[Java知识库] 【Spring】事务的执行原理（一）

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[Java知识库]【Spring】事务的执行原理（一）

在使用事务的时候需要添加@EnableTransactionManagement注解来开启事务，那么就从@EnableTransactionManagement入手查看一下事务的执行原理。

@EnableTransactionManagement

Spring事务底层是通过AOP来完成的，而Spring AOP基于动态代理实现，可以看到mode方法默认返回了PROXY代理模式，我们只需关注代理模式下的执行流程即可
使用@Import导入了TransactionManagementConfigurationSelector

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

    // 是否代理目标类
	boolean proxyTargetClass() default false;

	// 默认使用代理模式
	AdviceMode mode() default AdviceMode.PROXY;
    
	int order() default Ordered.LOWEST_PRECEDENCE;
}

TransactionManagementConfigurationSelector

在selectImports方法中可以看到对模式进行了判断：

如果是基于代理模式，返回AutoProxyRegistrar和ProxyTransactionManagementConfiguration类
如果是基于ASPECTJ，调用determineTransactionAspectClass方法

Spring默认使用的是代理模式，所以接下来看下AutoProxyRegistrar和ProxyTransactionManagementConfiguration里面都有什么。

public class TransactionManagementConfigurationSelector extends AdviceModeImportSelector<EnableTransactionManagement> {

    @Override
	protected String[] selectImports(AdviceMode adviceMode) {
		switch (adviceMode) {
			case PROXY:
                // 如果基于代理模式
				return new String[] {AutoProxyRegistrar.class.getName(),
						ProxyTransactionManagementConfiguration.class.getName()};
			case ASPECTJ:
                // 如果基于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);
	}

}

AutoProxyRegistrar

AutoProxyRegistrar实现了ImportBeanDefinitionRegistrar接口，ImportBeanDefinitionRegistrar可以向容器中注册Bean，跟着registerBeanDefinitions方法看下它会向容器中注册什么样的bean：

public class AutoProxyRegistrar implements ImportBeanDefinitionRegistrar {

    @Override
	public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
		boolean candidateFound = false;
		Set<String> annTypes = importingClassMetadata.getAnnotationTypes();
		for (String annType : annTypes) {
			AnnotationAttributes candidate = AnnotationConfigUtils.attributesFor(importingClassMetadata, annType);
			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) {
                    // 调用AopConfigUtils的registerAutoProxyCreatorIfNecessary向容器中注册bean
					AopConfigUtils.registerAutoProxyCreatorIfNecessary(registry);
					if ((Boolean) proxyTargetClass) {
						AopConfigUtils.forceAutoProxyCreatorToUseClassProxying(registry);
						return;
					}
				}
			}
		}
		if (!candidateFound && logger.isInfoEnabled()) {
			String name = getClass().getSimpleName();
			logger.info(String.format("%s was imported but no annotations were found " +
					"having both 'mode' and 'proxyTargetClass' attributes of type " +
					"AdviceMode and boolean respectively. This means that auto proxy " +
					"creator registration and configuration may not have occurred as " +
					"intended, and components may not be proxied as expected. Check to " +
					"ensure that %s has been @Import'ed on the same class where these " +
					"annotations are declared; otherwise remove the import of %s " +
					"altogether.", name, name, name));
		}
	}
}

AopConfigUtils

在AopConfigUtils中一共有三种自动代理创建器：

InfrastructureAdvisorAutoProxyCreator
AspectJAwareAdvisorAutoProxyCreator
AnnotationAwareAspectJAutoProxyCreator

在registerAutoProxyCreatorIfNecessary方法中，可以看到事务使用的是InfrastructureAdvisorAutoProxyCreator类型的创建器：

public abstract class AopConfigUtils {

	/**
	 * 自动代理创建器BeanName
	 */
	public static final String AUTO_PROXY_CREATOR_BEAN_NAME =
			"org.springframework.aop.config.internalAutoProxyCreator";

	/**
	 * 所有的自动代理创建器集合
	 */
	private static final List<Class<?>> APC_PRIORITY_LIST = new ArrayList<>(3);

	static {
		// 初始化
		APC_PRIORITY_LIST.add(InfrastructureAdvisorAutoProxyCreator.class);
		APC_PRIORITY_LIST.add(AspectJAwareAdvisorAutoProxyCreator.class); // AspectJ
		APC_PRIORITY_LIST.add(AnnotationAwareAspectJAutoProxyCreator.class); // 注解
	}

