简介:上次说到spring声明式事务管理的事务开始部分,按流程来讲,下面应该提交事务了, spring的声明式事务管理其实是比较复杂的,事实上这种复杂性正是由于事务本身的复杂性导致的,如果能用两三句话就把这部分内容说清楚是不现实的,也是不成熟的,而我对这部分的理解也可能是不全面的,还是那句话,希望大家和我一起把本贴的质量提交起来。
在下面的文章中,我讲会多次提到第一篇文章,第一篇文章的地址是:http://www.javaeye.com/topic/87426
如果要理解事务提交的话,理解事务开始是一个前提条件,所以请先看第一篇文章,再来看这篇
如果你仔细看下去,我想肯定是有很多收获,因为我们确实能从spring的代码和思想中学到很多东西。
正文:
其实俺的感觉就是事务提交要比事务开始复杂,看事务是否提交我们还是要回到TransactionInterceptor类的invoke方法
- public Object invoke(MethodInvocation invocation) throws Throwable {
- // Work out the target class: may be <code>null</code>.
- // The TransactionAttributeSource should be passed the target class
- // as well as the method, which may be from an interface
- Class targetClass = (invocation.getThis() != null) ? invocation.getThis().getClass() : null;
-
- // Create transaction if necessary.
- TransactionInfo txInfo = createTransactionIfNecessary(invocation.getMethod(), targetClass);
-
- 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.proceed();
- }
- catch (Throwable ex) {
- // target invocation exception
- doCloseTransactionAfterThrowing(txInfo, ex);
- throw ex;
- }
- finally {
- doFinally(txInfo);//业务方法出栈后必须先执行的一个方法
- }
- doCommitTransactionAfterReturning(txInfo);
- return retVal;
- }
public Object invoke(MethodInvocation invocation) throws Throwable {// Work out the target class: may be <code>null</code>.// The TransactionAttributeSource should be passed the target class// as well as the method, which may be from an interfaceClass targetClass = (invocation.getThis() != null) ? invocation.getThis().getClass() : null;// Create transaction if necessary.TransactionInfo txInfo = createTransactionIfNecessary(invocation.getMethod(), targetClass);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.proceed();}catch (Throwable ex) {// target invocation exceptiondoCloseTransactionAfterThrowing(txInfo, ex);throw ex;}finally {doFinally(txInfo);//业务方法出栈后必须先执行的一个方法}doCommitTransactionAfterReturning(txInfo);return retVal;}
其中的doFinally(txInfo)那一行很重要,也就是说不管如何,这个doFinally方法都是要被调用的,为什么它这么重要呢,举个例子:
我们还是以propregation_required来举例子吧,假设情况是这样的,AService中有一个方法调用了BService中的,这两个方法都处在事务体之中,他们的传播途径都是required。那么调用开始了,AService的方法首先入方法栈,并创建了TransactionInfo的实例,接着BService的方法入栈,又创建了一个TransactionInfo的实例,而重点要说明的是TransactionInfo是一个自身关联的内部类,第二个方法入栈时,会给新创建的TransactionInfo的实例设置一个属性,就是TransactionInfo对象中的private TransactionInfo oldTransactionInfo;属性,这个属性表明BService方法的创建的TransactionInfo对象是有一个old的transactionInfo对象的,这个oldTransactionInfo对象就是AService方法入栈时创建的TransactionInfo对象,我们还记得在createTransactionIfNecessary方法里有这样一个方法吧:
- protected TransactionInfo createTransactionIfNecessary(Method method, Class targetClass) {
- // We always bind the TransactionInfo to the thread, even if we didn't create
- // a new transaction here. This guarantees that the TransactionInfo stack
- // will be managed correctly even if no transaction was created by this aspect.
- txInfo.bindToThread();
- return txInfo;
- }
-
- 就是这个bindToThread()方法在作怪:
- private void bindToThread() {
- // Expose current TransactionStatus, preserving any existing transactionStatus for
- // restoration after this transaction is complete.
