Spring具体事务处理器的实现

Spring提供两种方式的编程式事务管理，分别是：使用TransactionTemplate和直接使用PlatformTransactionManager。

一.使用TransactionTemplate

1.TransactionTempale采用和其他Spring模板，如JdbcTempalte和HibernateTemplate一样的方法。它使用回调方法，把应用程序从处理取得和释放资源中解脱出来。如同其他模板，TransactionTemplate是线程安全的。代码片段：

public Object execute(TransactionCallback action); throws TransactionException {
TransactionStatus status = this.transactionManager.getTransaction(this);;
Object result = null;
try {
result = action.doInTransaction(status);;
}
catch (RuntimeException ex); {
// transactional code threw application exception -> rollback
rollbackOnException(status, ex);;
throw ex;
}
catch (Error err); {
// transactional code threw error -> rollback
rollbackOnException(status, err);;
throw err;
}
this.transactionManager.commit(status);;
return result;
}

使用TransactionTempale如下

Object result = tt.execute(new TransactionCallback(){
public Object doTransaction(TransactionStatus status){
updateOperation();
return resultOfUpdateOperation();
}
});

使用TransactionCallback()可以返回一个值。如果使用TransactionCallbackWithoutResult则没有返回值。

二.直接使用PlatformTransactionManager
Spring的事务处理中，通用的事务处理流程框架是由抽象事务管理器AbstractPlatformTransactionManager来提供的，而具体的底层事务处理实现，由PlatformTransactionManager的具体实现类来实现，如 DataSourceTransactionManager 、JtaTransactionManager和 HibernateTransactionManager等。
1.AbstractPlatformTransactionManager抽象事物处理器：
上一篇博客中，我们已经分析了抽象事物管理器AbstractPlatformTransactionManager的源码，了解它实现了PlatformTransactionManager平台事务管理器接口，提供了一系列设计好的事务模板方法，如事务提交、回滚等，这些模板方法的具体实现由具体的事务处理器来提供。

2.DataSourceTransactionManager事务处理器的实现：

针对JDBC连接提供的事务处理器实现，即数据源事务处理器把数据库Connection连接和当前线程进行绑定，通过直接调用数据库连接Connection的提交和回滚方法实现事务的提供和回滚处理。其部分源码如下：

