Spring-事务的源码分析（七）
都知道事务是通过spring AOP来实现的；前面文章已经分析过AOP的实现原理了，而事务只不过是AOP中的一个增强器而已；所以接下来将分析一下事务增强器的原理。
spring的核心就是IC依赖注入，那么就要先解析依赖配置，然后再注入。所以spring的功能都会出现两块，一块是解析mxl，一块是构建BeanDefinition。
事务增强器也是这样，先要解析事务的标签，然后才是执行事务。
解析tx事务标签
来看一下我们在xml中定义的事务标签是在那里被解析的？
public class TxNamespaceHandler extends NamespaceHandlerSupport {
static final String TRANSACTION_MANAGER_ATTRIBUTE = "transaction-manager";
static final String DEFAULT_TRANSACTION_MANAGER_BEAN_NAME = "transactionManager";
static String getTransactionManagerName(Element element) {
return (element.hasAttribute(TRANSACTION_MANAGER_ATTRIBUTE) ?
element.getAttribute(TRANSACTION_MANAGER_ATTRIBUTE) : DEFAULT_TRANSACTION_MANAGER_BEAN_NAME);
}
@Override
public void init() {
registerBeanDefinitionParser("advice", new TxAdviceBeanDefinitionParser());
registerBeanDefinitionParser("annotation-driven", new AnnotationDrivenBeanDefinitionParser());
registerBeanDefinitionParser("jta-transaction-manager", new JtaTransactionManagerBeanDefinitionParser());
}
}从代码中可以看出，事务的开始一切都从：AnnotationDrivenBeanDefinitionParser 类的parse开始了。
开始解析xml标签了
/**
* 解析annotation-driven开头的标签--> {@code <tx:annotation-driven/>} tag. Will
* {@link AopNamespaceUtils#registerAutoProxyCreatorIfNecessary register an AutoProxyCreator}
* with the container as necessary.
*/
@Override
public BeanDefinition parse(Element element, ParserContext parserContext) {
registerTransactionalEventListenerFactory(parserContext);
String mode = element.getAttribute("mode");
if ("aspectj".equals(mode)) {
// mode="aspectj"
registerTransactionAspect(element, parserContext);
}
else {
// mode="proxy"
AopAutoProxyConfigurer.configureAutoProxyCreator(element, parserContext);
}
return null;
}解析tx事务的标签的核心全部在InfrastructureAdvisorAutoProxyCreator这个里面，
InfrastructureAdvisorAutoProxyCreator -> AbstractAdvisorAutoProxyCreator -> AbstractAutoProxyCreator
所有的解析工作大部分都在AbstractAutoProxyCreator类里面解析完成的。前面已经分析了aop时已经分析完毕了，所以在这里就不继续分析了。
执行事务增强器
当判断某个bean适用于事务增强时，也就是用于增强器BeanFactoryTransactionAttributeSourceAdvisor这个类，所以在自定义标签解析时，注入的类成为了整个事务功能的基础。
BeanFactoryTransactionAttributeSourceAdvisor作为Advisor的实现类，需要遵从Advisor的处理方式，当代理被调用时会调用这个类的增强方法，也就是此bean的Advise，又因为在事务标签解析时我们把TransactionInterceptor类型的bean注入到了BeanFactoryTransactionAttributeSourceAdvisor中，所以在调用事务增强器增强的代理类时会先执行TransactionInterceptor进行增强，也就是执行TransactionInterceptor中的invoke方法完成整个事务的逻辑。
时序图：
从时序图中可以看出：
1. 解析注解事务中的属性；
2. 根据属性设置创建事务；
3. 在当前线程中执行用户真正的代码；
4. 如果执行过程中有异常则执行回滚逻辑；
5. 如果没有异常，则提交事务。
在前面讲解事务时说到过，spring事务是基于数据库本身的事务的，从时序图中也可以看出，创建事务，回滚事务，提交事务，操作都需要去操作数据库连接类DataSourceTransactionManager。
创建事务
看着时序图已经可以往下走了，下面将展示源码中个人觉得重要的点，其他不重要的由于篇幅问题就不帖出来了。
第一步：整个事务执行框架
TransactionInterceptor中的invoke执行了TransactionAspectSupport的invokeWithinTransaction方法。
protected Object invokeWithinTransaction(Method method, Class<?> targetClass, final InvocationCallback invocation)
throws Throwable {
//获取事务属性，如果没有属性，则该方法没有事务 ** getTransactionAttribute重点**
final TransactionAttribute txAttr = getTransactionAttributeSource().getTransactionAttribute(method, targetClass);
final PlatformTransactionManager tm = determineTransactionManager(txAttr); //获取beanFactory中的事务管理器
final String joinpointIdentification = methodIdentification(method, targetClass);
//声明式事务处理
if (txAttr == null || !(tm instanceof CallbackPreferringPlatformTransactionManager)) {
//创建事务
TransactionInfo txInfo = createTransactionIfNecessary(tm, txAttr, joinpointIdentification);
Object retVal = null;
try {
// This is an around advice: Invoke the next interceptor in the chain.