- oldTransactionInfo = (TransactionInfo) currentTransactionInfo.get();
- currentTransactionInfo.set(this);
- }
protected TransactionInfo createTransactionIfNecessary(Method method, Class targetClass) {// We always bind the TransactionInfo to the thread, even if we didn't create// a new transaction here. This guarantees that the TransactionInfo stack// will be managed correctly even if no transaction was created by this aspect.txInfo.bindToThread();return txInfo;}就是这个bindToThread()方法在作怪:private void bindToThread() {// Expose current TransactionStatus, preserving any existing transactionStatus for// restoration after this transaction is complete.oldTransactionInfo = (TransactionInfo) currentTransactionInfo.get();currentTransactionInfo.set(this);}
如果当前线程中已经有了一个TransactionInfo,则拿出来放到新建的transactionInfo对象的oldTransactionInfo属性中,然后再把新建的TransactionInfo设置到当前线程中。
这里有一个概念要搞清楚,就是TransactionInfo对象并不是表明事务状态的对象,表明事务状态的对象是TransactionStatus对象,这个对象同样是TransactionInfo的一个属性(这一点,我在前面一篇文章中并没有讲清楚)。
接下来BService中的那个方法返回,那么该它退栈了,它退栈后要做的就是doFinally方法,即把它的oldTransactionInfo设置到当前线程中(这个TransactionInfo对象显然就是AService方法入栈时创建的,怎么现在又要设置到线程中去呢,原因就是BService的方法出栈时并不提交事务,因为BService的传播途径是required,所以要把栈顶的方法所创建transactioninfo给设置到当前线程中),即调用AService的方法时所创建的TransactionInfo对象。那么在AServie的方法出栈时同样会设置TransactionInfo对象的oldTransactionInfo到当前线程,这时候显然oldTransactionInfo是空的,但AService中的方法会提交事务,所以它的oldTransactionInfo也应该是空了。
在这个小插曲之后,么接下来就应该是到提交事务了,之前在AService的方法出栈时,我们拿到了它入栈时创建的TransactionInfo对象,这个对象中包含了AService的方法事务状态。即TransactionStatus对象,很显然,太显然了,事务提交中的任何属性都和事务开始时的创建的对象息息相关,这个TransactionStatus对象哪里来的,我们再回头看看createTransactionIfNessary方法吧:
- protected TransactionInfo createTransactionIfNecessary(Method method, Class targetClass) {
- txInfo.newTransactionStatus(this.transactionManager.getTransaction(txAttr));
- }
protected TransactionInfo createTransactionIfNecessary(Method method, Class targetClass) {txInfo.newTransactionStatus(this.transactionManager.getTransaction(txAttr));}
再看看transactionManager.getTransaction(txAttr)方法吧:
- public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
-
- else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
- definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
- definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
- if (debugEnabled) {
- logger.debug("Creating new transaction with name [" + definition.getName() + "]");
- }
- doBegin(transaction, definition);
- boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);
- return newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, null);//注意这里的返回值,返回的就是一个TransactionStatus对象,这个对象表明了一个事务的状态,比如说是否是一个新的事务,事务是否已经结束,等等,这个对象是非常重要的,在事务提交的时候还是会用到它的。 }
- }
- }
public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {if (debugEnabled) {logger.debug("Creating new transaction with name [" + definition.getName() + "]");}doBegin(transaction, definition);boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);return newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, null);//注意这里的返回值,返回的就是一个TransactionStatus对象,这个对象表明了一个事务的状态,比如说是否是一个新的事务,事务是否已经结束,等等,这个对象是非常重要的,在事务提交的时候还是会用到它的。 }}}
还有一点需要说明的是,AService的方法在执行之前创建的transactionstatus确实是通过这个方法创建的,但是,BService的方法在执行之前创建transactionstatus的方法就与这个不一样了,下面会有详解。
回顾了事务开始时所调用的方法之后,是不是觉得现在对spring如何处理事务越来越清晰了呢。由于这么几个方法的调用,每个方法入栈之前它的事务状态就已经被设置好了。这个事务状态就是为了在方法出栈时被调用而准备的。
让我们再次回到BService中的方法出栈的那个时间段,看看spring都做了些什么,我们知道,后入栈的肯定是先出栈,BService中的方法后入栈,拿它肯定要先出栈了,它出栈的时候是要判断是否要提交事务,释放资源的,让我们来看看TransactionInterceptor的invoke的最后那个方法doCommitTransactionAfterReturning:
