Spring事务实现原理

代理对象在哪里创建

先从bean被创建后如何产生代理对象开始，在AbstractAutowireCapableBeanFactory.doCreateBean 初始化bean创建后，并且将依赖注入到bean中，在调用initializeBean 方法对刚刚完成依赖注入bean进行一次"初始化"

    protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd) {
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                invokeAwareMethods(beanName, bean);
                return null;
            }, getAccessControlContext());
        }
        else {
            invokeAwareMethods(beanName, bean);
        }

        Object wrappedBean = bean;
        if (mbd == null || !mbd.isSynthetic()) {
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }

        try {
            invokeInitMethods(beanName, wrappedBean, mbd);
        }
        catch (Throwable ex) {
        }
        if (mbd == null || !mbd.isSynthetic()) {
            //就是在这里对符合条件bean 转换成 代理对象  对象 -> AnnotationAwareAspectJAutoProxyCreator
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }

        return wrappedBean;
    }

AbstractAutoProxyCreator.postProcessAfterInitialization

    public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
        if (bean != null) {
            //判断Class FactoryBean 实现类，修改bean名字 在beanName前面加上&
            Object cacheKey = getCacheKey(bean.getClass(), beanName);
            if (this.earlyProxyReferences.remove(cacheKey) != bean) {
                return wrapIfNecessary(bean, beanName, cacheKey); //这里将会返回代理后的对象
            }
        }
        return bean;
    }

    protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
        //targetSourcedBeans 没有生成代理bean 缓存
        if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(beanName)) {
            return bean;
        }
        //advisedBeans 也是缓存，返回false 则不会生成代理对象
        if (Boolean.FALSE.equals(this.advisedBeans.get(cacheKey))) {
            return bean;
        }
        //ORIGINAL 后置表明bean 实例不会变，则不会生成代理对象
        if (isInfrastructureClass(bean.getClass()) || shouldSkip(bean.getClass(), beanName)) {
            this.advisedBeans.put(cacheKey, Boolean.FALSE);
            return bean;
        }

        // Create proxy if we have advice.
        //这里会返回 BeanFactoryTransactionAttributeSourceAdvisor,如果不创建代理对象，这里就会返回空数组
        Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
        if (specificInterceptors != DO_NOT_PROXY) {
            this.advisedBeans.put(cacheKey, Boolean.TRUE); //缓存已经解析过bean，后面就不用再解析一次class
            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;
    }

    protected Object[] getAdvicesAndAdvisorsForBean(
            Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {
        //使用Advisor 去处理class 是否需要生成代理对象，如果需要则返回 advisors 不为空
        List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
        if (advisors.isEmpty()) {
            return DO_NOT_PROXY;
        }
        return advisors.toArray();
    }


    protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
        //从容器中获取内置Advisor 使用一个Advisor 生成Advisor 通过代理工厂生成一堆代理对象
        //这里会返回 BeanFactoryTransactionAttributeSourceAdvisor
        List<Advisor> candidateAdvisors = findCandidateAdvisors();

        List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
        extendAdvisors(eligibleAdvisors);
        if (!eligibleAdvisors.isEmpty()) {
            eligibleAdvisors = sortAdvisors(eligibleAdvisors);
        }
        return eligibleAdvisors;
    }

上面判断主要做一些检查，当所有状态合法后才会进入getAdvicesAndAdvisorsForBean返回通过指定bean生成的通知，在通过Advisor数组生成代理对象。这个方法主要逻辑就是通过

BeanFactoryTransactionAttributeSourceAdvisor 工厂内置Advisor解析Class并且生成pointcut 切点。主要实现在AopUtils