// 执行被增强的方法
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// 异常回滚
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}
finally { //清除信息
cleanupTransactionInfo(txInfo);
} //提交事务
commitTransactionAfterReturning(txInfo);
return retVal;
}
else {
// 编程式事务处理It's a CallbackPreferringPlatformTransactionManager: pass a TransactionCallback in.
.......
.......
}
}可以看到整个事务在上面的方法中已经全部包含了，创建事务，执行代码，回滚，提交。spring把具体操作又拆分到各个业务类中了。
第二步：获取事务注解属性
就是解析方法上面的：@Transactional(propagation = Propagation.REQUIRES_NEW)这个里面的属性设置。
跟着上面的时序图直接来到parseTransactionAnnotation这个方法中看看。
//开始解析各种事务的标签
protected TransactionAttribute parseTransactionAnnotation(AnnotationAttributes attributes) {
RuleBasedTransactionAttribute rbta = new RuleBasedTransactionAttribute();
Propagation propagation = attributes.getEnum("propagation");
rbta.setPropagationBehavior(propagation.value());
Isolation isolation = attributes.getEnum("isolation");
rbta.setIsolationLevel(isolation.value());
rbta.setTimeout(attributes.getNumber("timeout").intValue());
rbta.setReadOnly(attributes.getBoolean("readOnly"));
rbta.setQualifier(attributes.getString("value"));
ArrayList<RollbackRuleAttribute> rollBackRules = new ArrayList<RollbackRuleAttribute>();
Class<?>[] rbf = attributes.getClassArray("rollbackFor");
for (Class<?> rbRule : rbf) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] rbfc = attributes.getStringArray("rollbackForClassName");
for (String rbRule : rbfc) {
RollbackRuleAttribute rule = new RollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
Class<?>[] nrbf = attributes.getClassArray("noRollbackFor");
for (Class<?> rbRule : nrbf) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
String[] nrbfc = attributes.getStringArray("noRollbackForClassName");
for (String rbRule : nrbfc) {
NoRollbackRuleAttribute rule = new NoRollbackRuleAttribute(rbRule);
rollBackRules.add(rule);
}
rbta.getRollbackRules().addAll(rollBackRules);
return rbta;
}第三步：根据上面获取的属性，创建事务
TransactionAspectSupport.createTransactionIfNecessary
protected TransactionInfo createTransactionIfNecessary(
PlatformTransactionManager tm, TransactionAttribute txAttr, final String joinpointIdentification) {
// If no name specified, apply method identification as transaction name.
if (txAttr != null && txAttr.getName() == null) {
txAttr = new DelegatingTransactionAttribute(txAttr) {
@Override
public String getName() {
return joinpointIdentification;
}
};
}
TransactionStatus status = null;
if (txAttr != null) {
if (tm != null) { //获取事务状态信息，**getTransaction重要**包活设置一些事务的传播性
status = tm.getTransaction(txAttr);
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Skipping transactional joinpoint [" + joinpointIdentification +
"] because no transaction manager has been configured");
}
}
} //准备事务信息**prepareTransactionInfo**
return prepareTransactionInfo(tm, txAttr, joinpointIdentification, status);
}设置事务的状态
AbstractPlatformTransactionManager.getTransaction
public final TransactionStatus getTransaction(TransactionDefinition definition) throws TransactionException {
Object transaction = doGetTransaction(); //获取事务**doGetTransaction**
// Cache debug flag to avoid repeated checks.
boolean debugEnabled = logger.isDebugEnabled();
if (definition == null) {
// Use defaults if no transaction definition given.