- protected void doCommitTransactionAfterReturning(TransactionInfo txInfo) {
- if (txInfo != null && txInfo.hasTransaction()) {
- if (logger.isDebugEnabled()) {
- logger.debug("Invoking commit for transaction on " + txInfo.joinpointIdentification());
- }
- this.transactionManager.commit(txInfo.getTransactionStatus());
- //瞧:提交事务时用到了表明事务状态的那个TransactionStatus对象了。
- }
- }
protected void doCommitTransactionAfterReturning(TransactionInfo txInfo) {if (txInfo != null && txInfo.hasTransaction()) {if (logger.isDebugEnabled()) {logger.debug("Invoking commit for transaction on " + txInfo.joinpointIdentification());}this.transactionManager.commit(txInfo.getTransactionStatus());//瞧:提交事务时用到了表明事务状态的那个TransactionStatus对象了。}}
看这个方法的名字就知道spring是要在业务方法出栈时提交事务,貌似很简单,但是事实是这样的吗? 我们接着往下看。
- public final void commit(TransactionStatus status) throws TransactionException {
- DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;
-
- if (defStatus.isCompleted()) {
- throw new IllegalTransactionStateException(
- "Transaction is already completed - do not call commit or rollback more than once per transaction");
- }
- if (defStatus.isLocalRollbackOnly()) {
- if (defStatus.isDebug()) {
- logger.debug("Transactional code has requested rollback");
- }
- processRollback(defStatus);
- return;
- }
- if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {
- if (defStatus.isDebug()) {
- logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");
- }
- processRollback(defStatus);
- throw new UnexpectedRollbackException(
- "Transaction has been rolled back because it has been marked as rollback-only");
- }
-
- processCommit(defStatus);
- }
public final void commit(TransactionStatus status) throws TransactionException {DefaultTransactionStatus defStatus = (DefaultTransactionStatus) status;if (defStatus.isCompleted()) {throw new IllegalTransactionStateException("Transaction is already completed - do not call commit or rollback more than once per transaction");}if (defStatus.isLocalRollbackOnly()) {if (defStatus.isDebug()) {logger.debug("Transactional code has requested rollback");}processRollback(defStatus);return;}if (!shouldCommitOnGlobalRollbackOnly() && defStatus.isGlobalRollbackOnly()) {if (defStatus.isDebug()) {logger.debug("Global transaction is marked as rollback-only but transactional code requested commit");}processRollback(defStatus);throw new UnexpectedRollbackException("Transaction has been rolled back because it has been marked as rollback-only");}processCommit(defStatus);}
上面这段代码就是transactionmanager中的commit,但是看上去,它又把自己的职责分配给别人了,从代码里我们看到,如果事务已经结束了就抛异常,如果事务是rollbackonly的,那么就rollback吧,但是按照正常流程,我们还是想来看一下,事务的提交,就是processCommit(status)这个方法吧。
- private void processCommit(DefaultTransactionStatus status) throws TransactionException {
- try {
- boolean beforeCompletionInvoked = false;
- try {
- triggerBeforeCommit(status);
- triggerBeforeCompletion(status);
- beforeCompletionInvoked = true;
- if (status.hasSavepoint()) {
- if (status.isDebug()) {
- logger.debug("Releasing transaction savepoint");
- }
- status.releaseHeldSavepoint();
- }
- else if (status.isNewTransaction()) {//这个判断非常重要,下面会详细讲解这个判断的作用
- if (status.isDebug()) {
- logger.debug("Initiating transaction commit");
- }
- boolean globalRollbackOnly = status.isGlobalRollbackOnly();
- doCommit(status);
- // Throw UnexpectedRollbackException if we have a global rollback-only
- // marker but still didn't get a corresponding exception from commit.
