Spring声明式事务管理源码解读之事务提交

/** 
*作者:张荣华(ahuaxuan) 
*2007-06-11 
*转载:http://www.iteye.com/wiki/Spring-source/1219-Spring声明式事务管理*源码解读之事务提交
*/ 


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.iteye.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 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;
	}


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。

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