definition = new DefaultTransactionDefinition();
}
//判断当时是否存在事务，判断依据为当前线程记录的连接不为空且连接中的事务属性不为空
if (isExistingTransaction(transaction)) {
// 当前线程已经存在事务 **handleExistingTransaction** Existing transaction found -> check propagation behavior to find out how to behave.
return handleExistingTransaction(definition, transaction, debugEnabled);
}
// 事务超时
if (definition.getTimeout() < TransactionDefinition.TIMEOUT_DEFAULT) {
throw new InvalidTimeoutException("Invalid transaction timeout", definition.getTimeout());
}
// 当前不存在事务，但是事务类型为：PROPAGATION_MANDATORY的时候会抛一个异常 -> check propagation behavior to find out how to proceed.
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_MANDATORY) {
throw new IllegalTransactionStateException(
"No existing transaction found for transaction marked with propagation 'mandatory'");
}
else if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRED ||
definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW ||
definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NESTED) {
SuspendedResourcesHolder suspendedResources = suspend(null);
if (debugEnabled) {
logger.debug("Creating new transaction with name [" + definition.getName() + "]: " + definition);
}
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
doBegin(transaction, definition); //**doBegin**构造事务，包括设置connectionHolder，隔离级别，timeout，如果是新连接，绑定到当前线程
prepareSynchronization(status, definition); //新同步事务的设置，针对当前线程的设置，这里用到了ThreadLocal哦。
return status;
}
catch (RuntimeException ex) {
resume(null, suspendedResources);
throw ex;
}
catch (Error err) {
resume(null, suspendedResources);
throw err;
}
}
else {
// Create "empty" transaction: no actual transaction, but potentially synchronization.
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT && logger.isWarnEnabled()) {
logger.warn("Custom isolation level specified but no actual transaction initiated; " +
"isolation level will effectively be ignored: " + definition);
}
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(definition, null, true, newSynchronization, debugEnabled, null);
}
}我们先抛开事务中已经存在事务的判断，先来继续创建事务。
走到了：doBegin。
进入到了DataSourceTransactionManager这个的doBegin
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
Connection con = null;
try {
if (txObject.getConnectionHolder() == null ||
txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
Connection newCon = this.dataSource.getConnection();
if (logger.isDebugEnabled()) {
logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
con = txObject.getConnectionHolder().getConnection();
//设置事务的隔离级别**prepareConnectionForTransaction**
Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
txObject.setPreviousIsolationLevel(previousIsolationLevel);
// Switch to manual commit if necessary. This is very expensive in some JDBC drivers,
// so we don't want to do it unnecessarily (for example if we've explicitly
// configured the connection pool to set it already).
if (con.getAutoCommit()) { //数据库的事务为自定提交，则进入
txObject.setMustRestoreAutoCommit(true);//改为spring控制提交
if (logger.isDebugEnabled()) {
logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
}
con.setAutoCommit(false); //取消数据库的自动提交
} //设置当前线程有事务，后续进来的线程判断的时候就可以按照这个来做判断了
txObject.getConnectionHolder().setTransactionActive(true);
int timeout = determineTimeout(definition);
if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
}
// Bind the session holder to the thread.
