springboot启动流程（十二）springboot事务自动配置

所有文章

https://www.cnblogs.com/lay2017/p/11478237.html

正文

在上一篇文章中，我们简单了解了aop的处理过程。代理增强之前，先生成Advisor，然后利用cglib或者jdk动态代理把可以应用到当前Bean的Advisor增强到Bean上。

springboot的事务，也是基于aop实现。那么我们就需要把事务相关的配置生成Advisor，然后一样地增强到Bean上。

生成Advisor

首先，我们先找到事务的自动配置类TransactionAutoConfiguration

@Configuration
@ConditionalOnClass(PlatformTransactionManager.class)
@AutoConfigureAfter({ JtaAutoConfiguration.class, HibernateJpaAutoConfiguration.class,
        DataSourceTransactionManagerAutoConfiguration.class, Neo4jDataAutoConfiguration.class })
@EnableConfigurationProperties(TransactionProperties.class)
public class TransactionAutoConfiguration {
    // 省略

    @Configuration
    @ConditionalOnBean(PlatformTransactionManager.class)
    @ConditionalOnMissingBean(AbstractTransactionManagementConfiguration.class)
    public static class EnableTransactionManagementConfiguration {

        // 省略

        @Configuration
        @EnableTransactionManagement(proxyTargetClass = true)
        @ConditionalOnProperty(prefix = "spring.aop", name = "proxy-target-class", havingValue = "true", matchIfMissing = true)
        public static class CglibAutoProxyConfiguration {

        }

    }

}

这里，我们注意到@EnableTransactionManagement这个注解。熟悉spring的我们都知道，这个意味着开启事务管理。我们打开这个注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Import(TransactionManagementConfigurationSelector.class)
public @interface EnableTransactionManagement {
    //
}

跟其它@Enable ** 形式的注解一样，通过@Import注解导入了一个类，跟进TransactionManagementConfigurationSelector看看

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

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

    //

}

这里将会导入两个类AutoProxyRegistrar和ProxyTransactionManagementConfiguration，前者是处理代理，后者是处理事务配置的。上一篇讲述aop的时候我们说过，创建代理将会通过AnnotationAwareAspectJAutoProxyCreator来处理，所以这里的AutoProxyRegistrar比AnnotationAwareAspectJAutoProxyCreator的优先级低。

ProxyTransactionManagementConfiguration是处理事务配置的，我们跟进它

@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

    @Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
        BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
        advisor.setTransactionAttributeSource(transactionAttributeSource());
        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();
    }

    @Bean
    @Role(BeanDefinition.ROLE_INFRASTRUCTURE)
    public TransactionInterceptor transactionInterceptor() {
        TransactionInterceptor interceptor = new TransactionInterceptor();
        interceptor.setTransactionAttributeSource(transactionAttributeSource());
        if (this.txManager != null) {
            interceptor.setTransactionManager(this.txManager);
        }
        return interceptor;
    }

}

正如我们开篇说的，事务是基于aop的，会去生成Advisor。我们看到transactionAdvisor将会返回一个BeanFactoryTransactionAttributeSourceAdvisor。我们看看它的uml类图

这里就是生成了一个Advisor，并作为Bean存在于BeanFactory当中。

增强到Bean

我们打开AbstractAutoProxyCreator的wrapIfNecessary方法

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    //

    // 获取可以增强到当前Bean的Advisor
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    if (specificInterceptors != DO_NOT_PROXY) {
        this.advisedBeans.put(cacheKey, Boolean.TRUE);
        // 创建代理
        Object proxy = createProxy(bean.getClass(), beanName, specificInterceptors, new SingletonTargetSource(bean));
        this.proxyTypes.put(cacheKey, proxy.getClass());
        return proxy;
    }

    this.advisedBeans.put(cacheKey, Boolean.FALSE);
    return bean;
}

getAdvicesAndAdvisorsForBean将会获取到事务生成的Advisor，然后createProxy将会进行代理增强，cglib或者jdk动态代理的方式。

跟进AbstractAdvisorAutoProxyCreator的findEligibleAdvisors方法

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    // 找到Bean工厂里所有Advisor
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    // 获取可以增强到当前Bean的Advisor    
    List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    extendAdvisors(eligibleAdvisors);
    if (!eligibleAdvisors.isEmpty()) {
        eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    }
    return eligibleAdvisors;
}

跟进findAdvisorsThatCanApply看看怎么判断是否可以增强

protected List<Advisor> findAdvisorsThatCanApply(
        List<Advisor> candidateAdvisors, Class<?> beanClass, String beanName) {

    ProxyCreationContext.setCurrentProxiedBeanName(beanName);
    try {
        // 找到可以增强的Advisor
        return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
    }
    finally {
        ProxyCreationContext.setCurrentProxiedBeanName(null);
    }
}

继续跟进findAdvisorsThatCanApply

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
    //

    List<Advisor> eligibleAdvisors = new ArrayList<>();

    //

    for (Advisor candidate : candidateAdvisors) {
        //
        if (canApply(candidate, clazz, hasIntroductions)) {
            eligibleAdvisors.add(candidate);
        }
    }
    
    return eligibleAdvisors;
}

判断逻辑落在了canApply方法上，跟进它

public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
    if (advisor instanceof IntroductionAdvisor) {
        return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
    }
    // 进入这里分支
    else if (advisor instanceof PointcutAdvisor) {
        PointcutAdvisor pca = (PointcutAdvisor) advisor;
        return canApply(pca.getPointcut(), targetClass, hasIntroductions);
    }
    else {
        return true;
    }
}

这里的canApply将会判断PointcutAdvisor是否能增强到targetClass上，继续跟进canApply

public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
    //

    MethodMatcher methodMatcher = pc.getMethodMatcher();
    //

    IntroductionAwareMethodMatcher introductionAwareMethodMatcher = null;
    if (methodMatcher instanceof IntroductionAwareMethodMatcher) {
        introductionAwareMethodMatcher = (IntroductionAwareMethodMatcher) methodMatcher;
    }

    Set<Class<?>> classes = new LinkedHashSet<>();
    if (!Proxy.isProxyClass(targetClass)) {
        classes.add(ClassUtils.getUserClass(targetClass));
    }
    classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));

    for (Class<?> clazz : classes) {
        // 获取所有方法
        Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
        for (Method method : methods) {
            // 判断是否能够应用在方法上
            if (introductionAwareMethodMatcher != null ?
                    introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
                    methodMatcher.matches(method, targetClass)) {
                return true;
            }
        }
    }

    return false;
}

总结

本文简单过了一下事务的自动配置到代理增强。事务基于自动配置机制和aop，自动配置机制将会生成Advisor，然后通过代理增强到Bean上，从而实现事务的代理增强。