【原创】002 | 搭上SpringBoot事务源码分析专车

前言

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专车介绍**

该趟专车是开往Spring Boot事务源码分析的专车

专车问题

• 为什么加上@Transactional注解就可以实现事务？
• 分析事务源码之后我们可以学到什么？

专车名词

事务

程序中通常使用事务来达到数据的一致性，从而避免脏数据

编程式事务

在业务方法开头开启事务，然后对我们的业务进行try-catch，假设没有异常则提交事务，如果出现异常，则在catch模块回滚事务

声明式事务由来

如果采用编程式事务，那么在任何需要事务的地方都要开启事务、try-catch、提交或者回滚事务，会导致重复编码、编写与业务无关的代码。基于Spring Aop思想，我们可以利用Aop的方式，对需要使用事务的方法进行增强，将公用的部分提取出来，那么就实现了声明式事务。

Spring提供的声明式事务

在需要使用事务的业务方法上添加@Transactional注解，那么就可以使用事务的特性，要么成功，要么失败

Spring Aop核心概念

• 切面：切面是由切点和通知组成
• 切点：用来匹配符合条件类或方法
• 通知：需要执行的操作

专车分析

基于Spring Boot自动配置原理，我们应该寻找xxxAutoConfiguration自动配置类，此处要寻找和事务相关的，那么自然是TransactionAutoConfiguration

自动配置

打开TransactionAutoConfiguration自动配置类

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

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

	}

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

	}

}

可以看到开启事务管理器的注解@EnableTransactionManagement

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

查看TransactionManagementConfigurationSelector导入的类

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;
	}
}

可以看到导入了AutoProxyRegistrar和ProxyTransactionManagementConfiguration

首先看看AutoProxyRegistrar，该类实现了ImportBeanDefinitionRegistrar

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) {
			    // 注册自动代理创建器
				AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
				if ((Boolean) proxyTargetClass) {
					AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
					return;
				}
			}
		}
	}
}

注册自动代理创建器，AopConfigUtils#registerAutoProxyCreatorIfNecessary

public static BeanDefinition registerAutoProxyCreatorIfNecessary(
			BeanDefinitionRegistry registry, @Nullable Object source) {
    // 注册了InfrastructureAdvisorAutoProxyCreator到IOC容器中
	return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
}

InfrastructureAdvisorAutoProxyCreator是AbstractAutoProxyCreator的子类，AbstractAutoProxyCreator又实现了BeanPostProcessor接口，那么在bean初始化完毕后就会调用postProcessAfterInstantiation()方法，postProcessAfterInstantiation()定义在AbstractAutoProxyCreator类中

BeanPostProcessor后置处理

打开AbstractAutoProxyCreator

@Override
public Object postProcessAfterInitialization(@Nullable Object bean, String beanName) {
	if (bean != null) {
		Object cacheKey = getCacheKey(bean.getClass(), beanName);
		if (!this.earlyProxyReferences.contains(cacheKey)) {
		    // 如果满足条件对bean进行包裹
			return wrapIfNecessary(bean, beanName, cacheKey);
		}
	}
	return bean;
}

该方法调用了wrapIfNecessary()方法

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
	// Create proxy if we have advice.
  	// 获取bean的切面和通知
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
  	// 需要代理
	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;
}

根据注释的意思就是如果存在advice，那么就创建代理，

寻找切面

进入AbstractAdvisorAutoProxyCreator#getAdvicesAndAdvisorsForBean

protected Object[] getAdvicesAndAdvisorsForBean(
			Class<?> beanClass, String beanName, @Nullable TargetSource targetSource) {
    // 查找符合条件的切面
	List<Advisor> advisors = findEligibleAdvisors(beanClass, beanName);
	// 不存在符合条件的切面，则不生成代理
	if (advisors.isEmpty()) {
		return DO_NOT_PROXY;
	}
	return advisors.toArray();
}