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
}
}
catch (Throwable ex) {
if (txObject.isNewConnectionHolder()) {
DataSourceUtils.releaseConnection(con, this.dataSource);
txObject.setConnectionHolder(null, false);
}
throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
}
}看到了怎么创建事务了嘛啊？就是连接数据库，然后更改当前数据库的连接的事务提交为false，当前执行了sql之后就不会自动提交了。
接下来进入到：
AbstractPlatformTransactionManager.prepareSynchronization
/**
* Initialize transaction synchronization as appropriate.，将当前事务记录到ThreadLocal中
*/
protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
if (status.isNewSynchronization()) {
TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
definition.getIsolationLevel() : null);
TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
TransactionSynchronizationManager.initSynchronization();
}
}这个方法里面主要是对：新同步事务的设置，针对当前线程的设置，这里用到了ThreadLocal哦。把刚建立好的事务信息存储到ThreadLocal里面。（这也就是下面在一个事务中开启多线程之后，多线程里面的数据库操作不在一个事务里面的原因了）
到这里，总算创建完毕一个事务了，整个过程就那么多了。
事务中已经存在事务（事务的传播属性）
如果当前事务中已经存在事务了，这就是事务的传播属性了。
private TransactionStatus handleExistingTransaction(
TransactionDefinition definition, Object transaction, boolean debugEnabled)
throws TransactionException {
//可以看出PROPAGATION_NEVER这个事务类型，如果当前有事务，直接抛异常，比较简单粗暴。
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NEVER) {
throw new IllegalTransactionStateException(
"Existing transaction found for transaction marked with propagation 'never'");
}
//忽略当前事务，以非事务方式执行操作，如果当前存在事务就把当前事务挂起，执行完后恢复事务（忽略当前事务）
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_NOT_SUPPORTED) {
if (debugEnabled) {
logger.debug("Suspending current transaction");
}
Object suspendedResources = suspend(transaction);
boolean newSynchronization = (getTransactionSynchronization() == SYNCHRONIZATION_ALWAYS);
return prepareTransactionStatus(
definition, null, false, newSynchronization, debugEnabled, suspendedResources);
}
//新事务处理
if (definition.getPropagationBehavior() == TransactionDefinition.PROPAGATION_REQUIRES_NEW) {
if (debugEnabled) {
logger.debug("Suspending current transaction, creating new transaction with name [" +
definition.getName() + "]");
}
SuspendedResourcesHolder suspendedResources = suspend(transaction); //将原事务挂起
try {
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, suspendedResources);
doBegin(transaction, definition); //连接数据库，设置当前线程连接数据库的信息
prepareSynchronization(status, definition); //将一些事务信息绑定到threadLocal线程中 //这两句代码和创建事务时的代码一样，说明开启一个新事务，相当于开启一个新的数据库连接
return status;
}
catch (RuntimeException beginEx) {
resumeAfterBeginException(transaction, suspendedResources, beginEx);
throw beginEx;
}
catch (Error beginErr) {
resumeAfterBeginException(transaction, suspendedResources, beginErr);
throw beginErr;
}
}
//嵌套事务处理
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()) {
// 创建一个保存点
// through the SavepointManager API implemented by TransactionStatus.
// Usually uses JDBC 3.0 savepoints. Never activates Spring synchronization.
DefaultTransactionStatus status =
prepareTransactionStatus(definition, transaction, false, false, debugEnabled, null);
status.createAndHoldSavepoint(); //创建一个保存点
return status;
}
else {
// 如果不能建立保存点，则创建一个新事务吧，和上面的创建一个新事务一样的处理。
// Usually only for JTA: Spring synchronization might get activated here
// in case of a pre-existing JTA transaction.
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
DefaultTransactionStatus status = newTransactionStatus(
definition, transaction, true, newSynchronization, debugEnabled, null);
doBegin(transaction, definition);
prepareSynchronization(status, definition);
return status;
}
}
// Assumably PROPAGATION_SUPPORTS or PROPAGATION_REQUIRED.
if (debugEnabled) {
logger.debug("Participating in existing transaction");
}
if (isValidateExistingTransaction()) {
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
Integer currentIsolationLevel = TransactionSynchronizationManager.getCurrentTransactionIsolationLevel();
if (currentIsolationLevel == null || currentIsolationLevel != definition.getIsolationLevel()) {
Constants isoConstants = DefaultTransactionDefinition.constants;
throw new IllegalTransactionStateException("Participating transaction with definition [" +
definition + "] specifies isolation level which is incompatible with existing transaction: " +
(currentIsolationLevel != null ?
isoConstants.toCode(currentIsolationLevel, DefaultTransactionDefinition.PREFIX_ISOLATION) :
"(unknown)"));
}
}
if (!definition.isReadOnly()) {
if (TransactionSynchronizationManager.isCurrentTransactionReadOnly()) {
throw new IllegalTransactionStateException("Participating transaction with definition [" +
definition + "] is not marked as read-only but existing transaction is");
}
}
}
boolean newSynchronization = (getTransactionSynchronization() != SYNCHRONIZATION_NEVER);
return prepareTransactionStatus(definition, transaction, false, newSynchronization, debugEnabled, null);
}回滚事务
在看事务回滚之前先来看一段代码：
try {
// This is an around advice: Invoke the next interceptor in the chain.
