声明：本文是自己在学习Spring注解事务处理源代码时所留下的笔记； 难免有错误，敬请读者谅解！！！

1、事务注解标签

    <tx:annotation-driven />

2、tx 命名空间解析器
事务tx命名空间解析器TxNamespaceHandler
org.springframework.transaction.config.TxNamespaceHandler#init

3、AnnotationDrivenBeanDefinitionParser#parse 解析事务标签

（1）、以下方法的核心逻辑主要是选择是否使用 Aspect 方式实现代理，默认方式为 JDK 的动态代理。
org.springframework.transaction.config.AnnotationDrivenBeanDefinitionParser#parse

public BeanDefinition parse(Element element, ParserContext parserContext) {
    String mode = element.getAttribute("mode");
    if ("aspectj".equals(mode)) {
        // mode="aspectj"
        registerTransactionAspect(element, parserContext);
    }
    else {
        // mode="proxy"   注意 AopAutoProxyConfigurer 为当前内的内部类
        AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
    }
    return null;
}

（2）、进入如下方法，该方法的核心逻辑通过硬编码的方式配置 Aop 动态代理的解析器
AopAutoProxyConfigurer#configureAutoProxyCreator
通过硬编码，Spring 为我们定义了如下的 Spring BeanDefinition 对象
(a)、AnnotationTransactionAttributeSource.class 事务注解属性解析器BeanDefinition 对象。
AnnotationTransactionAttributeSource 构造方法会初始化：

public AnnotationTransactionAttributeSource() {
        this(true);
}

public AnnotationTransactionAttributeSource(boolean publicMethodsOnly) {
    this.publicMethodsOnly = publicMethodsOnly;
    this.annotationParsers = new LinkedHashSet<TransactionAnnotationParser>(2);
    this.annotationParsers.add(new SpringTransactionAnnotationParser());// @Transactional 注解解析器
    if (ejb3Present) {
        this.annotationParsers.add(new Ejb3TransactionAnnotationParser());// Ejb 解析器
    }

(b)、TransactionInterceptor.class 事务拦截器BeanDefinition 对象
(c)、BeanFactoryTransactionAttributeSourceAdvisor.class 事务切面解析器
(d)、TransactionInterceptor.class 事务拦截器BeanDefinition 对象
(e)、容易忽略的第一行代码：AopNamespaceUtils.registerAutoProxyCreatorIfNecessary(parserContext, element); 在这个方法内部 Spring 为我们的注入了：InfrastructureAdvisorAutoProxyCreator.class

private static class AopAutoProxyConfigurer {

    public static void configureAutoProxyCreator(Element element, ParserContext parserContext) {

        // 非常重要的一行代码，在这个里面注册了：InfrastructureAdvisorAutoProxyCreator.class 该类实现了Spring BeanProcessor 的扩展接口
        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.
            RootBeanDefinition sourceDef = new RootBeanDefinition(AnnotationTransactionAttributeSource.class);// 事务注解解析器
            sourceDef.setSource(eleSource);
            sourceDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
            String sourceName = parserContext.getReaderContext().registerWithGeneratedName(sourceDef);

            // Create the TransactionInterceptor definition.
            RootBeanDefinition interceptorDef = new RootBeanDefinition(TransactionInterceptor.class);//事务拦截器
            interceptorDef.setSource(eleSource);
            interceptorDef.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
            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);//事务切面解析器
            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);
        }
    }
}

4、Spring Bean 实例化创建代理对象
（a）、AbstractAutowireCapableBeanFactory#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)

（b）、AbstractAutoProxyCreator#postProcessAfterInitialization

还记得上面我们提到的 InfrastructureAdvisorAutoProxyCreator 的类图吧，最后我们的 @Transactional 注解的类会执行该类中的 postProcessAfterInitialization 方法

（c）、Bean 的初始化后置处理，通过注释可以了解到，当前方法处理后会返回一个 bean 的代理对象

/**
 * Create a proxy with the configured interceptors if the bean is
 * identified as one to proxy by the subclass.
 * @see #getAdvicesAndAdvisorsForBean
 */
public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
    if (bean != null) {
        Object cacheKey = getCacheKey(bean.getClass(), beanName);
        if (!this.earlyProxyReferences.containsKey(cacheKey)) {
            return wrapIfNecessary(bean, beanName, cacheKey);// 创建代理类的核心方法
        }
    }
    return bean;
}

（d）、AbstractAutoProxyCreator#wrapIfNecessary

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
    if (beanName != null && this.targetSourcedBeans.containsKey(beanName)) {
        return bean;
    }
    if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
        return bean;
    }
    if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
        this.advisedBeans.put(cacheKey, Boolean.FALSE);
        return bean;
    }

    // Create proxy if we have advice.   获取切面   获取的过程是一个非常复杂的过程
    Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
    if (specificInterceptors != DO_NOT_PROXY) {
        this.advisedBeans.put(cacheKey, Boolean.TRUE);

        // 创建代理对象，默认的情况下会使用 JDK 的动态代理接口创建代理对象
        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;
}

（c）、获取到的事务切面
org.springframework.transaction.interceptor.AbstractFallbackTransactionAttributeSource#computeTransactionAttribute