- `````````````````````
- }
private void processCommit(DefaultTransactionStatus status) throws TransactionException {try {boolean beforeCompletionInvoked = false;try {triggerBeforeCommit(status);triggerBeforeCompletion(status);beforeCompletionInvoked = true;if (status.hasSavepoint()) {if (status.isDebug()) {logger.debug("Releasing transaction savepoint");}status.releaseHeldSavepoint();}else if (status.isNewTransaction()) {//这个判断非常重要,下面会详细讲解这个判断的作用if (status.isDebug()) {logger.debug("Initiating transaction commit");}boolean globalRollbackOnly = status.isGlobalRollbackOnly();doCommit(status);// Throw UnexpectedRollbackException if we have a global rollback-only// marker but still didn't get a corresponding exception from commit.`````````````````````}
我们注意到,在判断一个事务是否是新事务之前还有一个status.hasSavepoint()的判断,我认为这个判断事实上就是嵌套事务的判断,即判断这个事务是否是嵌套事务,如果不是嵌套事务,则再判断它是否是一个新事务,下面这段话就非常重要了,BService的中的方法是先出栈的,也就是说在调用BService之前的创建的那个事务状态对象在这里要先被判断,但是由于在调用BService的方法之前已经创建了一个Transaction和Session(假设我们使用的是hibernate3),这时候在创建第二个TransactionInfo(再强调一下吧,TransactionInfo并不是Transaction,Transaction是真正的事务对象,TransactionInfo只不过是一个辅助类而已,用来记录一系列状态的辅助类)的TransactionStatus的时候就会进入下面这个方法(当然在这之前会判断一下当前线程中是否已经有了一个SessionHolder对象,不清楚SessionHolder作用的同学情况第一篇文章),这个方法其实应该放到第一篇文章中讲的,但是想到如果不讲事务提交就讲这个方法好像没有这么贴切,废话少说,我们来看一下吧:
- private TransactionStatus handleExistingTransaction(
- TransactionDefinition definition, Object transaction, boolean debugEnabled)
- throws TransactionException {
-
- if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
- throw new IllegalTransactionStateException(
- "Transaction propagation 'never' but existing transaction found");
- }
-
- if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
- if (debugEnabled) {
- logger.debug("Suspending current transaction");
- }
- Object suspendedResources = suspend(transaction);
- boolean newSynchronization = (this.transactionSynchronization == SYNCHRONIZATION_ALWAYS);
- return newTransactionStatus(
- definition, null, false, newSynchronization, debugEnabled, suspendedResources);
- }
-
- if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
- if (debugEnabled) {
- logger.debug("Suspending current transaction, creating new transaction with name [" +
- definition.getName() + "]");
- }
- Object suspendedResources = suspend(transaction);
- doBegin(transaction, definition);
- boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);
- return newTransactionStatus(
- definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
- }
-
- if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
- if (!isNestedTransactionAllowed()) {
- throw new NestedTransactionNotSupportedException(
- "Transaction manager does not allow nested transactions by default - " +
- "specify 'nestedTransactionAllowed' property with value 'true'");
- }
- if (debugEnabled) {
- logger.debug("Creating nested transaction with name [" + definition.getName() + "]");
- }
- if (useSavepointForNestedTransaction()) {
- // Create savepoint within existing Spring-managed transaction,
- // through the SavepointManager API implemented by TransactionStatus.
- // Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
- DefaultTransactionStatus status =
- newTransactionStatus(definition, transaction, false, false, debugEnabled, null);
- status.createAndHoldSavepoint();
- return status;
- }
- else {
- // Nested transaction through nested begin and commit/rollback calls.
- // Usually only for JTA: Spring synchronization might get activated here
- // in case of a pre-existing JTA transaction.
- doBegin(transaction, definition);
- boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);
- return newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, null);
- }
- }
-
- // Assumably PROPAGATION_SUPPORTS.