// 1.执行被增强的方法
retVal = invocation.proceedWithInvocation();
}
catch (Throwable ex) {
// 2.异常回滚
completeTransactionAfterThrowing(txInfo, ex);
throw ex;
}try里面是执行我们的业务代码，就是说，只要业务代码里面有异常，都会出发回滚机制，这里只是说回滚机制哦！
TransactionAspectSupport. completeTransactionAfterThrowing
protected void completeTransactionAfterThrowing(TransactionInfo txInfo, Throwable ex) {
if (txInfo != null && txInfo.hasTransaction()) {
if (logger.isTraceEnabled()) {
logger.trace("Completing transaction for [" + txInfo.getJoinpointIdentification() +
"] after exception: " + ex);
} //判断异常是否回滚。DefaultTransactionAttribute的rollbackOn里面这个里面只有RuntimeException与error才会发生回滚
if (txInfo.transactionAttribute.rollbackOn(ex)) {
try {
//开始回滚了
txInfo.getTransactionManager().rollback(txInfo.getTransactionStatus());
}
catch (TransactionSystemException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
ex2.initApplicationException(ex);
throw ex2;
}
catch (RuntimeException ex2) {
logger.error("Application exception overridden by rollback exception", ex);
throw ex2;
}
catch (Error err) {
logger.error("Application exception overridden by rollback error", ex);
throw err;
}
}
else {
// We don't roll back on this exception.
// Will still roll back if TransactionStatus.isRollbackOnly() is true.
try {
//如果没有经过上面的回滚，那么还是会被执行提交的哦
txInfo.getTransactionManager().commit(txInfo.getTransactionStatus());
}
catch (TransactionSystemException ex2) {
logger.error("Application exception overridden by commit exception", ex);
ex2.initApplicationException(ex);
throw ex2;
}
catch (RuntimeException ex2) {
logger.error("Application exception overridden by commit exception", ex);
throw ex2;
}
catch (Error err) {
logger.error("Application exception overridden by commit error", ex);
throw err;
}
}
}
}rollbackOn()里面是这样写的：
return (ex instanceof RuntimeException || ex instanceof Error);所以默认情况下面只会回滚RuntimeException 和Error的异常，而Exception异常则不会被回滚。
回滚：AbstractPlatformTransactionManager.rollback -> processRollback
private void processRollback(DefaultTransactionStatus status) {
try {
try {
triggerBeforeCompletion(status);
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Rolling back transaction to savepoint");
} //如果有保存点，当前事务为单独的线程则回退到保存点 **rollbackToHeldSavepoint**
status.rollbackToHeldSavepoint();
}
else if (status.isNewTransaction()) {
if (status.isDebug()) {
logger.debug("Initiating transaction rollback");
} //如果当前事务为独立的新事务，则直接回退
doRollback(status); //直接执行数据库的callback回滚
}
else if (status.hasTransaction()) {
if (status.isLocalRollbackOnly() || isGlobalRollbackOnParticipationFailure()) {
if (status.isDebug()) {
logger.debug("Participating transaction failed - marking existing transaction as rollback-only");
} //如果当前事务不是独立的事务，那么只能标记状态，等待事务链执行完毕后一起回滚
doSetRollbackOnly(status);
}
else {
if (status.isDebug()) {
logger.debug("Participating transaction failed - letting transaction originator decide on rollback");
}
}
}
else {
logger.debug("Should roll back transaction but cannot - no transaction available");
}
}
catch (RuntimeException ex) {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
throw ex;
}
catch (Error err) {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
throw err;
}
triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
}
finally { //清空前面记录的资源，并将挂起的线程恢复，前面有挂起，这里有恢复哦！
cleanupAfterCompletion(status);
}
}1.如果有保存点，则直接回滚到保存点
public void rollbackToHeldSavepoint() throws TransactionException {
if (!hasSavepoint()) {
throw new TransactionUsageException(
"Cannot roll back to savepoint - no savepoint associated with current transaction");
}
getSavepointManager().rollbackToSavepoint(getSavepoint()); //**rollbackToSavepoint**数据库保存点回滚
getSavepointManager().releaseSavepoint(getSavepoint());
setSavepoint(null);
}JdbcTransactionObjectSupport.rollbackToSavepoint
@Override
public void rollbackToSavepoint(Object savepoint) throws TransactionException {
ConnectionHolder conHolder = getConnectionHolderForSavepoint();
try {
conHolder.getConnection().rollback((Savepoint) savepoint);
}
catch (Throwable ex) {
throw new TransactionSystemException("Could not roll back to JDBC savepoint", ex);
}
}2.回滚数据，还是的靠操作数据库,所以执行的是DataSourceTransactionManager.doRollback;
@Override
protected void doRollback(DefaultTransactionStatus status) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
Connection con = txObject.getConnectionHolder().getConnection();
if (status.isDebug()) {
logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
}
try {
con.rollback();
}
catch (SQLException ex) {
throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
}
}提交事务
AbstractPlatformTransactionManager.processCommit
private void processCommit(DefaultTransactionStatus status) throws TransactionException {
try {
boolean beforeCompletionInvoked = false;
try {
prepareForCommit(status);
triggerBeforeCommit(status);
triggerBeforeCompletion(status);
beforeCompletionInvoked = true;
boolean globalRollbackOnly = false;
if (status.isNewTransaction() || isFailEarlyOnGlobalRollbackOnly()) {
globalRollbackOnly = status.isGlobalRollbackOnly();
}
if (status.hasSavepoint()) {
if (status.isDebug()) {
logger.debug("Releasing transaction savepoint");
} //如果存在保存点，则清除保存点信息
status.releaseHeldSavepoint();
}
else if (status.isNewTransaction()) {
if (status.isDebug()) {
logger.debug("Initiating transaction commit");
} //独立事务则直接提交
doCommit(status);
}
// Throw UnexpectedRollbackException if we have a global rollback-only
// marker but still didn't get a corresponding exception from commit.