该代码第一句最重要，如果不存在符合条件的切面，那么最终的结果返回null，根据上面分析的，如果为null就不创建代理，否则创建代理。接下来看看第一句的实现

protected List<Advisor> findEligibleAdvisors(Class<?> beanClass, String beanName) {
    // 获取所有候选的切面，也就是类型为Advisor的切面，此处获取到的候选切面为BeanFactoryTransactionAttributeSourceAdvisor
    List<Advisor> candidateAdvisors = findCandidateAdvisors();
    // 从候选的切面中获取可以解析当前bean的切面，最终符合条件的切面为BeanFactoryTransactionAttributeSourceAdvisor
    List<Advisor> eligibleAdvisors = findAdvisorsThatCanApply(candidateAdvisors, beanClass, beanName);
    extendAdvisors(eligibleAdvisors);
    if (!eligibleAdvisors.isEmpty()) {
    	eligibleAdvisors = sortAdvisors(eligibleAdvisors);
    }
    return eligibleAdvisors;
}

为什么上面获取到的切面是BeanFactoryTransactionAttributeSourceAdvisor？是否还记得之前导入配置类的时候还有一个配置类没有分析？那就是ProxyTransactionManagementConfiguration

打开ProxyTransactionManagementConfiguration

@Configuration
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

    // 创建BeanFactoryTransactionAttributeSourceAdvisor
	@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
		advisor.setTransactionAttributeSource(transactionAttributeSource());
		// 设置切面对应的通知，后面分析会用到
		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();
	}

    // 创建通知
	@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;
	}

}

通过上面的自动配置，可得知获取到的候选切面为什么是BeanFactoryTransactionAttributeSourceAdvisor

接下来看看如何从候选切面中找到可以解析当前bean的切面？

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

	ProxyCreationContext.setCurrentProxiedBeanName(beanName);
	try {
	    // 查找可以解析当前bean对应的切面
		return AopUtils.findAdvisorsThatCanApply(candidateAdvisors, beanClass);
	}
	finally {
		ProxyCreationContext.setCurrentProxiedBeanName(null);
	}
}

查找可以解析当前bean对应的切面，AopUtils#findAdvisorsThatCanApply

public static List<Advisor> findAdvisorsThatCanApply(List<Advisor> candidateAdvisors, Class<?> clazz) {
	if (candidateAdvisors.isEmpty()) {
		return candidateAdvisors;
	}
	List<Advisor> eligibleAdvisors = new ArrayList<>();
	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;
		}
		// 当前切面是否可以解析bean
		if (canApply(candidate, clazz, hasIntroductions)) {
			eligibleAdvisors.add(candidate);
		}
	}
	return eligibleAdvisors;
}

候选切面是否可以解析bean

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) {
	    // 由上面分析知道最终的候选切面为BeanFactoryTransactionAttributeSourceAdvisor
	    // 该类实现了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;
	}
}

候选切面是否可以解析bean

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;
	}

    // 获取切面切点方法匹配对象，用来匹配方法是否符合
	MethodMatcher methodMatcher = pc.getMethodMatcher();
	if (methodMatcher == MethodMatcher.TRUE) {
		// 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)) {
		classes.add(ClassUtils.getUserClass(targetClass));
	}
	classes.addAll(ClassUtils.getAllInterfacesForClassAsSet(targetClass));

	for (Class<?> clazz : classes) {
	    // 通过反射获取当前类所有的Method对象
		Method[] methods = ReflectionUtils.getAllDeclaredMethods(clazz);
		for (Method method : methods) {
			if (introductionAwareMethodMatcher != null ?
					introductionAwareMethodMatcher.matches(method, targetClass, hasIntroductions) :
					// 匹配方法是否符合
					methodMatcher.matches(method, targetClass)) {
				return true;
			}
		}
	}

	return false;
}

匹配方法TransactionAttributeSourcePointcut#matches

public boolean matches(Method method, Class<?> targetClass) {
	if (TransactionalProxy.class.isAssignableFrom(targetClass) ||
			PlatformTransactionManager.class.isAssignableFrom(targetClass) ||
			PersistenceExceptionTranslator.class.isAssignableFrom(targetClass)) {
		return false;
	}
	TransactionAttributeSource tas = getTransactionAttributeSource();
	// 如果事务属性源对象为空或者事务属性对象不为null返回true，代表匹配成功；否则返回false，匹配失败
	return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
}

获取事务属性对象，AbstractFallbackTransactionAttributeSource#getTransactionAttribute

public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
	if (method.getDeclaringClass() == Object.class) {
		return null;
	}