    // AutoProxyRegistrar中调用的registerAutoProxyCreatorIfNecessary方法
	@Nullable
	public static BeanDefinition registerAutoProxyCreatorIfNecessary(BeanDefinitionRegistry registry) {
	    // 会调用下面那个registerAutoProxyCreatorIfNecessary方法
		return registerAutoProxyCreatorIfNecessary(registry, null);
	}
	
	@Nullable
	public static BeanDefinition registerAutoProxyCreatorIfNecessary(
			BeanDefinitionRegistry registry, @Nullable Object source) {
		// 调用registerOrEscalateApcAsRequired进行注册，这里传入的是InfrastructureAdvisorAutoProxyCreator类型的
		return registerOrEscalateApcAsRequired(InfrastructureAdvisorAutoProxyCreator.class, registry, source);
	}

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

		Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
        // 判断容器中是否已经包含代理创建器
		if (registry.containsBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME)) {
            // 从容器中获取
			BeanDefinition apcDefinition = registry.getBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME);
			if (!cls.getName().equals(apcDefinition.getBeanClassName())) {
                // 判断容器中已经存在的创建器的优先级
				int currentPriority = findPriorityForClass(apcDefinition.getBeanClassName());
                // 需要的创建器的优先级
				int requiredPriority = findPriorityForClass(cls);
                // 如果容器中已经存在的创建器的优先级小于需要创建的
				if (currentPriority < requiredPriority) {
                    // 使用优先级高的
					apcDefinition.setBeanClassName(cls.getName());
				}
			}
			return null;
		}
        // 创建RootBeanDefinition
		RootBeanDefinition beanDefinition = new RootBeanDefinition(cls);
        // 设置source
		beanDefinition.setSource(source);
		beanDefinition.getPropertyValues().add("order", Ordered.HIGHEST_PRECEDENCE);
		beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
        // 注册代理创建器
		registry.registerBeanDefinition(AUTO_PROXY_CREATOR_BEAN_NAME, beanDefinition);
		return beanDefinition;
	}
}

总结

AutoProxyRegistrar实现ImportBeanDefinitionRegistrar是为了向容器中注册代理创建器，事务默认使用的是InfrastructureAdvisorAutoProxyCreator类型的。

ProxyTransactionManagementConfiguration

1. AOP概念

**Advice通知：**定义在切点上需要执行什么样的操作

PointCut切点：定义在哪些方法上使用通知

**Advisor：**Advice和Pointcut加起来组成了Advisor

2. 事务中的Advisor

我们已经知道事务是基于AOP实现的，在transactionAdvisor方法中可以看到创建了Advisor，然后设置了事务属性TransactionAttributeSource和事务拦截器TransactionInterceptor：

TransactionAttributeSource，从名字上可以看出是和事务的属性设置相关的
TransactionInterceptor事务拦截器相当于Advice通知
BeanFactoryTransactionAttributeSourceAdvisor是Advisor

Advisor由Advice和PointCut组成，现在Advice已经有了，接下来看下Pointcut在哪里。

@Configuration(proxyBeanMethods = false)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public class ProxyTransactionManagementConfiguration extends AbstractTransactionManagementConfiguration {

  @Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor(
			TransactionAttributeSource transactionAttributeSource, TransactionInterceptor transactionInterceptor) {
        // 创建Advisor
		BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
        // 设置TransactionAttributeSource，类型为AnnotationTransactionAttributeSource
		advisor.setTransactionAttributeSource(transactionAttributeSource);
        // 设置事务拦截器，相当于Advice
		advisor.setAdvice(transactionInterceptor);
		if (this.enableTx != null) {
			advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
		}
		return advisor;
	}

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

	@Bean
	@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
	public TransactionInterceptor transactionInterceptor(TransactionAttributeSource transactionAttributeSource) {
        // 创建事务拦截器
		TransactionInterceptor interceptor = new TransactionInterceptor();
		interceptor.setTransactionAttributeSource(transactionAttributeSource);
		if (this.txManager != null) {
			interceptor.setTransactionManager(this.txManager);
		}
		return interceptor;
	}
}