if (globalRollbackOnly) {
throw new UnexpectedRollbackException(
"Transaction silently rolled back because it has been marked as rollback-only");
}
}
catch (UnexpectedRollbackException ex) {
// can only be caused by doCommit
triggerAfterCompletion(status, TransactionSynchronization.STATUS_ROLLED_BACK);
throw ex;
}
catch (TransactionException ex) {
// can only be caused by doCommit
if (isRollbackOnCommitFailure()) {
doRollbackOnCommitException(status, ex);
}
else {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_UNKNOWN);
}
throw ex;
}
catch (RuntimeException ex) {
if (!beforeCompletionInvoked) {
triggerBeforeCompletion(status);
} //提交过程中出现异常回滚
doRollbackOnCommitException(status, ex);
throw ex;
}
catch (Error err) {
if (!beforeCompletionInvoked) {
triggerBeforeCompletion(status);
}
doRollbackOnCommitException(status, err);
throw err;
}
// Trigger afterCommit callbacks, with an exception thrown there
// propagated to callers but the transaction still considered as committed.
try {
triggerAfterCommit(status);
}
finally {
triggerAfterCompletion(status, TransactionSynchronization.STATUS_COMMITTED);
}
}
finally {
cleanupAfterCompletion(status);
}
}同样的提交事务也得靠数据库来操作
DataSourceTransactionManager.doCommit
protected void doCommit(DefaultTransactionStatus status) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
Connection con = txObject.getConnectionHolder().getConnection();
if (status.isDebug()) {
logger.debug("Committing JDBC transaction on Connection [" + con + "]");
}
try {
con.commit();
}
catch (SQLException ex) {
throw new TransactionSystemException("Could not commit JDBC transaction", ex);
}
}事务开启多线程后无效原因
上面源码分析了那么多，来聊聊工作中遇到的在事务中开启多线程后，事务无效的问题。
示例：
public static void main(String args[]){
@Transactional
public void serviceHI() {
userInfoExtendService.serviceI();
try {
new Thread(new Runnable() {
public void run() {
userInfoExtendService.serviceH();
}
}).start();
} catch (Exception e) {
e.printStackTrace();
}
userInfoExtendService.serviceJ();
}
}执行serviceH的时候被放入多线程里面了额，那么serviceH里面是否抛异常都和外面的serviceI,serviceJ()无关了。
相当于serviceH和serviceI,serviceJ()不在一个事务里面了。加入serviceJ抛异常了，serviceI和serviceJ会被一起回滚，而serviceH则不会。
为什么呢？
看源码
AbstractPlatformTransactionManager.prepareSynchronization
/**
* Initialize transaction synchronization as appropriate.，将当前事务记录到ThreadLocal中
*/
protected void prepareSynchronization(DefaultTransactionStatus status, TransactionDefinition definition) {
if (status.isNewSynchronization()) {
TransactionSynchronizationManager.setActualTransactionActive(status.hasTransaction());
TransactionSynchronizationManager.setCurrentTransactionIsolationLevel(
definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT ?