    /**
    * candidateAdvisors 内置Advisor 也就是BeanFactoryTransactionAttributeSourceAdvisor
    *  给定Class 找寻可用Advisor
    */
    public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
        if (candidateAdvisors.isEmpty()) {
            return candidateAdvisors;
        }
        List<Advisor> eligibleAdvisors = new ArrayList<>();
        // IntroductionAdvisor只能应用于类级别 事务一般定位到Method上，不会使用这种类型Advisor
        for (Advisor candidate : candidateAdvisors) {
            if (candidate instanceof IntroductionAdvisor && canApply(candidate, clazz)) {
                eligibleAdvisors.add(candidate);
            }
        }
        boolean hasIntroductions = !eligibleAdvisors.isEmpty();
        for (Advisor candidate : candidateAdvisors) {
            if (candidate instanceof IntroductionAdvisor) {
                // already processed
                continue;
            }
                        // 这里会使用candidate 去解析Class 如果需要生成代理方法或者代理对象 将会返回true
            if (canApply(candidate, clazz, hasIntroductions)) {
                eligibleAdvisors.add(candidate);
            }
        }
        return eligibleAdvisors;
    }        
    public static boolean canApply(Advisor advisor, Class<?> targetClass, boolean hasIntroductions) {
        if (advisor instanceof IntroductionAdvisor) {
            return ((IntroductionAdvisor) advisor).getClassFilter().matches(targetClass);
        }
        else if (advisor instanceof PointcutAdvisor) {//代理一般都是方法层面，选用PointcutAdvisor
            PointcutAdvisor pca = (PointcutAdvisor) advisor;
            return canApply(pca.getPointcut(), targetClass, hasIntroductions);
        }
        else {
            // It doesn't have a pointcut so we assume it applies.
            return true;
        }
    }
        //pointcut 为 TransactionAttributeSourcePointcut 内部类
    public static boolean canApply(Pointcut pc, Class<?> targetClass, boolean hasIntroductions) {
        Assert.notNull(pc, "Pointcut must not be null");
        if (!pc.getClassFilter().matches(targetClass)) {
            return false;
        }
                //方法匹配器，用于解析Method 是否需要生成代理方法
        MethodMatcher methodMatcher = pc.getMethodMatcher();
        if (methodMatcher == MethodMatcher.TRUE) { //没有任何逻辑，没有方法都会生成代理方法
            // No need to iterate the methods if we're matching any method anyway...
            return true;
        }

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

        Set<Class<?>> classes = new LinkedHashSet<>();
        if (!Proxy.isProxyClass(targetClass)) { //向上获取父类class，排除掉代理class
            classes.add(ClassUtils.getUserClass(targetClass));
        }
        classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));

        for (Class<?> clazz : classes) {
            Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
            for (Method method : methods) {
                if (introductionAwareMethodMatcher != null ?
                        introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
                        methodMatcher.matches(method, targetClass)) { //最终调用方法匹配器找到适合方法
                    return true;
                }
            }
        }

        return false;
    }

本质依然是使用BeanFactoryTransactionAttributeSourceAdvisor 内部对象来匹配Class 或Method，并且生成Advisor。
主要流程使用BeanFactoryTransactionAttributeSourceAdvisor.Pointcut(TransactionAttributeSourcePointcut 抽象类) -> TransactionAttributeSourcePointcut.matches ->AbstractFallbackTransactionAttributeSource.getTransactionAttribute
-> AnnotationTransactionAttributeSource.findTransactionAttribute ->AnnotationTransactionAttributeSource.determineTransactionAttribute -> TransactionAnnotationParser.TransactionAttribute

其中在AnnotationTransactionAttributeSource.determineTransactionAttribute 方法会使用Spring 支持TransactionAnnotationParser 数组去解析method并且返回TransactionAttribute
TransactionAnnotationParser是Spring 事务注解解析器接口在Class、Method 上解析注解并且将声明注解解析成TransactionAttribute 支持3种实现

• SpringTransactionAnnotationParser Spring自身数据库事务 解析@Transactional
• Ejb3TransactionAnnotationParser EJB事务 解析 javax.ejb.TransactionAttribute
• JtaTransactionAnnotationParser JTA1.2 事务 解析javax.transaction.Transactional

我们一起看下如何生成代理对象的createProxy

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

        if (this.beanFactory instanceof ConfigurableListableBeanFactory) {
            AutoProxyUtils.exposeTargetClass((ConfigurableListableBeanFactory) this.beanFactory, beanName, beanClass);
        }

        ProxyFactory proxyFactory = new ProxyFactory();
        proxyFactory.copyFrom(this);

        if (!proxyFactory.isProxyTargetClass()) { // 如果还没有设置代理目标类这里在设置一次
            if (shouldProxyTargetClass(beanClass, beanName)) {
                proxyFactory.setProxyTargetClass(true);
            }
            else {
                evaluateProxyInterfaces(beanClass, proxyFactory);
            }
        }

