这一次搞懂Spring事务注解的解析

这一次搞懂Spring事务注解的解析

前言

事务我们都知道是什么,而Spring事务就是在数据库之上利用AOP提供声明式事务编程式事务帮助我们简化开发,解耦业务逻辑和系统逻辑。但是Spring事务原理是怎样?事务在方法间是如何传播的?为什么有时候事务会失效?接下来几篇文章将重点分析Spring事务源码,让我们彻底搞懂Spring事务的原理。

正文

XML标签的解析

<tx:annotation-driven transaction-manager="transactionManager"/>

配置过事务的应该都不陌生,上面这个配置就是Spring开启事务注解(@Transactional)支持的配置,而看过我之前文章的应该知道,这个带前缀的标签叫自定义标签,我在之前的文章也分析过自定义标签的解析过程,所以这里我直接找到对应的handler:

public class TxNamespaceHandler extends NamespaceHandlerSupport {

    static final String TRANSACTION_MANAGER_ATTRIBUTE = "transaction-manager";

    static final String DEFAULT_TRANSACTION_MANAGER_BEAN_NAME = "transactionManager";


    static String getTransactionManagerName(Element element) {
        return (element.hasAttribute(TRANSACTION_MANAGER_ATTRIBUTE) ?
                element.getAttribute(TRANSACTION_MANAGER_ATTRIBUTE) : DEFAULT_TRANSACTION_MANAGER_BEAN_NAME);
    }


    @Override
    public void init() {
        registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
        registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
        registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
    }

}

可以看到对应的注解解析器就是AnnotationDrivenBeanDefinitionParser类,在该类中一定会有一个parse方法:

    public BeanDefinition parse(Element element, ParserContext parserContext) {
        registerTransactionalEventListenerFactory(parserContext);
        String mode = element.getAttribute("mode");
        if ("aspectj".equals(mode)) {
            // mode="aspectj"
            registerTransactionAspect(element, parserContext);
            if (ClassUtils.isPresent("javax.transaction.Transactional", getClass().getClassLoader())) {
                registerJtaTransactionAspect(element, parserContext);
            }
        }
        else {
            // mode="proxy"
            AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
        }
        return null;
    }

首先拿到mode属性的值判断是使用AspectJ生成代理还是JDK生成代理,这里我们主要看proxy模式,进入configureAutoProxyCreator方法:

        public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {
            // 注册AOP的入口类
            AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element);

            String txAdvisorBeanName = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME;
            if (!parserContext.getRegistry().containsBeanDefinition(txAdvisorBeanName)) {
                Object eleSource = parserContext.extractSource(element);

                // Create the TransactionAttributeSource definition.
                // @Transactional注解的属性封装
                RootBeanDefinition sourceDef = new RootBeanDefinition(
                        "org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
                sourceDef.setSource(eleSource);
                sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);

                // Create the TransactionInterceptor definition.
                // AOP执行链
                RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);
                interceptorDef.setSource(eleSource);
                interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                // 拿到transaction-manager属性的值
                registerTransactionManager(element, interceptorDef);
                interceptorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                String interceptorName = parserContext.getReaderContext().registerWithGeneratedName(interceptorDef);

                // Create the TransactionAttributeSourceAdvisor definition.
                RootBeanDefinition advisorDef = new RootBeanDefinition(BeanFactoryTransactionAttributeSourceAdvisor.class);
                advisorDef.setSource(eleSource);
                advisorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
                advisorDef.getPropertyValues().add("transactionAttributeSource", new RuntimeBeanReference(sourceName));
                advisorDef.getPropertyValues().add("adviceBeanName", interceptorName);
                if (element.hasAttribute("order")) {
                    advisorDef.getPropertyValues().add("order", element.getAttribute("order"));
                }
                parserContext.getRegistry().registerBeanDefinition(txAdvisorBeanName, advisorDef);

                CompositeComponentDefinition compositeDef = new CompositeComponentDefinition(element.getTagName(), eleSource);
                compositeDef.addNestedComponent(new BeanComponentDefinition(sourceDef, sourceName));
                compositeDef.addNestedComponent(new BeanComponentDefinition(interceptorDef, interceptorName));
                compositeDef.addNestedComponent(new BeanComponentDefinition(advisorDef, txAdvisorBeanName));
                parserContext.registerComponent(compositeDef);
            }
        }

这里的流程比较长,但逻辑很简单。首先来看注册事务AOP入口类是哪个:

    public static void registerAutoProxyCreatorIfNecessary(
            ParserContext parserContext, Element sourceElement) {