definition.getIsolationLevel() : null);
TransactionSynchronizationManager.setCurrentTransactionReadOnly(definition.isReadOnly());
TransactionSynchronizationManager.setCurrentTransactionName(definition.getName());
TransactionSynchronizationManager.initSynchronization();
}
}来看看这个TransactionSynchronizationManager类里面定义的成员变量
public abstract class TransactionSynchronizationManager {
private static final Log logger = LogFactory.getLog(TransactionSynchronizationManager.class);
private static final ThreadLocal<Map<Object, Object>> resources =
new NamedThreadLocal<Map<Object, Object>>("Transactional resources");
private static final ThreadLocal<Set<TransactionSynchronization>> synchronizations =
new NamedThreadLocal<Set<TransactionSynchronization>>("Transaction synchronizations");
private static final ThreadLocal<String> currentTransactionName =
new NamedThreadLocal<String>("Current transaction name");
private static final ThreadLocal<Boolean> currentTransactionReadOnly =
new NamedThreadLocal<Boolean>("Current transaction read-only status");
private static final ThreadLocal<Integer> currentTransactionIsolationLevel =
new NamedThreadLocal<Integer>("Current transaction isolation level");
private static final ThreadLocal<Boolean> actualTransactionActive =
new NamedThreadLocal<Boolean>("Actual transaction active");
.....
}spring与数据库建立连接，每个线程都会有一个自己的连接，而这些连接的状态和连接属性，全部被存放到了ThreadLocal里面了，都知道ThreadLoacl的key是使用当前线程的值来作为key值的。
上面的代码中：main方法启动默认就是一个线程，serviceI，serviceJ都在这个主线程中，而现在searchH单独开启了一个线程，serviceH就在自线程里面了。
所以在开启事务，回滚事务时，serviceI，serviceJ在同一个连接里面，而serviceH在另外一个连接里面。
事务说白了就是“绑架”一个数据库连接，更改数据库的自动提交功能，使用手动提交机制。spring把这一系列操作属性及数据库连接放到了ThreadLocal中，而Thread Local的key为“当前线程的值”，可以说一个线程一个连接一个事务，在一个事务中开启多线程后，多线程就不在当前事务中了，所以serviceH在serviceHI这个的事务中是无效的。
事务注意事项
- @Transactional 只能被应用到public方法上, 对于其它非public的方法,如果标记了@Transactional也不会报错,但方法没有事务功能.
- 用 spring 事务管理器,由spring来负责数据库的打开,提交,回滚.默认遇到运行期例外(throw new RuntimeException(“注释”);)会回滚，即遇到不受检查（unchecked）的例外时回滚；而遇到需要捕获的例外(throw new Exception(“注释”);)不会回滚,即遇到受检查的例外（就是非运行时抛出的异常，编译器会检查到的异常叫受检查例外或说受检查异常）时，需我们指定方式来让事务回滚 要想所有异常都回滚,要加上
@Transactional( rollbackFor={Exception.class,其它异常}) .如果让unchecked例外不回滚： @Transactional(notRollbackFor=RunTimeException.class)
如下:
@Transactional(rollbackFor=Exception.class) //指定回滚,遇到异常Exception时回滚
public void methodName() {
throw new Exception("注释");
}
@Transactional(noRollbackFor=Exception.class)//指定不回滚,遇到运行期例外(throw new RuntimeException("注释");)会回滚
public ItimDaoImpl getItemDaoImpl() {
throw new RuntimeException("注释");
}- Spring团队的建议是你在具体的类（或类的方法）上使用 @Transactional 注解，而不要使用在类所要实现的任何接口上。你当然可以在接口上使用 @Transactional 注解，但是这将只能当你设置了基于接口的代理时它才生效。因为注解是 不能继承 的，这就意味着如果你正在使用基于类的代理时，那么事务的设置将不能被基于类的代理所识别，而且对象也将不会被事务代理所包装（将被确认为严重的）。因 此，请接受Spring团队的建议并且在具体的类上使用 @Transactional 注解。
- @Transactional 注解可以被应用于接口定义和接口方法、类定义和类的 public 方法上。然而，请注意仅仅 @Transactional 注解的出现不足于开启事务行为，它仅仅 是一种元数据，能够被可以识别 @Transactional 注解和上述的配置适当的具有事务行为的beans所使用。上面的例子中，其实正是 元素的出现 开启 了事务行为。
参考
《Spring源码深度解析》书