spring事务注解@Transactional

之前文章springAop原理之查找和原始bean关联的Advisor.已经分析了spring如何查找和判断Advisor是否和原始Bean关联，这里以
BeanFactoryTransactionAttributeSourceAdvisor(事务注解关联的Advisor)为例，具体分析下最后一个canApply方法。

ClassFilter

BeanFactoryTransactionAttributeSourceAdvisor的切点为TransactionAttributeSourcePointcut 继承了StaticMethodMatcherPointcut，StaticMethodMatcherPointcut对应的ClassFilter为TrueClassFilter，所以类匹配成功。

MethodMatcher

StaticMethodMatcherPointcut继承StaticMethodMatcher，所以TransactionAttributeSourcePointcut的MethodMatcher 为TransactionAttributeSourcePointcut自身。

TransactionAttributeSourcePointcut的matches源码

@Override
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();
   return (tas == null || tas.getTransactionAttribute(method, targetClass) != null);
}

• 先判断TransactionalProxy、PlatformTransactionManager、PersistenceExceptionTranslator是否为当前class的父类，是的话返回false。一般都是false,自己写的sevice不会继承这些类。

• 获取TransactionAttributeSource。我们发现TransactionAttributeSourcePointcut其实是个抽象类，而TransactionAttributeSourcePointcut中的getTransactionAttributeSource也是一个抽象方法。从BeanFactoryTransactionAttributeSourceAdvisor源码，我们可以看到，BeanFactoryTransactionAttributeSourceAdvisor定义TransactionAttributeSourcePointcut时，已经重写了该方法

private final TransactionAttributeSourcePointcut pointcut = new TransactionAttributeSourcePointcut() {
  @Override
  @Nullable
  protected TransactionAttributeSource getTransactionAttributeSource() {
     return transactionAttributeSource;
  }
};

返回的TransactionAttributeSource是BeanFactoryTransactionAttributeSourceAdvisor的一个成员变量。而BeanFactoryTransactionAttributeSourceAdvisor是spring通过@bean配置的（ProxyTransactionManagementConfiguration的transactionAdvisor()方法）

@Bean(name = TransactionManagementConfigUtils.TRANSACTION_ADVISOR_BEAN_NAME)
@Role(BeanDefinition.ROLE_INFRASTRUCTURE)
public BeanFactoryTransactionAttributeSourceAdvisor transactionAdvisor() {
   BeanFactoryTransactionAttributeSourceAdvisor advisor = new BeanFactoryTransactionAttributeSourceAdvisor();
   advisor.setTransactionAttributeSource(transactionAttributeSource());
   advisor.setAdvice(transactionInterceptor());
   if (this.enableTx != null) {
      advisor.setOrder(this.enableTx.<Integer>getNumber("order"));
   }
   return advisor;
}

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

所以最终的TransactionAttributeSource为AnnotationTransactionAttributeSource。

• 最后由AnnotationTransactionAttributeSource调用getTransactionAttribute(method, targetClass)实际上是调用父类AbstractFallbackTransactionAttributeSource中的方法

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 {
     // We need to work it out.
     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;
  }
}

computeTransactionAttribute(method, targetClass)继续向下就会调用AnnotationTransactionAttributeSource中findTransactionAttribute(Method method)、findTransactionAttribute(Class<?> clazz)，查找方法或类上是否有Transactional注解。