	// First, see if we have a cached value.
	Object cacheKey = getCacheKey(method, targetClass);
	TransactionAttribute cached = this.attributeCache.get(cacheKey);
	if (cached != null) {
		// Value will either be canonical value indicating there is no transaction attribute,
		// or an actual transaction attribute.
		if (cached == NULL_TRANSACTION_ATTRIBUTE) {
			return null;
		}
		else {
			return cached;
		}
	}
	else {
		// 计算事务属性对象
		TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
		// Put it in the cache.
		if (txAttr == null) {
			this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
		}
		else {
			String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
			if (txAttr instanceof DefaultTransactionAttribute) {
				((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
			}
			if (logger.isTraceEnabled()) {
				logger.trace("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
			}
			this.attributeCache.put(cacheKey, txAttr);
		}
		return txAttr;
	}
}

计算事务属性对象

protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
	// Don't allow no-public methods as required.
	if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
		return null;
	}

	// The method may be on an interface, but we need attributes from the target class.
	// If the target class is null, the method will be unchanged.
	Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);

	// First try is the method in the target class.
	// 首先根据Method对象获取事务属性对象
	TransactionAttribute txAttr = findTransactionAttribute(specificMethod);
	if (txAttr != null) {
		return txAttr;
	}

	// Second try is the transaction attribute on the target class.
	// 如果根据Method对象获取不到事务属性对象，那么根据Class来获取属性对象
	txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
	if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
		return txAttr;
	}

	if (specificMethod != method) {
		// Fallback is to look at the original method.
		txAttr = findTransactionAttribute(method);
		if (txAttr != null) {
			return txAttr;
		}
		// Last fallback is the class of the original method.
		txAttr = findTransactionAttribute(method.getDeclaringClass());
		if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
			return txAttr;
		}
	}

	return null;
}

获取属性对象AnnotationTransactionAttributeSource#findTransactionAttribute

protected TransactionAttribute findTransactionAttribute(Class<?> clazz) {
	return determineTransactionAttribute(clazz);
}

决定事务属性对象

protected TransactionAttribute determineTransactionAttribute(AnnotatedElement element) {
	for (TransactionAnnotationParser annotationParser : this.annotationParsers) {
		TransactionAttribute attr = annotationParser.parseTransactionAnnotation(element);
		if (attr != null) {
			return attr;
		}
	}
	return null;
}

解析事务属性对象，SpringTransactionAnnotationParser#parseTransactionAnnotation

public TransactionAttribute parseTransactionAnnotation(AnnotatedElement element) {
    // 判断元素是否含有@Transactional注解，通过前面的分析我们可以得出如下结论：
    // 1、首选判断类的方法上是否含有@Transactional注解，如果有就解析
    // 2、如果所有的方法都不含有@Transactional注解，那么判断当前类是否含有@Transactional注解，如果有就解析
    // 3、如果类或者类的某个方法含有@Transactional注解，那么事务属性对象就不为空，则说明次切面可以解析当前bean
	AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
			element, Transactional.class, false, false);
	if (attributes != null) {
		return parseTransactionAnnotation(attributes);
	}
	else {
		return null;
	}
}

回到AbstractAutoProxyCreator#wrapIfNecessary

protected Object wrapIfNecessary(Object bean, String beanName, Object cacheKey) {
	if (StringUtils.hasLength(beanName) && this.targetSourcedBeans.contains(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.
	// 此处有值返回，进行代理，否则不进行代理
	Object[] specificInterceptors = getAdvicesAndAdvisorsForBean(bean.getClass(), beanName, null);
	// 需要进行代理
	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;
}

创建代理

创建代理AbstractAutoProxyCreator#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);
	}

    // 创建代理工厂
	ProxyFactory proxyFactory = new ProxyFactory();
	proxyFactory.copyFrom(this);

	if (!proxyFactory.isProxyTargetClass()) {
		if (shouldProxyTargetClass(beanClass, beanName)) {
			proxyFactory.setProxyTargetClass(true);
		}
		else {
			evaluateProxyInterfaces(beanClass, proxyFactory);
		}
	}