        //这里返回Advisor 所以仍然是返回BeanFactoryTransactionAttributeSourceAdvisor
        Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
        proxyFactory.addAdvisors(advisors);
        proxyFactory.setTargetSource(targetSource);
        customizeProxyFactory(proxyFactory);

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

        // Use original ClassLoader if bean class not locally loaded in overriding class loader
        ClassLoader classLoader = getProxyClassLoader();
        if (classLoader instanceof SmartClassLoader && classLoader != beanClass.getClassLoader()) {
            classLoader = ((SmartClassLoader) classLoader).getOriginalClassLoader();
        }
        // 这里有两个逻辑，一根据需求创建AopProxy 二 调用getProxy 创建代理对象
        return proxyFactory.getProxy(classLoader);
    }

调用AopProxy创建代理目标类，根据不同情况初始化不同AopProxy

public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {
    @Override
    public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
        // GraalVM Native Image 只支持 Dynamic proxy (java.lang.reflect.Proxy)
        if (!NativeDetector.inNativeImage() &&
                (config.isOptimize() || config.isProxyTargetClass() || hasNoUserSuppliedProxyInterfaces(config))) {
            Class<?> targetClass = config.getTargetClass();
            if (targetClass == null) {
                throw new AopConfigException("TargetSource cannot determine target class: " +
                        "Either an interface or a target is required for proxy creation.");
            }
            if (targetClass.isInterface() || Proxy.isProxyClass(targetClass)) {
                return new JdkDynamicAopProxy(config);
            }
            return new ObjenesisCglibAopProxy(config);
        }
        else {
            return new JdkDynamicAopProxy(config);
        }
    }

    /**
     * Determine whether the supplied {@link AdvisedSupport} has only the
     * {@link org.springframework.aop.SpringProxy} interface specified
     * (or no proxy interfaces specified at all).
     */
    private boolean hasNoUserSuppliedProxyInterfaces(AdvisedSupport config) {
        Class<?>[] ifcs = config.getProxiedInterfaces();
        return (ifcs.length == 0 || (ifcs.length == 1 && SpringProxy.class.isAssignableFrom(ifcs[0])));
    }
}

其中JdkDynamicAopProxy 是通过InvocationHandler 接口实现，ObjenesisCglibAopProxy就是通过Cglib实现，这次只有看下Cglib如何创建动态对象的

    public Object getProxy(@Nullable ClassLoader classLoader) {

        try {
            Class<?> rootClass = this.advised.getTargetClass();
            Assert.state(rootClass != null, "Target class must be available for creating a CGLIB proxy");

            Class<?> proxySuperClass = rootClass;
            if (rootClass.getName().contains(ClassUtils.CGLIB_CLASS_SEPARATOR)) {
                proxySuperClass = rootClass.getSuperclass();
                Class<?>[] additionalInterfaces = rootClass.getInterfaces();
                for (Class<?> additionalInterface : additionalInterfaces) {
                    this.advised.addInterface(additionalInterface);
                }
            }

            // Validate the class, writing log messages as necessary.
            validateClassIfNecessary(proxySuperClass, classLoader);

            // Configure CGLIB Enhancer...
            //Cglib 核心就是通过Enhancer 对象去创建代理
            Enhancer enhancer = createEnhancer();
            if (classLoader != null) {
                enhancer.setClassLoader(classLoader);
                if (classLoader instanceof SmartClassLoader &&
                        ((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
                    enhancer.setUseCache(false);
                }
            }
            enhancer.setSuperclass(proxySuperClass);
            enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
            enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
            enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(classLoader));
            //这里将Spring 内置7MethodInterceptor 实现
            Callback[] callbacks = getCallbacks(rootClass);
            Class<?>[] types = new Class<?>[callbacks.length];
            for (int x = 0; x < types.length; x++) {
                types[x] = callbacks[x].getClass();
            }
            // fixedInterceptorMap only populated at this point, after getCallbacks call above
            enhancer.setCallbackFilter(new ProxyCallbackFilter(
                    this.advised.getConfigurationOnlyCopy(), this.fixedInterceptorMap, this.fixedInterceptorOffset));
            enhancer.setCallbackTypes(types);

