Spring中@Transactional什么时候开启事务？

Spring中@Transactional什么时候开启事务？

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在使用注解事务的情况时，Spring会把每个方法的执行封装为AOP执行过程。每次执行被事务注解方法时，会判断是否有必要创建事务。如果创建事务成功，则继续调用业务逻辑方法。事务是在方法执行最开始阶段创建的，因此方法要越小越好。

事务的执行入口是TransactionInterceptor的invoke方法

@Overridepublic 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 = (This() != null ? This()) : null);// Adapt to TransactionAspectSupport'urn Method(), targetClass, new InvocationCallback() {@Overridepublic Object proceedWithInvocation() throws Throwable {return invocation.proceed();}});}

而invoke是实现MethodInterceptor接口，而MethodInterceptor最后会封装成ReflectiveMethodInvocation实例执行，调用proceed方法。

public interface MethodInterceptor extends Interceptor {Object invoke(MethodInvocation invocation) throws Throwable;
}

TransactionInterceptor是继承扩展了

public class TransactionInterceptor extends TransactionAspectSupport implements MethodInterceptor, Serializable {}

TransactionAspectSupport是比较关键的类，里面实现了事务很多业务逻辑。包括是否必要创建事务createTransactionIfNecessary，执行事务调用invokeWithinTransaction，选用什么事务管理器determineTransactionManager。

从invokeWithinTransaction方法名可以看出，在事务中执行调用，这个方法很关键，实现事务主要逻辑。

public abstract class TransactionAspectSupport implements BeanFactoryAware, InitializingBean {protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)throws Throwable {// If the transaction attribute is null, the method is non-transactional.final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);final PlatformTransactionManager tm = determineTransactionManager(txAttr);final String joinpointIdentification = methodIdentification(method, targetClass);if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {// Standard transaction demarcation with getTransaction and commit/rollback calls.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 Val = invocation.proceedWithInvocation();//执行真正的业务操作，也就是DAO方法}catch (Throwable ex) {// target invocation exceptioncompleteTransactionAfterThrowing(txInfo, ex);throw ex;}finally {cleanupTransactionInfo(txInfo);}commitTransactionAfterReturning(txInfo);return retVal;}//省略...}

createTransactionIfNecessary告诉我们在必要的时候才创建事务

protected TransactionInfo createTransactionIfNecessary(PlatformTransactionManager tm, TransactionAttribute txAttr, final String joinpointIdentification) {// If no name specified, apply method identification as transaction name.if (txAttr != null && Name() == null) {txAttr = new DelegatingTransactionAttribute(txAttr) {@Overridepublic String getName() {return joinpointIdentification;}};}TransactionStatus status = null;if (txAttr != null) {if (tm != null) {status = tm.getTransaction(txAttr);}else {if (logger.isDebugEnabled()) {logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +"] because no transaction manager has been configured");}}}return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);}

其中invocation.proceedWithInvocation()方法是一个around advice，会进行链式调用。DAO类方法就在这里执行的，执行完后，真正事务提交在commitTransactionAfterReturning中。

protected void commitTransactionAfterReturning(TransactionInfo txInfo) {if (txInfo != null && txInfo.hasTransaction()) {if (logger.isTraceEnabled()) {ace("Completing transaction for [" + JoinpointIdentification() + "]");}TransactionManager()TransactionStatus());}}

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