(d)、事务切面获取逻辑

(e)、委托 ProxyFactory 创建代理对象

protected Object createProxy(
        Class<?> beanClass, String beanName, Object[] specificInterceptors, TargetSource targetSource) {

    ProxyFactory proxyFactory = new ProxyFactory();
    // Copy our properties (proxyTargetClass etc) inherited from ProxyConfig.
    proxyFactory.copyFrom(this);

    if (!shouldProxyTargetClass(beanClass, beanName)) {
        // Must allow for introductions; can't just set interfaces to
        // the target's interfaces only.
        Class<?>[] targetInterfaces = ClassUtils.getAllInterfacesForClass(beanClass, this.proxyClassLoader);
        for (Class<?> targetInterface : targetInterfaces) {
            proxyFactory.addInterface(targetInterface);
        }
    }

    Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
    for (Advisor advisor : advisors) {
        proxyFactory.addAdvisor(advisor);
    }

    proxyFactory.setTargetSource(targetSource);
    customizeProxyFactory(proxyFactory);

    proxyFactory.setFrozen(this.freezeProxy);
    if (advisorsPreFiltered()) {
        proxyFactory.setPreFiltered(true);
    }

    // 最终会使用：JdkDynamicAopProxy  创建事务的Aop 代理对象
    return proxyFactory.getProxy(this.proxyClassLoader);
}

(f)、最终生成代理对象

5、代理类执行
JdkDynamicAopProxy#invoke

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    MethodInvocation invocation;
    Object oldProxy = null;
    boolean setProxyContext = false;

    TargetSource targetSource = this.advised.targetSource;
    Class<?> targetClass = null;
    Object target = null;

    try {
        // 如果目标方法没有实现equals
        if (!this.equalsDefined && AopUtils.isEqualsMethod(method)) {
            // The target does not implement the equals(Object) method itself.
            return equals(args[0]);
        }
        // 如果目标方法没有实现hashcode
        if (!this.hashCodeDefined && AopUtils.isHashCodeMethod(method)) {
            // The target does not implement the hashCode() method itself.
            return hashCode();
        }
        // 根据代理对象的配置来调用服务
        if (!this.advised.opaque && method.getDeclaringClass().isInterface() &&
                method.getDeclaringClass().isAssignableFrom(Advised.class)) {
            // Service invocations on ProxyConfig with the proxy config...
            return AopUtils.invokeJoinpointUsingReflection(this.advised, method, args);
        }

        Object retVal;

        if (this.advised.exposeProxy) {
            // Make invocation available if necessary.
            oldProxy = AopContext.setCurrentProxy(proxy);
            setProxyContext = true;
        }

        // May be null. Get as late as possible to minimize the time we "own" the target,
        // in case it comes from a pool.
        // 获取目标对象
        target = targetSource.getTarget();
        if (target != null) {
            targetClass = target.getClass();
        }

        // 获取定义好的拦截器链
        // Get the interception chain for this method.
        List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);

        // Check whether we have any advice. If we don't, we can fallback on direct
        // reflective invocation of the target, and avoid creating a MethodInvocation.
        if (chain.isEmpty()) {
            // We can skip creating a MethodInvocation: just invoke the target directly
            // Note that the final invoker must be an InvokerInterceptor so we know it does
            // nothing but a reflective operation on the target, and no hot swapping or fancy proxying.
            // 没有拦截链则直接调用target方法
            retVal = AopUtils.invokeJoinpointUsingReflection(target, method, args);
        }
        else {
            // We need to create a method invocation...
            //对拦截链进行封装  得到对象ReflectiveMethodInvocation 调用 proceed 方法
            invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
            // Proceed to the joinpoint through the interceptor chain.
            retVal = invocation.proceed();// 方法内部将执行拦截器的切面直到目标方法被执行
        }

        // Massage return value if necessary.
        Class<?> returnType = method.getReturnType();
        if (retVal != null && retVal == target && returnType.isInstance(proxy) &&
                !RawTargetAccess.class.isAssignableFrom(method.getDeclaringClass())) {
            // Special case: it returned "this" and the return type of the method
            // is type-compatible. Note that we can't help if the target sets
            // a reference to itself in another returned object.
            retVal = proxy;
        }
        else if (retVal == null && returnType != Void.TYPE && returnType.isPrimitive()) {
            throw new AopInvocationException(
                    "Null return value from advice does not match primitive return type for: " + method);
        }
        return retVal;
    }
    finally {
        if (target != null && !targetSource.isStatic()) {
            // Must have come from TargetSource.
            targetSource.releaseTarget(target);
        }
        if (setProxyContext) {
            // Restore old proxy.
            AopContext.setCurrentProxy(oldProxy);
        }
    }
}

7、代理会执行到： ReflectiveMethodInvocation#proceed 方法

8、最终会执行到：TransactionInterceptor#invoke 方法

9、执行 TransactionAspectSupport 事务方法
TransactionInterceptor#invoke 方法会调用到父类的 TransactionAspectSupport#invokeWithinTransaction 方法

10、业务方法执行

11、Spring 事务处理的流程

（a）、开启事务

（b）、事务回滚

（c）、事务提交