            // Generate the proxy class and create a proxy instance.
            return createProxyClassAndInstance(enhancer, callbacks);
        }

    }

Cglib样例

编写一个简单Demo类，对方法进行前后增强。

public class Demo {

    public void  call(){
        System.out.println("我就是目标类原始方法");
    }
}

编写拦截器

public class CglibMethodInterceptor implements MethodInterceptor {

        /**
         * 通过在intercept 上重写方法达到通知增强逻辑
         * @param o 代表Cglib 生成的动态代理类 对象本身
         * @param method 代理类中被拦截的接口方法 Method 实例
         * @param objects 接口方法参数
         * @param methodProxy 用于调用父类真正的业务类方法。可以直接调用被代理类接口方法 原始方法
         * @return
         * @throws Throwable
         */
        @Override
        public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
            System.out.println("invoke before....");
            MonitorUtil.start();
            Object invoke = methodProxy.invokeSuper(o, objects);
            //Object invoke = method.invoke(o,objects); 这样会导致栈溢出
            System.out.println("invoken after");
            MonitorUtil.finish();
            return invoke;
        }
    }

最后创建代理对象

    @Test
    public void cglibTest(){
        Enhancer enhancer = new Enhancer();
        enhancer.setClassLoader(this.getClass().getClassLoader());
        enhancer.setSuperclass(Demo.class);
        enhancer.setCallback(new CglibMethodInterceptor());
        Demo proxyInst = (Demo) enhancer.create();
        proxyInst.call();
    }

执行结果

invoke before....
我就是目标类原始方法
invoken after

其实跟JDK动态代理写法差不多，都是通过在原始方法前后插入代码，达到增强。CGLIB支持多个MethodInterceptor，组成一个拦截器链，按照一定顺序执行intercept。这种方法有利于AOP结构和代理业务代码解耦。

事务如何通过代理来实现的

通过上面一个小例子，我们已经了解到实现代理逻辑核心就是getCallbacks(rootClass) 返回拦截器，内置拦截器有7种，事务实现类就是在CglibAopProxy.DynamicAdvisedInterceptor。

private static class DynamicAdvisedInterceptor implements MethodInterceptor, Serializable {

        private final AdvisedSupport advised;

        public DynamicAdvisedInterceptor(AdvisedSupport advised) {
            this.advised = advised;
        }

        @Override
        @Nullable
        public Object intercept(Object proxy, Method method, Object[] args, MethodProxy methodProxy) throws Throwable {
            Object oldProxy = null;
            boolean setProxyContext = false;
            Object target = null;
            TargetSource targetSource = this.advised.getTargetSource();
            try {
                if (this.advised.exposeProxy) {
                    // Make invocation available if necessary.
                    oldProxy = AopContext.setCurrentProxy(proxy);
                    setProxyContext = true;
                }
                // Get as late as possible to minimize the time we "own" the target, in case it comes from a pool...
                target = targetSource.getTarget();
                Class<?> targetClass = (target != null ? target.getClass() : null);
                //这里会返回TransactionInterceptor 事务执行核心类
                List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);
                Object retVal;
                // Check whether we only have one InvokerInterceptor: that is,
                // no real advice, but just reflective invocation of the target.
                if (chain.isEmpty() && Modifier.isPublic(method.getModifiers())) { //没有代理拦截器
                    // 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.
                    Object[] argsToUse = AopProxyUtils.adaptArgumentsIfNecessary(method, args);
                    retVal = methodProxy.invoke(target, argsToUse);
                }
                else {
                    // We need to create a method invocation...  在这里执行事务
                    retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
                }
                retVal = processReturnType(proxy, target, method, retVal); //包装返回类型
                return retVal;
            }
            finally {
                if (target != null && !targetSource.isStatic()) {
                    targetSource.releaseTarget(target);
                }
                if (setProxyContext) {
                    // Restore old proxy.
                    AopContext.setCurrentProxy(oldProxy);
                }
            }
        }

