Spring注解式事务解析

---

1. 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。
2. 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。
3. 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();

1. 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();            }        });    }

1. 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());            }

1. 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;            }

1. 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。