BeanFactoryTransactionAttributeSourceAdvisor

BeanFactoryTransactionAttributeSourceAdvisor继承关系如下：
在这里插入图片描述

除了继承父类的属性和方法，它自己还有两个成员变量：

transactionAttributeSource，实际传入的是AnnotationTransactionAttributeSource类型的对象
TransactionAttributeSourcePointcut类型的切点pointcut
- 切点在实例化时实现了getTransactionAttributeSource方法，返回了transactionAttributeSource，后面的方法中需要调用此方法获取transactionAttributeSource

由上面的分析可知，在创建BeanFactoryTransactionAttributeSourceAdvisor的时候，设置了TransactionInterceptor和TransactionAttributeSource，TransactionInterceptor相当于Advice，而这里我们看到了它还有一个TransactionAttributeSourcePointcut切点:

public class BeanFactoryTransactionAttributeSourceAdvisor extends AbstractBeanFactoryPointcutAdvisor {

	@Nullable
	private TransactionAttributeSource transactionAttributeSource; // 实际传入的是AnnotationTransactionAttributeSource类型的

    // 创建切点
	private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
    
        // 实现了getTransactionAttributeSource方法，返回的是AnnotationTransactionAttributeSource
		@Override
		@Nullable
		protected TransactionAttributeSource getTransactionAttributeSource() {
			return transactionAttributeSource;
		}
	};

	public void setTransactionAttributeSource(TransactionAttributeSource transactionAttributeSource) {
    // 设置TransactionAttributeSource
		this.transactionAttributeSource = transactionAttributeSource;
	}

	@Override
	public Pointcut getPointcut() {
		return this.pointcut;
	}

}

TransactionAttributeSourcePointcut

TransactionAttributeSourcePointcut是一个切点，它的继承关系如下：

在这里插入图片描述

Pointcut和MethodMatcher

Pointcut接口中定义了两个方法：

获取ClassFilter，ClassFilter是一个接口，里面定义了matches方法，检查切点是否与类匹配
获取MethodMatcher，它也是一个接口，并且定义了matches方法，检查切点是否与方法匹配

public interface Pointcut {

	/**
	 * 返回类过滤器ClassFilter
	 */
	ClassFilter getClassFilter();

	/**
	 * 返回MethodMatcher
	 */
	MethodMatcher getMethodMatcher();

}

public interface MethodMatcher {

	/**
	 * 检查方法是否匹配pointcut
	 */
	boolean matches(Method method, Class<?> targetClass);

	/**
	 * 检查方法是否匹配pointcut
	 */
	boolean matches(Method method, Class<?> targetClass, Object... args);

}

@FunctionalInterface
public interface ClassFilter {

	/**
	 * 检查类是否与pointcut匹配
	 */
	boolean matches(Class<?> clazz);

}

TransactionAttributeSourcePointcut是Pointcut和MethodMatcher的子类：

在构造函数中设置了ClassFilter，类型为TransactionAttributeSourceClassFilter，它是TransactionAttributeSourcePointcut的一个内部类，实现了ClassFilter接口中定义的matches方法，检查pointcut与类是否匹配：
- 如果是TransactionalProxy、PlatformTransactionManager或者PersistenceExceptionTranslator的子类，则不匹配
- 获取TransactionAttributeSource，调用它的isCandidateClass方法判断是否匹配
实现了MethodMatcher接口中定义的matches方法，检查pointcut是否匹配当前的方法
- 获取TransactionAttributeSource判断是否为空，如果不为空则调用getTransactionAttribute获取事务属性，TransactionAttributeSource为空，或者从TransactionAttributeSource获取到的事务属性不为空都会返回true

所以一个方法执行时开启事务，需要满足两个条件，当前的方法和类都需要与事务的pointcut匹配，对应的方法分别是MethodMatcher的matches和ClassFilter的matches方法。

abstract class TransactionAttributeSourcePointcut extends StaticMethodMatcherPointcut implements Serializable {

  
	protected TransactionAttributeSourcePointcut() {
        // 设置ClassFilter
		setClassFilter(new TransactionAttributeSourceClassFilter());
	}

    // 方法是否与切点匹配
	@Override
	public boolean matches(Method method, Class<?> targetClass) {
        // 获取TransactionAttributeSource，由上面的步骤可知返回的是AnnotationTransactionAttributeSource
		TransactionAttributeSource tas = getTransactionAttributeSource();
        // 如果TransactionAttributeSource为空，或者从TransactionAttributeSource获取到的事务属性不为空都会返回true
		return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
	}
  
  // ClassFilter过滤器
  private class TransactionAttributeSourceClassFilter implements ClassFilter {