这个内部类主要就是两个核心方法getInterceptorsAndDynamicInterceptionAdvice，从BeanFactoryTransactionAttributeSourceAdvisor.pointcut 返回MethodInterceptor 实现类TransactionInterceptor 并且使用InterceptorAndDynamicMethodMatcher 将返回MethodInterceptor、MethodMatcher 包装起来，下面会使用到的。
将拦截器执行链作为构造器参数初始化CglibMethodInvocation，调用proceed 执行事务，proceed 会调用父类ReflectiveMethodInvocation.proceed，核心逻辑就在里面了。

    public Object proceed() throws Throwable {
        // We start with an index of -1 and increment early.
        // currentInterceptorIndex 默认是-1 相等表示拦截器链里面没有代理方法，直接执行原方法
        if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
            return invokeJoinpoint();
        }
        //从第一个开始执行
        Object interceptorOrInterceptionAdvice =
                this.interceptorsAndDynamicMethodMatchers.get(++this.currentInterceptorIndex);
        if (interceptorOrInterceptionAdvice instanceof InterceptorAndDynamicMethodMatcher) {
            // Evaluate dynamic method matcher here: static part will already have
            // been evaluated and found to match.
            InterceptorAndDynamicMethodMatcher dm =
                    (InterceptorAndDynamicMethodMatcher) interceptorOrInterceptionAdvice;
            Class<?> targetClass = (this.targetClass != null ? this.targetClass : this.method.getDeclaringClass());
            //dm 就是上面写到TransactionAnnotationParser
            if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
                return dm.interceptor.invoke(this); //调用TransactionInterceptor.invoke
            }
            else {
                // Dynamic matching failed.
                // Skip this interceptor and invoke the next in the chain.
                return proceed();
            }
        }
        else {
            // It's an interceptor, so we just invoke it: The pointcut will have
            // been evaluated statically before this object was constructed.
            return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
        }
    }

这里使用递归方式执行调用链
TransactionInterceptor 可以说是Spring事务执行器了，它负责执行事务，它自己本身没有任务事务实现代码，都是通TransactionManager 事务管理器来实现事务开始、回滚、提交。
直接从TransactionInterceptor.invoke 开始分析

    public Object invoke(MethodInvocation invocation) throws Throwable {
        // Work out the target class: may be {@code null}.
        // The TransactionAttributeSource should be passed the target class
        // as well as the method, which may be from an interface.
        Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

        // Adapt to TransactionAspectSupport's invokeWithinTransaction...
        //这里调用父类TransactionAspectSupport.invokeWithinTransaction
        return invokeWithinTransaction(invocation.getMethod(), targetClass, new CoroutinesInvocationCallback() {
            @Override
            @Nullable
            public Object proceedWithInvocation() throws Throwable {
                return invocation.proceed();
            }
            @Override
            public Object getTarget() {
                return invocation.getThis();
            }
            @Override
            public Object[] getArguments() {
                return invocation.getArguments();
            }
        });
    }

invokeWithinTransaction 看方法吗方法命名就知道干什么的，执行事务方法。三个参数
Method 要执行方法，主要是获取事务注解上属性
Class 被代理Class，作业跟Method差不多
InvocationCallback 被实现的class，主要用于执行代理方法。要知道Spring AOP是代理chain 方式执行，一个类不单止是被事务代理的，还有会因为其他业务被代理了，保证代理链能全部执行下去。

protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
            final InvocationCallback invocation) throws Throwable {

        // If the transaction attribute is null, the method is non-transactional.
        TransactionAttributeSource tas = getTransactionAttributeSource();
        //TransactionAttribute 是执行事务必须实体 包含很多重要信息 事务隔离级别、事务传播级别、异常回滚等
        final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
        //获取TransactionManager 事务管理器，再平常开发中可以存在自己配置事务管理器情况，先读取@Transactional value 属性，
        //没有从transactionManagerBeanName 可以从配置文件指定
        //最后就是Spring默认事务实现
        final TransactionManager tm = determineTransactionManager(txAttr);

        if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager) {
            //这里是反应式事务 Mongdb NOSQL使用，这里略过
        }

        PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
        final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
        
        // CallbackPreferringPlatformTransactionManager 是 PlatformTransactionManager  扩展接口提供在执行事务时暴露一个回调方法
        if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
            // Standard transaction demarcation with getTransaction and commit/rollback calls.
            // 获取正在执行的事务状态或者创建一个事务 TransactionInfo 事务状态 会记录事务执行，用于回滚
            TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