    // 构建切面，此处的切面为BeanFactoryTransactionAttributeSourceAdvisor
	Advisor[] advisors = buildAdvisors(beanName, specificInterceptors);
	// 设置切面
	proxyFactory.addAdvisors(advisors);
	proxyFactory.setTargetSource(targetSource);
	customizeProxyFactory(proxyFactory);

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

	return proxyFactory.getProxy(getProxyClassLoader());
}

获取代理ProxyFactory#getProxy

public Object getProxy(@Nullable ClassLoader classLoader) {
	return createAopProxy().getProxy(classLoader);
}

创建aop代理

protected final synchronized AopProxy createAopProxy() {
	if (!this.active) {
		activate();
	}
	// 此处的this实际上就是ProxyFactory
	return getAopProxyFactory().createAopProxy(this);
}

aop代理工厂创建aop代理DefaultAopProxyFactory#createAopProxy

public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
	if (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);
		}
		// 创建cglib aop代理
		return new ObjenesisCglibAopProxy(config);
	}
	else {
		return new JdkDynamicAopProxy(config);
	}
}

实例化ObjenesisCglibAopProxy对象

public ObjenesisCglibAopProxy(AdvisedSupport config) {
	super(config);
}

父类实例化

public CglibAopProxy(AdvisedSupport config) throws AopConfigException {
	Assert.notNull(config, "AdvisedSupport must not be null");
	if (config.getAdvisors().length == 0 && config.getTargetSource() == AdvisedSupport.EMPTY_TARGET_SOURCE) {
		throw new AopConfigException("No advisors and no TargetSource specified");
	}
	// 此处的config就是之前的ProxyFactory
	this.advised = config;
	this.advisedDispatcher = new AdvisedDispatcher(this.advised);
}

回到之前获取代理的地方

public Object getProxy(@Nullable ClassLoader classLoader) {
	return createAopProxy().getProxy(classLoader);
}

通过上面的分析可以得知createAopProxy()返回的是CglibAopProxy

通过CglibAopProxy获取代理，CglibAopProxy#getProxy

public Object getProxy(@Nullable ClassLoader classLoader) {
	if (logger.isTraceEnabled()) {
		logger.trace("Creating CGLIB proxy: " + this.advised.getTargetSource());
	}

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

		Class<?> proxySuperClass = rootClass;
		if (ClassUtils.isCglibProxyClass(rootClass)) {
			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...
		// 创建Enhancer对象
		Enhancer enhancer = createEnhancer();
		if (classLoader != null) {
			enhancer.setClassLoader(classLoader);
			if (classLoader instanceof SmartClassLoader &&
					((SmartClassLoader) classLoader).isClassReloadable(proxySuperClass)) {
				enhancer.setUseCache(false);
			}
		}
		// 设置父类
		enhancer.setSuperclass(proxySuperClass);
		// 设置接口
		enhancer.setInterfaces(AopProxyUtils.completeProxiedInterfaces(this.advised));
		enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
		enhancer.setStrategy(new ClassLoaderAwareUndeclaredThrowableStrategy(classLoader));

        // 获取回调，重点分析
		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));
		// 设置回调类型
		enhancer.setCallbackTypes(types);

		// Generate the proxy class and create a proxy instance.
		// 生成代理并创建代理实例
		return createProxyClassAndInstance(enhancer, callbacks);
	}
	catch (CodeGenerationException | IllegalArgumentException ex) {
		throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() +
				": Common causes of this problem include using a final class or a non-visible class",
				ex);
	}
	catch (Throwable ex) {
		// TargetSource.getTarget() failed
		throw new AopConfigException("Unexpected AOP exception", ex);
	}
}

获取回调

private Callback[] getCallbacks(Class<?> rootClass) throws Exception {
	// Parameters used for optimization choices...
	boolean exposeProxy = this.advised.isExposeProxy();
	boolean isFrozen = this.advised.isFrozen();
	boolean isStatic = this.advised.getTargetSource().isStatic();

	// Choose an "aop" interceptor (used for AOP calls).
	// 实例化回调，在调用目标对象方法的时候执行
	Callback aopInterceptor = new DynamicAdvisedInterceptor(this.advised);
	return callbacks;
}

实例化回调部分

private static class DynamicAdvisedInterceptor implements MethodInterceptor, Serializable {

	private final AdvisedSupport advised;

	public DynamicAdvisedInterceptor(AdvisedSupport advised) {
	    // 设置切面信息，也就是之前的ProxyFactory
		this.advised = advised;
	}

