Spring注解式事务解析
增加一个Advisor
首先往Spring容器新增一个Advisor，BeanFactoryTransactionAttributeSourceAdvisor，它包含了TransactionInterceptor通知和TransactionAttributeSourcePointcut切点。TransactionAttributeSourcePointcut切点实际匹配使用了AnnotationTransactionAttributeSource这个类，它的作用是方法的切点匹配，解析Transactional注解，它尝试从当前类的方法，当前类，父接口方法，父接口查找Transactional注解，有则匹配到。AOP动态代理
当普通bean实例化的时候，Spring通过AbstractAdvisorAutoProxyCreator的其中一个子类进行postProcessAfterInitialization进行AOP这个bean，织入匹配的Advisor，并生成动态代理。动态代理有2种，JdkDynamicAopProxy和ObjenesisCglibAopProxy。-
拦截过程
方法调用的时候，拿JdkDynamicAopProxy动态代理来讲，被动态代理的bean的公有方法调用会走invoke方法拦截，首先确定当前方法的拦截器链。List<Object> chain = this.advised.getInterceptorsAndDynamicInterceptionAdvice(method, targetClass);如果上面的chain为空，不走拦截，否则走拦截责任链
invocation = new ReflectiveMethodInvocation(proxy, target, method, args, targetClass, chain);
retVal = invocation.proceed(); -
TransactionInterceptor事务拦截器
内部拿到我们的TransactionInterceptor进行方法调用@Override
public Object invoke(final 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...
return invokeWithinTransaction(invocation.getMethod(), targetClass, new InvocationCallback() {
@Override
public Object proceedWithInvocation() throws Throwable {
return invocation.proceed();
}
});
}
5.创建TransactionInfo对象
他会先获取TransactionAttribute和PlatformTransactionManager，然后调用
```
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
```
内部逻辑为拿DataSourceTransactionManager举例，
尝试从ThreadLocal获取ConnectionHolder（包含了一个Connection）
```
protected Object doGetTransaction() {
DataSourceTransactionObject txObject = new DataSourceTransactionObject();
txObject.setSavepointAllowed(isNestedTransactionAllowed());
ConnectionHolder conHolder =
(ConnectionHolder) TransactionSynchronizationManager.getResource(this.dataSource);
txObject.setConnectionHolder(conHolder, false);
return txObject;
}
```
然后判断是不是新事务
```
if (isExistingTransaction(transaction)) {
// Existing transaction found -> check propagation behavior to find out how to behave.
return handleExistingTransaction(definition, transaction, debugEnabled);
}
```
不是新事务，走handleExistingTransaction逻辑，内部会根据我们配置的传递规则做不同的处理，默认的Propagation.REQUIRED就会返回TransactionStatus对象标志为已存在的事务。
是新事务则返回TransactionStatus对象标志为新事务，代码如下：
```
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
```
doBegin方法会从dataSource获取一个新连接，并设置到DataSourceTransactionObject的ConnectionHolder字段里，并且设置synchronizedWithTransaction和transactionActive为true代表新事务，然后设置con.setAutoCommit(false)，最后设置ThreadLocal值，为当前ConnectionHolder。
```
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
}
```
6.责任链调用下一个拦截器
获取了当前的TransactionInfo开始走下一个拦截器
```
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.
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// target invocation exception
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
```
7.提交或回滚
处理完了之后如果抛异常根据匹配规则是否需要回滚，默认对RuntimeException和Error回滚。如果正常返回则调用commitTransactionAfterReturning方法
```
protected void commitTransactionAfterReturning(TransactionInfo txInfo) {
if (txInfo != null && txInfo.hasTransaction()) {
if (logger.isTraceEnabled()) {
logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() + "]");
}
txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
}
}
```
进行commit，最后调用doCleanupAfterCompletion方法，清空ThreadLocal，设置con.setAutoCommit(true)，并关闭连接。