        // 切点是否与类匹配
		@Override
		public boolean matches(Class<?> clazz) {
            // 如果TransactionalProxy、PlatformTransactionManager或者PersistenceExceptionTranslator的子类，则不匹配
			if (TransactionalProxy.class.isAssignableFrom(clazz) ||
					PlatformTransactionManager.class.isAssignableFrom(clazz) ||
					PersistenceExceptionTranslator.class.isAssignableFrom(clazz)) {
				return false;
			}
            // 获取TransactionAttributeSource，由上文可知是AnnotationTransactionAttributeSource类型的
			TransactionAttributeSource tas = getTransactionAttributeSource();
            // 调用isCandidateClass方法判断是否是匹配
			return (tas == null || tas.isCandidateClass(clazz));
		}
	}

}

AnnotationTransactionAttributeSource

条件一：检查类是否匹配事务切点

上面分析可知，检查类是否与切点匹配时获取了TransactionAttributeSource，调用它的isCandidateClass方法进行判断，TransactionAttributeSource的具体实现是AnnotationTransactionAttributeSource：

在构造方法中，添加了注解解析器：
- Spring事务注解解析器的实现类为SpringTransactionAnnotationParser，也是默认的注解解析器。
- 如果开启了JTA或者EJB，将会分别添加对应的解析器。
实现了isCandidateClass方法，实际又是调用注解解析器的isCandidateClass判断是否是候选类的。

@SuppressWarnings("serial")
public class AnnotationTransactionAttributeSource extends AbstractFallbackTransactionAttributeSource
		implements Serializable {

	private static final boolean jta12Present;

	private static final boolean ejb3Present;

	static {
		ClassLoader classLoader = AnnotationTransactionAttributeSource.class.getClassLoader();
		jta12Present = ClassUtils.isPresent("javax.transaction.Transactional", classLoader);
		ejb3Present = ClassUtils.isPresent("javax.ejb.TransactionAttribute", classLoader);
	}

	private final boolean publicMethodsOnly;
    // 注解解析器集合
	private final Set<TransactionAnnotationParser> annotationParsers;

	public AnnotationTransactionAttributeSource(boolean publicMethodsOnly) {
		this.publicMethodsOnly = publicMethodsOnly;
        // 添加注解解析器
		if (jta12Present || ejb3Present) {
			this.annotationParsers = new LinkedHashSet<>(4);
            // 添加Spring事务注解解析器
			this.annotationParsers.add(new SpringTransactionAnnotationParser());
			if (jta12Present) {
                // JTA事务注解解析器
				this.annotationParsers.add(new JtaTransactionAnnotationParser());
			}
			if (ejb3Present) {
                // EJB3事务注解解析器
				this.annotationParsers.add(new Ejb3TransactionAnnotationParser());
			}
		}
		else {
            // 添加Spring事务注解解析器
			this.annotationParsers = Collections.singleton(new SpringTransactionAnnotationParser());
		}
	}

    // 判断是否是候选类
	@Override
	public boolean isCandidateClass(Class<?> targetClass) {
		for (TransactionAnnotationParser parser : this.annotationParsers) {
            // 调用解析器的isCandidateClass方法判断是否是候选类
			if (parser.isCandidateClass(targetClass)) {
				return true;
			}
		}
		return false;
	}

}

SpringTransactionAnnotationParser

SpringTransactionAnnotationParser实现了isCandidateClass方法，它又调用了AnnotationUtils的isCandidateClass判断目标类是否是Transactional注解的候选类，AnnotationUtils中isCandidateClass的具体判断逻辑如下：
- 如果注解类路径以java.开头，返回true，这里Transactional注解不是java.开头，它是Spring的注解类，所以这个条件不会成立
- 如果目标类的类路径以java.开头，或者是Ordered类型，isCandidateClass返回false，说明目标类不是某个注解的候选类
- 除去以上两种情况之外，isCandidateClass都返回true
  
  总结：如果目标类的类路径不以java.开头（也就是说它不是java的相关类），也不是Ordered类型，说明目标类是Transactional注解的候选类。
SpringTransactionAnnotationParser实现了parseTransactionAnnotation方法，里面包含对事物属性的解析。

public class SpringTransactionAnnotationParser implements TransactionAnnotationParser, Serializable {

	@Override
	public boolean isCandidateClass(Class<?> targetClass) {
        // 是否是Transactional注解的候选类
		return AnnotationUtils.isCandidateClass(targetClass, Transactional.class);
	}