        // 将优先级更高的AOP入口类放入到IOC容器中
        BeanDefinition beanDefinition = AopConfigUtils.registerAutoProxyCreatorIfNecessary(
                parserContext.getRegistry(), parserContext.extractSource(sourceElement));
        // 设置代理生成的方式以及是否缓存代理类到当前线程
        useClassProxyingIfNecessary(parserContext.getRegistry(), sourceElement);
        registerComponentIfNecessary(beanDefinition, parserContext);
    }

主要看registerAutoProxyCreatorIfNecessary方法:

    public static BeanDefinition registerAutoProxyCreatorIfNecessary(
            BeanDefinitionRegistry registry, @Nullable Object source) {

        return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
    }

    private static BeanDefinition registerOrEscalateApcAsRequired(
            Class<?> cls, BeanDefinitionRegistry registry, @Nullable Object source) {

        Assert.notNull(registry, "BeanDefinitionRegistry must not be null");

        // 判断传进来的类和ICO中当前存在的类哪个优先级更高,将更高的放入IOC中
        if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
            BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
            if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
                int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
                int requiredPriority = findPriorityForClass(cls);
                if (currentPriority < requiredPriority) {
                    apcDefinition.setBeanClassName(cls.getName());
                }
            }
            return null;
        }

        //把AOP入口类封装成beanDefinition对象,要实例化
        RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
        beanDefinition.setSource(source);
        beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
        beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        //注解aop入口类的beanName名称 AopConfigUtils.AUTO_PROXY_CREATOR_BEAN_NAME
        registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
        return beanDefinition;
    }

首先判断容器中是否已经存在AOP入口类,如果不存在则直接创建InfrastructureAdvisorAutoProxyCreator的BeanDefinition对象注册到容器中,这个类也是我上一篇文章分析的AOP入口类AbstractAutoProxyCreator的子类,再来看看其继承关系:
在这里插入图片描述
你会不会疑惑,这么多子类,到底会使用哪一个呢?回到刚刚的代码中,可以看到如果已经存在一个入口类了,就会通过findPriorityForClass获取两个类的优先级,最终就会使用优先级更大的那个,那么它们的优先级顺序是怎样的呢?

    private static final List<Class<?>> APC_PRIORITY_LIST = new ArrayList<>(3);

    static {
        // Set up the escalation list...
        APC_PRIORITY_LIST.add(InfrastructureAdvisorAutoProxyCreator.class);
        APC_PRIORITY_LIST.add(AspectJAwareAdvisorAutoProxyCreator.class);
        APC_PRIORITY_LIST.add(AnnotationAwareAspectJAutoProxyCreator.class);
    }

    private static int findPriorityForClass(@Nullable String className) {
        // 索引即是优先级,越大优先级越高,IOC中只会存在一个事务AOP入口类
        for (int i = 0; i < APC_PRIORITY_LIST.size(); i++) {
            Class<?> clazz = APC_PRIORITY_LIST.get(i);
            if (clazz.getName().equals(className)) {
                return i;
            }
        }
        throw new IllegalArgumentException(
                "Class name [" + className + "] is not a known auto-proxy creator class");
    }

可以看到,InfrastructureAdvisorAutoProxyCreator是优先级最低的,基本上不会起作用;AspectJAwareAdvisorAutoProxyCreator是当我们配置了<aop:config>标签时会注册,也就是xml配置的AOP的入口类;而AnnotationAwareAspectJAutoProxyCreator是当我们配置了<aop:aspectj-autoproxy>或使用@EnableAspectJAutoProxy注解时注册,因此大部分情况下都是使用的AnnotationAwareAspectJAutoProxyCreator
注册完AOP的入口类后,回到configureAutoProxyCreator方法:

RootBeanDefinition sourceDef = new RootBeanDefinition(
        "org.springframework.transaction.annotation.AnnotationTransactionAttributeSource");
sourceDef.setSource(eleSource);
sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
String sourceName =parserContext.getReaderContext().registerWithGeneratedName(sourceDef);

AnnotationTransactionAttributeSource类的作用就是封装事务注解@Transactional的属性,这里需要记住其继承体系以及熟悉该类和其父类的属性和方法,对后面分析事物切面执行原理有帮助:
在这里插入图片描述
紧接着就是创建了TransactionInterceptor对象,专门的事务拦截器,并且该类是MethodInterceptor的子类,看到这个应该不陌生了,我们知道AOP调用链在执行过程中主要就是调用该类的invoke的方法,因此它是事务切面执行的入口。既然有了Interceptor,那么必不可少的还应该有Advisor,而Advisor又是由AdvicePoincut组成的,这样才能构成一个完整的切面,所以该方法后面就是创建这两个对象。以上就是xml配置AOP注解支持的原理,很简单,下面再来看看零配置又是如何实现的。

