存放校驗結果
  五、DataBinder的validator默認在ConfigurableWebBindingInitializer中初始化，默認使用OptionalValidatorFactoryBean，該Bean繼承了LocalValidatorFactoryBean，LocalValidatorFactoryBean組合了ValidatorFactory、自定義校驗屬性等各類校驗會用到的信息，默認使用ValidatorFactoryImpl來獲取validator

至此，全部的線索都指向了ValidatorFactoryImpl，下面分析下該類

public Validator `getValidator`() {
	return `createValidator`(
		constraintValidatorManager.getDefaultConstraintValidatorFactory(),
		valueExtractorManager,
		validatorFactoryScopedContext,
		methodValidationConfiguration
	);
}
Validator `createValidator`(ConstraintValidatorFactory constraintValidatorFactory,
	ValueExtractorManager valueExtractorManager,
	ValidatorFactoryScopedContext validatorFactoryScopedContext,
	MethodValidationConfiguration methodValidationConfiguration) {
	
	BeanMetaDataManager beanMetaDataManager = beanMetaDataManagers.computeIfAbsent(
		new BeanMetaDataManagerKey( validatorFactoryScopedContext.getParameterNameProvider(), valueExtractorManager, methodValidationConfiguration ),
		key -> new BeanMetaDataManager(
			`constraintHelper`,
			executableHelper,
			typeResolutionHelper,
			validatorFactoryScopedContext.getParameterNameProvider(),
			valueExtractorManager,
			validationOrderGenerator,
			buildDataProviders(),
			methodValidationConfiguration
		)
	 );
   
        return `new ValidatorImpl`(
			constraintValidatorFactory,
			beanMetaDataManager,
			valueExtractorManager,
			constraintValidatorManager,
			validationOrderGenerator,
			validatorFactoryScopedContext
	);
}
public final <T> Set<ConstraintViolation<T>> validate(T object, Class<?>... groups) {
	Contracts.assertNotNull( object, MESSAGES.validatedObjectMustNotBeNull() );
	sanityCheckGroups( groups );

	ValidationContext<T> validationContext = `getValidationContextBuilder().forValidate( object )`;

	if ( !validationContext.getRootBeanMetaData().hasConstraints() ) {
		return Collections.emptySet();
	}

	ValidationOrder validationOrder = determineGroupValidationOrder( groups );
	ValueContext<?, Object> valueContext = `ValueContext.getLocalExecutionContext`(
			validatorScopedContext.getParameterNameProvider(),
			object,
			validationContext.getRootBeanMetaData(),
			PathImpl.createRootPath()
	);

	return validateInContext( validationContext, valueContext, validationOrder );
}
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一、getValidator->createValidator->ValidatorImpl->validate
在執行過程當中封裝了beanMetaDataManager、validationContext、valueContext等內容，都是校驗時會用到的上下文信息，如待校驗bean的全部校驗項（含父類和接口）、property、method parameter的校驗信息，從ValidatorFactoryScopedContext繼承過來的validator通用的各類工具類（如message、script等的處理）等，內容比較複雜
二、分組（group）校驗忽略，來到默認分組處理validateConstraintsForDefaultGroup->validateConstraintsForSingleDefaultGroupElement->validateMetaConstraint(注：metaConstraints維護了該bean類型及其父類、接口的全部校驗，須要遍歷調用validateMetaConstraint)
三、繼續調用MetaConstraint的doValidateConstraint方法，根據不一樣的annotation type走不一樣的ConstraintTree

public static <U extends Annotation> ConstraintTree<U> of(ConstraintDescriptorImpl<U> composingDescriptor, Type validatedValueType) {
	if ( composingDescriptor.getComposingConstraintImpls().isEmpty() ) {
		return new SimpleConstraintTree<>( composingDescriptor, validatedValueType );
	}
	else {
		return new ComposingConstraintTree<>( composingDescriptor, validatedValueType );
	}
}
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四、具體哪些走simple，哪些走composing暫且無論，由於兩者都調用了ConstraintTree的'getInitializedConstraintValidator'方法，該步用來獲取校驗annotation（如DecimalMax、NotEmpty等）對應的validator並初始化validator 五、ConstraintHelper類維護了全部builtin的validator，並根據校驗annotation（如DecimalMax）分類，validator的描述類中維護了該validator的泛型模板（如BigDecimal），以下：

putConstraints( tmpConstraints, DecimalMax.class,  Arrays.asList(
	DecimalMaxValidatorForBigDecimal.class,
	DecimalMaxValidatorForBigInteger.class,
	DecimalMaxValidatorForDouble.class,
	DecimalMaxValidatorForFloat.class,
	DecimalMaxValidatorForLong.class,
	DecimalMaxValidatorForNumber.class,
	DecimalMaxValidatorForCharSequence.class,
	DecimalMaxValidatorForMonetaryAmount.class
) );
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在獲取具體bean類的validator時，先根據annotation獲取全部的validator，對應方法是ConstraintManager.findMatchingValidatorDescriptor，而後根據被校驗對象的類型獲取惟一的validator
六、而後根據上下文信息initializeValidator，進而調用validator的isValid方法校驗

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