	@Override
	@Nullable
	// 调用目标方法的时候执行
	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);
		    // 获取通知，此处的通知为TransactionInterceptor
			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())) {
				// 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...
				retVal = new CglibMethodInvocation(proxy, target, method, args, targetClass, chain, methodProxy).proceed();
			}
			retVal = processReturnType(proxy, target, method, retVal);
			return retVal;
		}
		finally {
			if (target != null && !targetSource.isStatic()) {
				targetSource.releaseTarget(target);
			}
			if (setProxyContext) {
				// Restore old proxy.
				AopContext.setCurrentProxy(oldProxy);
			}
		}
	}

	@Override
	public boolean equals(Object other) {
		return (this == other ||
				(other instanceof DynamicAdvisedInterceptor &&
						this.advised.equals(((DynamicAdvisedInterceptor) other).advised)));
	}

	/**
	 * CGLIB uses this to drive proxy creation.
	 */
	@Override
	public int hashCode() {
		return this.advised.hashCode();
	}
}

调用invocation的处理方法，ReflectiveMethodInvocation#proceed

public Object proceed() throws Throwable {
	//	We start with an index of -1 and increment early.
	if (this.currentInterceptorIndex == this.interceptorsAndDynamicMethodMatchers.size() - 1) {
		return invokeJoinpoint();
	}

    // 此处的通知TransactionInterceptor
	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());
		if (dm.methodMatcher.matches(this.method, targetClass, this.arguments)) {
			return dm.interceptor.invoke(this);
		}
		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.
		// 调用TransactionInterceptor#invoke
		return ((MethodInterceptor) interceptorOrInterceptionAdvice).invoke(this);
	}
}

调用TransactionInterceptor#invoke

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...
	// 以事务的方式进行调用
	return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
}

事务方式调用

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();
	final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
	final PlatformTransactionManager tm = determineTransactionManager(txAttr);
	final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);

	if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
		// Standard transaction demarcation with getTransaction and commit/rollback calls.
		// 创建事务信息对象
		TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
		Object retVal = null;
		try {
			// This is an around advice: Invoke the next interceptor in the chain.
			// This will normally result in a target object being invoked.
			// 调用被代理对象方法
			retVal = invocation.proceedWithInvocation();
		}
		catch (Throwable ex) {
			// target invocation exception
			// 业务方法执行异常，进行事务回滚
			completeTransactionAfterThrowing(txInfo, ex);
			throw ex;
		}
		finally {
		    // 清除事务信息对象
			cleanupTransactionInfo(txInfo);
		}
		// 提交事务
		commitTransactionAfterReturning(txInfo);
		return retVal;
	}

	else {
		final ThrowableHolder throwableHolder = new ThrowableHolder();

		// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
		try {
			Object result = ((CallbackPreferringPlatformTransactionManager) tm).execute(txAttr, status -> {
				TransactionInfo txInfo = prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
				try {
					return invocation.proceedWithInvocation();
				}
				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);
				}
			});

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

到此事务的源码分析就结束了

专车总结

• 导入AutoProxyRegistrar、ProxyTransactionManagementConfiguration配置类
• AutoProxyRegistrar用来注册InfrastructureAdvisorAutoProxyCreator到IOC中，InfrastructureAdvisorAutoProxyCreator实现了BeanPostProcessor
• 执行BeanPostProcessor的后置处理
• 获取由ProxyTransactionManagementConfiguration配置类创建的切面
• 通过切面解析bean是否需要创建代理，需要就创建代理
• 执行代理的回调，在回调中拿到通知
• 执行通知，通知里面逻辑：开启事务、执行目标方法、提交或回滚事务

专车回顾

回顾下开头的两个问题：

• 为什么加上@Transactional注解就可以实现事务？
• 分析事务源码之后我们可以学到什么？

通过以上分析，第一个问题应该就迎刃而解了，那么通过以上学到的知识我们可以实现什么功能呢？在下一篇我们会在此基础上进行实战，通过@SystemLog注解实现系统日志功能。感谢各位撸友乘坐此趟专车，欢迎下次继续乘坐

最后

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