AOP零配置原理

使用过SpringBoot的都知道,如果需要开启事务注解的支持,只需要一个注解就能搞定:@EnableTransactionManagement,不用再配置xml文件,这个又是怎么做到的呢?不多说,我们直接来看其源码:

@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {

    boolean proxyTargetClass() default false;

    AdviceMode mode() default AdviceMode.PROXY;

    int order() default Ordered.LOWEST_PRECEDENCE;

}

在该注解下使用@Import导入了一个类TransactionManagementConfigurationSelector,首先该注解的作用就是导入一个类的实例到IOC容器中,你可能会说不是在类上加@Component注解就行了么,但是有些类它并不在你扫描的路径下,而该注解依然可以将其导入进来,所以我么主要看TransactionManagementConfigurationSelector类中做了些啥:

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

    @Override
    protected String[] selectImports(AdviceMode adviceMode) {
        switch (adviceMode) {
            case PROXY:
                return new String[] {AutoProxyRegistrar.class.getName(),
                        ProxyTransactionManagementConfiguration.class.getName()};
            case ASPECTJ:
                return new String[] {determineTransactionAspectClass()};
            default:
                return null;
        }
    }

    private String determineTransactionAspectClass() {
        return (ClassUtils.isPresent("javax.transaction.Transactional", getClass().getClassLoader()) ?
                TransactionManagementConfigUtils.JTA_TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME :
                TransactionManagementConfigUtils.TRANSACTION_ASPECT_CONFIGURATION_CLASS_NAME);
    }

}

可以看到在selectImports方法中返回了AutoProxyRegistrarProxyTransactionManagementConfiguration类,返回后会被封装为BeanDefinition对象,那这个方法是在哪里调用的呢?这个在之前的文章中也分析过,ConfigurationClassPostProcessor类中会调用ConfigurationClassParser类的parse方法解析@Configuration、@Import、@ImportSource等注解,具体过程这里就不再赘述了。我们继续来分别看看AutoProxyRegistrarProxyTransactionManagementConfiguration类:

public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

    private final Log logger = LogFactory.getLog(getClass());

    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        boolean candidateFound = false;
        Set<String> annoTypes = importingClassMetadata.getAnnotationTypes();
        for (String annoType : annoTypes) {
            AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annoType);
            if (candidate == null) {
                continue;
            }
            Object mode = candidate.get("mode");
            Object proxyTargetClass = candidate.get("proxyTargetClass");
            if (mode != null && proxyTargetClass != null && AdviceMode.class == mode.getClass() &&
                    Boolean.class == proxyTargetClass.getClass()) {
                candidateFound = true;
                if (mode == AdviceMode.PROXY) {
                    //注册事务AOP的入口类InfrastructureAdvisorAutoProxyCreator,实际上这个AOP入口类起不了作用
                    AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
                    if ((Boolean) proxyTargetClass) {
                        AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
                        return;
                    }
                }
            }
        }
    }
}

public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

    /*
    * 明显是创建事务切面实例
    * BeanFactoryTransactionAttributeSourceAdvisor
    *
    * */
    @Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
        BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
        advisor.setTransactionAttributeSource(transactionAttributeSource());
        //设置通知类
        advisor.setAdvice(transactionInterceptor());
        if (this.enableTx != null) {
            advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
        }
        return advisor;
    }

    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionAttributeSource transactionAttributeSource() {
        return new AnnotationTransactionAttributeSource();
    }

    /*
    * 创建事务advice
    * TransactionInterceptor
    * */
    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionInterceptor transactionInterceptor() {
        TransactionInterceptor interceptor = new TransactionInterceptor();
        interceptor.setTransactionAttributeSource(transactionAttributeSource());
        //事务管理器要跟数据源挂钩,所以需要自己定义
        if (this.txManager != null) {
            interceptor.setTransactionManager(this.txManager);
        }
        return interceptor;
    }

}

看到这就很清楚了,前者是注册AOP的入口类(这里注册的入口类依然是InfrastructureAdvisorAutoProxyCreator),后者则是创建事务AOP的组件的实例到IOC中,到这里相信不仅仅是对于事务的零配置,而是整个SpringBoot的零配置实现原理都心中有数了。

总结

本篇结合之前所学分析了事务配置解析的原理,也带出了SpringBoot零配置实现的原理,下一篇就是事务的执行调用过程。我们需要在脑海将加载、解析和调用串联起来,从微观到宏观整体把握Spring,才能真正的理解Spring。






转自夜勿语的blog,感谢夜勿语博主的分享

blog地址: https://blog.csdn.net/l6108003/article/details/106650023


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