- if (debugEnabled) {
- logger.debug("Participating in existing transaction");
- }
- boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);
- return newTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
- }
private TransactionStatus handleExistingTransaction(TransactionDefinition definition, Object transaction, boolean debugEnabled)throws TransactionException {if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {throw new IllegalTransactionStateException("Transaction propagation 'never' but existing transaction found");}if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {if (debugEnabled) {logger.debug("Suspending current transaction");}Object suspendedResources = suspend(transaction);boolean newSynchronization = (this.transactionSynchronization == SYNCHRONIZATION_ALWAYS);return newTransactionStatus(definition, null, false, newSynchronization, debugEnabled, suspendedResources);}if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {if (debugEnabled) {logger.debug("Suspending current transaction, creating new transaction with name [" +definition.getName() + "]");}Object suspendedResources = suspend(transaction);doBegin(transaction, definition);boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);return newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);}if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {if (!isNestedTransactionAllowed()) {throw new NestedTransactionNotSupportedException("Transaction manager does not allow nested transactions by default - " +"specify 'nestedTransactionAllowed' property with value 'true'");}if (debugEnabled) {logger.debug("Creating nested transaction with name [" + definition.getName() + "]");}if (useSavepointForNestedTransaction()) {// Create savepoint within existing Spring-managed transaction,// through the SavepointManager API implemented by TransactionStatus.// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.DefaultTransactionStatus status =newTransactionStatus(definition, transaction, false, false, debugEnabled, null);status.createAndHoldSavepoint();return status;}else {// Nested transaction through nested begin and commit/rollback calls.// Usually only for JTA: Spring synchronization might get activated here// in case of a pre-existing JTA transaction.doBegin(transaction, definition);boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);return newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, null);}}// Assumably PROPAGATION_SUPPORTS.if (debugEnabled) {logger.debug("Participating in existing transaction");}boolean newSynchronization = (this.transactionSynchronization != SYNCHRONIZATION_NEVER);return newTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);}
我们看到这个方法其实很明了,就是什么样的传播途径就创建什么样的transactionstatus,这个方法是在事务开始时被调用的,拿到我们之前举的例子中来看下,我们就恍然大悟了,原来,如果之前已经创建过事务,那个这个新建的transactionstauts就不应该是属于一个newTransaction了,所以第3个参数就是false了。
也就是说,在BService的方法出栈要要执行processcommit,但是由于BService的那个TransactionStatus不是一个newTransaction,所以它根本不会触发这个动作:
- else if (status.isNewTransaction()) {//这个判断非常重要,下面会详细讲解这个判断的作用
- if (status.isDebug()) {
- logger.debug("Initiating transaction commit");
- }
- boolean globalRollbackOnly = status.isGlobalRollbackOnly();
- doCommit(status);
- }
else if (status.isNewTransaction()) {//这个判断非常重要,下面会详细讲解这个判断的作用if (status.isDebug()) {logger.debug("Initiating transaction commit");}boolean globalRollbackOnly = status.isGlobalRollbackOnly();doCommit(status);}
也就是说在BService的方法出栈后,事务是不会提交的。这完全符合propragation_required的模型。
而在AService的方法出栈后,AService的方法所对应的那个TransactionStatus对象的newTransaction属性是为true的,即它会触发上面这段代码,进行真正的事务提交。让我们回想一下AService方法入栈之前创建TransactionStatus对象的情形吧:
newTransactionStatus(definition, transaction, true, newSynchronization, debugEnabled, null);看到第3个参数为true没有。
那么事务该提交了吧,事务的提交我想使用过hibernate的人都知道怎么提交了:
txObject.getSessionHolder().getTransaction().commit();
从当前线程中拿到SessionHolder,再拿到开始事务的那个Transaction对象,然后再commit事务。在没有用spring之前,我们经常这么做。呵呵。
好吧,我已经说到了spring声明式事务管理的70%到80%的内容了,这70%到80%的内容看上去还是非常容易理解的,如果把这两篇文章认真看过,我相信会有所收获的,剩下的内容需要靠大家自己去挖掘了,因为另剩下的内容可是需要花费很多时间的,因为牵扯的东西实在是太多了,呵呵。最后祝大家阅读愉快,因为我的文笔实在是让大家的眼睛受罪了。