    // 解析注解属性
	@Override
	@Nullable
	public TransactionAttribute parseTransactionAnnotation(AnnotatedElement element) {
		AnnotationAttributes attributes = AnnotatedElementUtils.findMergedAnnotationAttributes(
				element, Transactional.class, false, false);
		if (attributes != null) {
            // 解析注解属性
			return parseTransactionAnnotation(attributes);
		}
		else {
			return null;
		}
	}
    
   /**
    * 解析事务注解
    */
	public TransactionAttribute parseTransactionAnnotation(Transactional ann) {
		return parseTransactionAnnotation(AnnotationUtils.getAnnotationAttributes(ann, false, false));
	}

   /**
    * 解析事务注解
    */
	protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
		RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();
        // 事务的传播行为
		Propagation propagation = attributes.getEnum("propagation");
		rbta.setPropagationBehavior(propagation.value());
		Isolation isolation = attributes.getEnum("isolation");
		rbta.setIsolationLevel(isolation.value());
		rbta.setTimeout(attributes.getNumber("timeout").intValue());
		rbta.setReadOnly(attributes.getBoolean("readOnly"));
		rbta.setQualifier(attributes.getString("value"));

		List<RollbackRuleAttribute> rollbackRules = new ArrayList<>();
		for (Class<?> rbRule : attributes.getClassArray("rollbackFor")) {
			rollbackRules.add(new RollbackRuleAttribute(rbRule));
		}
		for (String rbRule : attributes.getStringArray("rollbackForClassName")) {
			rollbackRules.add(new RollbackRuleAttribute(rbRule));
		}
		for (Class<?> rbRule : attributes.getClassArray("noRollbackFor")) {
			rollbackRules.add(new NoRollbackRuleAttribute(rbRule));
		}
		for (String rbRule : attributes.getStringArray("noRollbackForClassName")) {
			rollbackRules.add(new NoRollbackRuleAttribute(rbRule));
		}
		rbta.setRollbackRules(rollbackRules);

		return rbta;
	}

}

// AnnotationUtils
public abstract class AnnotationUtils {
     
    /**
	 * 检查目标类clazz是否是注解的候选类
	 */
	public static boolean isCandidateClass(Class<?> clazz, Collection<Class<? extends Annotation>> annotationTypes) {
		for (Class<? extends Annotation> annotationType : annotationTypes) {
             // 传入目标类和注解，SpringTransactionAnnotationParser传入的注解是Transactional
			if (isCandidateClass(clazz, annotationType)) {
				return true;
			}
		}
		return false;
	}
    
    /**
	 * 检查目标类clazz是否是注解的候选类
	 */
    public static boolean isCandidateClass(Class<?> clazz, Class<? extends Annotation> annotationType) {
        // 传入目标类和注解类路径
		return isCandidateClass(clazz, annotationType.getName());
	}

	/**
	 * 检查目标类clazz是否是注解的候选类
	 */
	public static boolean isCandidateClass(Class<?> clazz, String annotationName) {
        // 注解类路径是否是java.开头
		if (annotationName.startsWith("java.")) {
			return true;
		}
        // 调用AnnotationsScanner的hasPlainJavaAnnotationsOnly方法判断
        // hasPlainJavaAnnotationsOnly方法的判断逻辑是：如果目标类的类路径以java.开头或者是Ordered类会返回true
        // 所以如果目标类的类路径以java.开头或者是Ordered类，isCandidateClass会返回false，说明目标类不是注解的候选类
		if (AnnotationsScanner.hasPlainJavaAnnotationsOnly(clazz)) {
			return false;
		}
        // 如果注解类路径不是java.开头，并且目标类的类路径不以java.开头，也不是Ordered类型，返回true，说明目标类是某个注解的候选类
		return true;
	}
}

// AnnotationsScanner
abstract class AnnotationsScanner {
    static boolean hasPlainJavaAnnotationsOnly(Class<?> type) {
        // 如果目标类的类路径以java.开头或者是Ordered类
		return (type.getName().startsWith("java.") || type == Ordered.class);
	}
}

条件二：检查方法是否匹配事务切点

AbstractFallbackTransactionAttributeSource

如果从TransactionAttributeSource获取到的事务属性不为空将会满足切点的匹配条件，获取事务属性的方法实现在AbstractFallbackTransactionAttributeSource类中：

