小架构step系列27：Hibernate提供的validator

1 概述

直接使用Spring提供的Validation机制来校验请求参数比较麻烦，还好hibernate-validator包提供了注解的方式，在参数对象的属性上加上对应的注解，即可进行校验，这样就方便多了。本文来了解一下hibernate-validator是如何把Spring的Validation机制扩展成用注解这种便利方式的。

2 原理

2.1 加载过程

1) 先依赖hibernate-validator包：

<dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator</artifactId>
</dependency>

<!-- 在spring-boot-dependencies里有管理配置 -->
<properties>
    <hibernate-validator.version>6.2.5.Final</hibernate-validator.version>
</properties>
<dependency>
    <groupId>org.hibernate.validator</groupId>
    <artifactId>hibernate-validator</artifactId>
    <version>${hibernate-validator.version}</version>
</dependency>

2) 在hibernate-validator包里包含了文件/META-INF/services/javax.validation.spi.ValidationProvider，文件里配置了HibernateValidator类

org.hibernate.validator.HibernateValidator

Spring里提供了专门加载/META-INF/services目录的方法，正是通过这种方式可以支持第三方包来扩展。

3) 加载过程：

// 在spring-boot-autoconfigure包里有ValidationAutoConfiguration类，是一个带@AutoConfiguration注解的配置类。
// 当检测到classpath下有META-INF/services/javax.validation.spi.ValidationProvider时，
// 会触发ValidationAutoConfiguration这个配置类的加载。
// 源码位置：org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration
@AutoConfiguration
@ConditionalOnClass(ExecutableValidator.class)
@ConditionalOnResource(resources = "classpath:META-INF/services/javax.validation.spi.ValidationProvider")
@Import(PrimaryDefaultValidatorPostProcessor.class)
public class ValidationAutoConfiguration {
    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    @ConditionalOnMissingBean(Validator.class)
    public static LocalValidatorFactoryBean defaultValidator(ApplicationContext applicationContext) {
        // 1. 创建一个LocalValidatorFactoryBean对象，把对象作为bean
        //    LocalValidatorFactoryBean实现了InitializingBean接口，bean创建完成后执行afterPropertiesSet()
        LocalValidatorFactoryBean factoryBean = new LocalValidatorFactoryBean();
        MessageInterpolatorFactory interpolatorFactory = new MessageInterpolatorFactory(applicationContext);
        factoryBean.setMessageInterpolator(interpolatorFactory.getObject());
        return factoryBean;
    }
    // 省略其它代码
}

// 源码位置：org.springframework.validation.beanvalidation.LocalValidatorFactoryBean
public void afterPropertiesSet() {
    Configuration<?> configuration;
    if (this.providerClass != null) {
        ProviderSpecificBootstrap bootstrap = Validation.byProvider(this.providerClass);
        if (this.validationProviderResolver != null) {
            bootstrap = bootstrap.providerResolver(this.validationProviderResolver);
        }
        configuration = bootstrap.configure();
    }
    else {
        // 2. 加载默认的GenericBootstrap，其实现类为GenericBootstrapImpl
        GenericBootstrap bootstrap = Validation.byDefaultProvider();
        if (this.validationProviderResolver != null) {
            bootstrap = bootstrap.providerResolver(this.validationProviderResolver);
        }
        // 3. 进行配置
        configuration = bootstrap.configure();
    }

    if (this.applicationContext != null) {
        try {
            Method eclMethod = configuration.getClass().getMethod("externalClassLoader", ClassLoader.class);
            ReflectionUtils.invokeMethod(eclMethod, configuration, this.applicationContext.getClassLoader());
        }
        catch (NoSuchMethodException ex) {
        }
    }

    MessageInterpolator targetInterpolator = this.messageInterpolator;
    if (targetInterpolator == null) {
        targetInterpolator = configuration.getDefaultMessageInterpolator();
    }
    configuration.messageInterpolator(new LocaleContextMessageInterpolator(targetInterpolator));

    if (this.traversableResolver != null) {
        configuration.traversableResolver(this.traversableResolver);
    }

    ConstraintValidatorFactory targetConstraintValidatorFactory = this.constraintValidatorFactory;
    if (targetConstraintValidatorFactory == null && this.applicationContext != null) {
        targetConstraintValidatorFactory =
                new SpringConstraintValidatorFactory(this.applicationContext.getAutowireCapableBeanFactory());
    }
    if (targetConstraintValidatorFactory != null) {
        configuration.constraintValidatorFactory(targetConstraintValidatorFactory);
    }

    if (this.parameterNameDiscoverer != null) {
        configureParameterNameProvider(this.parameterNameDiscoverer, configuration);
    }

    List<InputStream> mappingStreams = null;
    if (this.mappingLocations != null) {
        mappingStreams = new ArrayList<>(this.mappingLocations.length);
        for (Resource location : this.mappingLocations) {
            try {
                InputStream stream = location.getInputStream();
                mappingStreams.add(stream);
                configuration.addMapping(stream);
            }
            catch (IOException ex) {
                closeMappingStreams(mappingStreams);
                throw new IllegalStateException("Cannot read mapping resource: " + location);
            }
        }
    }

    this.validationPropertyMap.forEach(configuration::addProperty);

    if (this.configurationInitializer != null) {
        this.configurationInitializer.accept(configuration);
    }
    postProcessConfiguration(configuration);

    try {
        this.validatorFactory = configuration.buildValidatorFactory();
        setTargetValidator(this.validatorFactory.getValidator());
    }
    finally {
        closeMappingStreams(mappingStreams);
    }
}