            Object retVal;
            try {
                // This is an around advice: Invoke the next interceptor in the chain.
                // This will normally result in a target object being invoked.
                // invocation 就是被实现类，调用原始方法或者代理方法
                // 其实这里就是around advice 环绕通知 前面开启事务，下面方法负责回滚或提交
                retVal = invocation.proceedWithInvocation();
            }
            catch (Throwable ex) {
                // target invocation exception
                //处理业务异常，如果异常符合回滚，就会回滚否则就是commit
                completeTransactionAfterThrowing(txInfo, ex);
                throw ex;
            }
            finally {
                 // 在ThreadLocal 清除事务状态txInfo
                 //要知道所有事务都通过ThreadLocal 进行传递
                 //在正常或者异常情况下，清除线程绑定事务
                cleanupTransactionInfo(txInfo);
            }

            //处理下返回值，使用了io.vavr.control.Try 来处理，没用过 略过下面
            if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
                // Set rollback-only in case of Vavr failure matching our rollback rules...
                TransactionStatus status = txInfo.getTransactionStatus();
                if (status != null && txAttr != null) {
                    retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
                }
            }

            //这个说的很明白 处理完返回值后再进行提交事务
            commitTransactionAfterReturning(txInfo);
            return retVal;
        }

        else { //这个是CallbackPreferringPlatformTransactionManager 执行事务方式
               //跟上面处理差不多就是多了一个execute 方法，在回调函数去执行事务，相当于将事务执行交给调用者去实现
            Object result;
            final ThrowableHolder throwableHolder = new ThrowableHolder();

            // It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
            try {
                result = ((CallbackPreferringPlatformTransactionManager) ptm).execute(txAttr, status -> {
                    TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
                    try {
                        Object retVal = invocation.proceedWithInvocation();
                        if (retVal != null && vavrPresent && VavrDelegate.isVavrTry(retVal)) {
                            // Set rollback-only in case of Vavr failure matching our rollback rules...
                            retVal = VavrDelegate.evaluateTryFailure(retVal, txAttr, status);
                        }
                        return retVal;
                    }
                    catch (Throwable ex) {
                        if (txAttr.rollbackOn(ex)) {
                            // A RuntimeException: will lead to a rollback.
                            if (ex instanceof RuntimeException) {
                                throw (RuntimeException) ex;
                            }
                            else {
                                throw new ThrowableHolderException(ex);
                            }
                        }
                        else {
                            // A normal return value: will lead to a commit.
                            throwableHolder.throwable = ex;
                            return null;
                        }
                    }
                    finally {
                        cleanupTransactionInfo(txInfo);
                    }
                });
            }
            catch (ThrowableHolderException ex) {
                throw ex.getCause();
            }
            catch (TransactionSystemException ex2) {
                if (throwableHolder.throwable != null) {
                    logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
                    ex2.initApplicationException(throwableHolder.throwable);
                }
                throw ex2;
            }
            catch (Throwable ex2) {
                if (throwableHolder.throwable != null) {
                    logger.error("Application exception overridden by commit exception", throwableHolder.throwable);
                }
                throw ex2;
            }

            // Check result state: It might indicate a Throwable to rethrow.
            if (throwableHolder.throwable != null) {
                throw throwableHolder.throwable;
            }
            return result;
        }
    }

总结

在这篇文章中我们简单学习了Spring初始化bean时，如何将bean创建成一个代理对象，并且使用Cglib技术创建一个代理bean，在结合事务管理器分析了代理如何去实现Spring事务的。使用一个小例子演示了Cglib代理如何实现的，方便理解Spring AOP代理是通过一个拦截器去实现的，一个对象的多个代理封装到调用链里面，执行方法时顺序执行，保证每一个代理与代理之间没有任何联系，相互独立。这次我还了解了Spring代理机制原理，通过Advisor实现类去解析Class、Method，通过PointcutAdvisor(匹配Class、Method)、IntroductionAdvisor (支持Class)是否需要生成代理对象。然后在通过专门代理工程去生成对应代理对象。
我们简单了解事务实现，其实就是环绕通知实现而已，还了解到事务状态传递是通过ThreadLocal来实现的。