如果当前方法是Object中的方法，返回空
根据当前的方法和类的信息构建缓存key，从缓存中获取
- 如果获取不为空，判断是否为空的事务属性NULL_TRANSACTION_ATTRIBUTE，如果是则返回null，否则返回从缓存中获取到的事务属性
- 如果获取为空，调用解析事务属性的方法进行解析，然后放入缓存中并返回

public abstract class AbstractFallbackTransactionAttributeSource implements TransactionAttributeSource {
  
    // 空的TransactionAttribute
	@SuppressWarnings("serial")
	private static final TransactionAttribute NULL_TRANSACTION_ATTRIBUTE = new DefaultTransactionAttribute() {
		@Override
		public String toString() {
			return "null";
		}
	};
    // 缓存
	private final Map<Object, TransactionAttribute> attributeCache = new ConcurrentHashMap<>(1024);

    // 获取事务属性
    @Override
	@Nullable
	public TransactionAttribute getTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
        // 如果当前方法所在的类是Object
		if (method.getDeclaringClass() == Object.class) {
			return null;
		}
		// 构建cacheKey
		Object cacheKey = getCacheKey(method, targetClass);
        // 首先根据cacheKey从缓存中获取
		TransactionAttribute cached = this.attributeCache.get(cacheKey);
		if (cached != null) {
			// 判断是否为空的TRANSACTION_ATTRIBUTE
			if (cached == NULL_TRANSACTION_ATTRIBUTE) {
                // 返回空
				return null;
			}
			else {
                // 返回事务属性TransactionAttribute
				return cached;
			}
		}
		else {
			// 解析TransactionAttribute
			TransactionAttribute txAttr = computeTransactionAttribute(method, targetClass);
			// 如果为空，也加入缓存，但是value是NULL_TRANSACTION_ATTRIBUTE
			if (txAttr == null) {
				this.attributeCache.put(cacheKey, NULL_TRANSACTION_ATTRIBUTE);
			}
			else {
				String methodIdentification = ClassUtils.getQualifiedMethodName(method, targetClass);
                // 如果是DefaultTransactionAttribute类型的
				if (txAttr instanceof DefaultTransactionAttribute) {
					((DefaultTransactionAttribute) txAttr).setDescriptor(methodIdentification);
				}
				if (logger.isTraceEnabled()) {
					logger.trace("Adding transactional method '" + methodIdentification + "' with attribute: " + txAttr);
				}
                // 加入缓存
				this.attributeCache.put(cacheKey, txAttr);
			}
			return txAttr;
		}
	}

    // 获取缓存KEY
	protected Object getCacheKey(Method method, @Nullable Class<?> targetClass) {
		return new MethodClassKey(method, targetClass);
	}

	/**
	 * 解析TransactionAttribute
	 */
	@Nullable
	protected TransactionAttribute computeTransactionAttribute(Method method, @Nullable Class<?> targetClass) {
		// 判断方法是否是public的
		if (allowPublicMethodsOnly() && !Modifier.isPublic(method.getModifiers())) {
			return null;
		}

		// 获取目标方法
		Method specificMethod = AopUtils.getMostSpecificMethod(method, targetClass);

		// 在方法上查找事务属性的设置，findTransactionAttribute方法在AnnotationTransactionAttributeSource中实现
		TransactionAttribute txAttr = findTransactionAttribute方法(specificMethod);
		if (txAttr != null) {
			return txAttr;
		}

		// 在目标类上面查找事务属性的设置
		txAttr = findTransactionAttribute(specificMethod.getDeclaringClass());
		if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
			return txAttr;
		}

		if (specificMethod != method) {
			// 使用方法上配置的事务属性
			txAttr = findTransactionAttribute(method);
			if (txAttr != null) {
				return txAttr;
			}
			// 使用类上面配置的事务属性
			txAttr = findTransactionAttribute(method.getDeclaringClass());
			if (txAttr != null && ClassUtils.isUserLevelMethod(method)) {
				return txAttr;
			}
		}
		return null;
	}

}

总结

事务是基于AOP实现的，事务的Advisor是BeanFactoryTransactionAttributeSourceAdvisor，Advisor判断方法是否匹配时，是通过Pointcut的matches方法判断的，事务的Pointcut是TransactionAttributeSourcePointcut，里面实现了方法是否与事务切点匹配的判断：