// 源码位置：javax.validation.Validation.GenericBootstrapImpl
public Configuration<?> configure() {
    // ValidationProviderResolver为DefaultValidationProviderResolver
    ValidationProviderResolver resolver = this.resolver == null ? this.getDefaultValidationProviderResolver() : this.resolver;

    List<ValidationProvider<?>> validationProviders;
    try {
        // 4. 加载所有ValidationProvider
        validationProviders = resolver.getValidationProviders();
    } catch (ValidationException e) {
        throw e;
    } catch (RuntimeException re) {
        throw new ValidationException("Unable to get available provider resolvers.", re);
    }

    if (validationProviders.isEmpty()) {
        String msg = "Unable to create a Configuration, because no Jakarta Bean Validation provider could be found. Add a provider like Hibernate Validator (RI) to your classpath.";
        throw new NoProviderFoundException(msg);
    } else {
        try {
            Configuration<?> config = ((ValidationProvider)resolver.getValidationProviders().get(0)).createGenericConfiguration(this);
            return config;
        } catch (RuntimeException re) {
            throw new ValidationException("Unable to instantiate Configuration.", re);
        }
    }
}

// 源码位置：javax.validation.Validation.DefaultValidationProviderResolver
public List<ValidationProvider<?>> getValidationProviders() {
    // 5. 加载ValidationProvider列表
    return Validation.GetValidationProviderListAction.getValidationProviderList();
}

// 源码位置：javax.validation.Validation.GetValidationProviderListAction
public static synchronized List<ValidationProvider<?>> getValidationProviderList() {
    // 5. 执行run()方法加载，INSTANCE就是Validation.GetValidationProviderListAction的对象
    return System.getSecurityManager() != null ? (List)AccessController.doPrivileged(INSTANCE) : INSTANCE.run();
}

// 源码位置：javax.validation.Validation.GetValidationProviderListAction
public List<ValidationProvider<?>> run() {
    ClassLoader classloader = Thread.currentThread().getContextClassLoader();
    List<ValidationProvider<?>> cachedContextClassLoaderProviderList = this.getCachedValidationProviders(classloader);
    if (cachedContextClassLoaderProviderList != null) {
        return cachedContextClassLoaderProviderList;
    } else {    
        // 6. 使用ClassLoader去加载
        List<ValidationProvider<?>> validationProviderList = this.loadProviders(classloader);
        if (validationProviderList.isEmpty()) {
            classloader = DefaultValidationProviderResolver.class.getClassLoader();
            List<ValidationProvider<?>> cachedCurrentClassLoaderProviderList = this.getCachedValidationProviders(classloader);
            if (cachedCurrentClassLoaderProviderList != null) {
                return cachedCurrentClassLoaderProviderList;
            }

            validationProviderList = this.loadProviders(classloader);
        }

        // 加载到的类会缓存起来
        this.cacheValidationProviders(classloader, validationProviderList);
        return validationProviderList;
    }
}

// 源码位置：javax.validation.Validation.GetValidationProviderListAction
private List<ValidationProvider<?>> loadProviders(ClassLoader classloader) {
    // 7. ServiceLoader就是Spring提供的专门加载所有jar内META-INF/services/目录指定类型类的加载器，
    //    hibernate-validator包里有/META-INF/services/javax.validation.spi.ValidationProvider文件，
    //    该文件里配置的org.hibernate.validator.HibernateValidator类就是ValidationProvider类型的
    ServiceLoader<ValidationProvider> loader = ServiceLoader.load(ValidationProvider.class, classloader);
    Iterator<ValidationProvider> providerIterator = loader.iterator();
    List<ValidationProvider<?>> validationProviderList = new ArrayList();

    while(providerIterator.hasNext()) {
        try {
            validationProviderList.add(providerIterator.next());
        } catch (ServiceConfigurationError var6) {
        }
    }

    // 这里列表里只有一个HibernateValidator对象
    return validationProviderList;
}

// 回到GenericBootstrapImpl的configure()继续处理
// 源码位置：javax.validation.Validation.GenericBootstrapImpl
public Configuration<?> configure() {
    // ValidationProviderResolver为DefaultValidationProviderResolver
    ValidationProviderResolver resolver = this.resolver == null ? this.getDefaultValidationProviderResolver() : this.resolver;

    List<ValidationProvider<?>> validationProviders;
    try {
        // 4. 加载所有ValidationProvider
        validationProviders = resolver.getValidationProviders();
    } catch (ValidationException e) {
        throw e;
    } catch (RuntimeException re) {
        throw new ValidationException("Unable to get available provider resolvers.", re);
    }

    if (validationProviders.isEmpty()) {
        String msg = "Unable to create a Configuration, because no Jakarta Bean Validation provider could be found. Add a provider like Hibe