对类的匹配是通过判断目标类是否是Transactional注解的候选类实现的，我们创建的类一般不会以java.开头，所以说可以与Transactional注解匹配成功。
对方法的匹配是通过解析方法上面配置的事务属性判断的，如果解析到了事务属性，则满足匹配条件。

TransactionInterceptor

TransactionInterceptor是事务Advisor的Advice，执行目标方法时，方法会被拦截，进入到TransactionInterceptor中，在TransactionInterceptor的invoke方法中实际是调用invokeWithinTransaction执行的：

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {
    @Override
	@Nullable
	public Object invoke(MethodInvocation invocation) throws Throwable {
		Class<?> targetClass = (invocation.getThis() != null ? AopUtils.getTargetClass(invocation.getThis()) : null);

		// 通过事务执行目标方法，实现在TransactionAspectSupport方法中
		return invokeWithinTransaction(invocation.getMethod(), targetClass, invocation::proceed);
	}
}

TransactionAspectSupport

TransactionAspectSupport中实现了invokeWithinTransaction方法：

获取事务属性TransactionAttribute和TransactionManager事务管理器
对响应式事务、声明式事务和编程式事务分别进行判断，以声明式事务为例步骤如下：
- 创建事务
- 执行方法
- 捕捉异常，如果抛出异常进行回滚
- 清除事务信息
- 提交事务

public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {
    @Nullable
	protected Object invokeWithinTransaction(Method method, @Nullable Class<?> targetClass,
			final InvocationCallback invocation) throws Throwable {

		// 获取TransactionAttributeSource
		TransactionAttributeSource tas = getTransactionAttributeSource();
		final TransactionAttribute txAttr = (tas != null ? tas.getTransactionAttribute(method, targetClass) : null);
        // 获取事务管理器TransactionManager
		final TransactionManager tm = determineTransactionManager(txAttr);
        // 响应式事务处理
		if (this.reactiveAdapterRegistry != null && tm instanceof ReactiveTransactionManager) {
			ReactiveTransactionSupport txSupport = this.transactionSupportCache.computeIfAbsent(method, key -> {
				if (KotlinDetector.isKotlinType(method.getDeclaringClass()) && KotlinDelegate.isSuspend(method)) {
					throw new TransactionUsageException(
							"Unsupported annotated transaction on suspending function detected: " + method +
							". Use TransactionalOperator.transactional extensions instead.");
				}
				ReactiveAdapter adapter = this.reactiveAdapterRegistry.getAdapter(method.getReturnType());
				if (adapter == null) {
					throw new IllegalStateException("Cannot apply reactive transaction to non-reactive return type: " +
							method.getReturnType());
				}
				return new ReactiveTransactionSupport(adapter);
			});
			return txSupport.invokeWithinTransaction(
					method, targetClass, invocation, txAttr, (ReactiveTransactionManager) tm);
		}

		PlatformTransactionManager ptm = asPlatformTransactionManager(tm);
		final String joinpointIdentification = methodIdentification(method, targetClass, txAttr);
        // 声明式事务的处理
		if (txAttr == null || !(ptm instanceof CallbackPreferringPlatformTransactionManager)) {
			// 创建事务
			TransactionInfo txInfo = createTransactionIfNecessary(ptm, txAttr, joinpointIdentification);

			Object retVal;
			try {
				// This is an around advice: Invoke the next interceptor in the chain.
                // 这是一个环绕通知，将会在拦截链中执行下一个拦截
				retVal = invocation.proceedWithInvocation();
			}
			catch (Throwable ex) {
				// 捕捉异常，进行回滚
				completeTransactionAfterThrowing(txInfo, ex);
				throw ex;
			}
			finally {
                // 清除事务
				cleanupTransactionInfo(txInfo);
			}

			if (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);
				}
			}
            // 提交事务
			commitTransactionAfterReturning(txInfo);
			return retVal;
		}
        // 编程式事务的处理
		else {
			final ThrowableHolder throwableHolder = new ThrowableHolder();

			// It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
			try {
				Object result = ((CallbackPreferringPlatformTransactionManager) ptm).execute(txAttr, status -> {
                    // 获取事务信息
					TransactionInfo txInfo = prepareTransactionInfo(ptm, txAttr, joinpointIdentification, status);
					try {
                        // 执行方法
						Object retVal = invocation.proceedWithInvocation();
						if (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 {
                        // 清除事务信息
						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;
			}
		}
	}
}