public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
implements ResourceTransactionManager, InitializingBean {
//注入数据源
private DataSource dataSource;
//数据源事务处理器默认构造方法，创建一个数据源事务处理器实例，并设置允许嵌套事务
public DataSourceTransactionManager() {
setNestedTransactionAllowed(true);
}
//根据给定数据源，创建一个数据源事务处理器实例
public DataSourceTransactionManager(DataSource dataSource) {
this();
setDataSource(dataSource);
afterPropertiesSet();
}
//设置数据源
public void setDataSource(DataSource dataSource) {
if (dataSource instanceof TransactionAwareDataSourceProxy) {
//如果数据源是一个事务包装数据源代理，则获取事务包装代理的目标数据源
this.dataSource = ((TransactionAwareDataSourceProxy) dataSource).getTargetDataSource();
}
else {
this.dataSource = dataSource;
}
}
//获取数据源
public DataSource getDataSource() {
return this.dataSource;
}
//数据源事务处理器对象构造方法的回调函数
public void afterPropertiesSet() {
if (getDataSource() == null) {
throw new IllegalArgumentException("Property 'dataSource' is required");
}
}
public Object getResourceFactory() {
return getDataSource();
}
//创建事务，对数据库而言，是由Connection来完成事务工作的。该方法把数据库的//Connection对象放到一个ConnectionHolder对象中，然后封装到一个
//DataSourceTransactionObject对象中
protected Object doGetTransaction() {
//创建数据源事务对象
DataSourceTransactionObject txObject = new DataSourceTransactionObject();
//设置数据源事务对象对嵌套事务使用保存点
txObject.setSavepointAllowed(isNestedTransactionAllowed());
//从事务管理容器中获取存放数据库Connection的对象
ConnectionHolder conHolder =
(ConnectionHolder) TransactionSynchronizationManager.getResource(this.dataSource);
txObject.setConnectionHolder(conHolder, false);
return txObject;
}
//判断是否已经存在事务
protected boolean isExistingTransaction(Object transaction) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
//根据存放数据库连接的ConnectionHolder的isTransactionActive属性来判断
return (txObject.getConnectionHolder() != null && txObject.getConnectionHolder().isTransactionActive());
}
//处理事务开始的方法
protected void doBegin(Object transaction, TransactionDefinition definition) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
Connection con = null;
try {
//如果数据源事务对象的ConnectionHolder为null或者是事务同步的
if (txObject.getConnectionHolder() == null ||
txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
//获取当前数据源的数据库连接
Connection newCon = this.dataSource.getConnection();
if (logger.isDebugEnabled()) {
logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
}
//为数据源事务对象设置ConnectionHolder
txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
}
//设置数据源事务对象的事务同步	txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
//获取数据源事务对象的数据库连接
con = txObject.getConnectionHolder().getConnection();
//根据数据连接和事务属性，获取数据库连接的事务隔离级别
Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
//为数据源事务对象设置事务隔离级别
txObject.setPreviousIsolationLevel(previousIsolationLevel);
//如果数据库连接设置了自动事务提交属性，则关闭自动提交
if (con.getAutoCommit()) {
//保存数据库连接设置的自动连接到数据源事务对象中
txObject.setMustRestoreAutoCommit(true);
if (logger.isDebugEnabled()) {
logger.debug("Switching JDBC Connection [" + con + "] to manual commit");
}
//设置数据库连接自动事务提交属性为false，即禁止自动事务提交
con.setAutoCommit(false);
}
//激活当前数据源事务对象的事务配置
txObject.getConnectionHolder().setTransactionActive(true);
//获取事务配置的超时时长
int timeout = determineTimeout(definition);
//如果事务配置的超时时长不等于事务的默认超时时长
if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
//数据源事务对象设置超时时长
txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
}
//把当前数据库Connection和线程绑定
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
}
}
catch (Exception ex) {
DataSourceUtils.releaseConnection(con, this.dataSource);
throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
}
}
//事务挂起
protected Object doSuspend(Object transaction) {
//获取事务对象
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
//将事务对象中的ConnectionHolders设置为null
txObject.setConnectionHolder(null);
ConnectionHolder conHolder = (ConnectionHolder)
//解除事务对象和当前线程的绑定	TransactionSynchronizationManager.unbindResource(this.dataSource);
return conHolder;
}
//事务恢复
protected void doResume(Object transaction, Object suspendedResources) {
//获取已暂停事务的ConnectionHolder
ConnectionHolder conHolder = (ConnectionHolder) suspendedResources;
//重新将事务对象和当前线程绑定
TransactionSynchronizationManager.bindResource(this.dataSource, conHolder);
}
//事务提交
protected void doCommit(DefaultTransactionStatus status) {
//获取事务对象
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
//通过事务对象获取数据库连接
Connection con = txObject.getConnectionHolder().getConnection();
if (status.isDebug()) {
logger.debug("Committing JDBC transaction on Connection [" + con + "]");
}
try {
//使用数据库连接手动进行事务提交
con.commit();
}
catch (SQLException ex) {
throw new TransactionSystemException("Could not commit JDBC transaction", ex);
}
}
//事务回滚
protected void doRollback(DefaultTransactionStatus status) {
//获取事务对象
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
//通过事务对象获取数据库连接
Connection con = txObject.getConnectionHolder().getConnection();
if (status.isDebug()) {
logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
}
try {
//通过调用数据库连接的回滚方法完成事务回滚操作
con.rollback();
}
catch (SQLException ex) {
throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
}
}
//设置回滚
protected void doSetRollbackOnly(DefaultTransactionStatus status) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
if (status.isDebug()) {
logger.debug("Setting JDBC transaction [" + txObject.getConnectionHolder().getConnection() +
"] rollback-only");
}
txObject.setRollbackOnly();
}
//操作完成之后清除操作
protected void doCleanupAfterCompletion(Object transaction) {
DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
//移除当前线程绑定的ConnectionHolder
if (txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.unbindResource(this.dataSource);
}
Connection con = txObject.getConnectionHolder().getConnection();
try {
//如果事务对象保存了自动事务提交属性，则设置数据库连接的自动事务提交属性
if (txObject.isMustRestoreAutoCommit()) {
con.setAutoCommit(true);
}
//事务结束后重置数据库连接
DataSourceUtils.resetConnectionAfterTransaction(con, txObject.getPreviousIsolationLevel());
}
catch (Throwable ex) {
logger.debug("Could not reset JDBC Connection after transaction", ex);
}
//如果事务对象中有新的ConnectionHolder
if (txObject.isNewConnectionHolder()) {
if (logger.isDebugEnabled()) {
logger.debug("Releasing JDBC Connection [" + con + "] after transaction");
}
//释放数据库连接
DataSourceUtils.releaseConnection(con, this.dataSource);
}
//清除事务对象的ConnectionHolder
txObject.getConnectionHolder().clear();
}
//数据源事务对象，内部类
private static class DataSourceTransactionObject extends JdbcTransactionObjectSupport {
//是否有新的ConnectionHolder
private boolean newConnectionHolder;
//是否保存自动提交
private boolean mustRestoreAutoCommit;
//设置ConnectionHolder
public void setConnectionHolder(ConnectionHolder connectionHolder, boolean newConnectionHolder) {
//为父类JdbcTransactionObjectSupport设置ConnectionHolder
super.setConnectionHolder(connectionHolder);
this.newConnectionHolder = newConnectionHolder;
}
public boolean isNewConnectionHolder() {
return this.newConnectionHolder;
}
//调用父类JdbcTransactionObjectSupport的相关方法，查询收费存在事务
public boolean hasTransaction() {
return (getConnectionHolder() != null && getConnectionHolder().isTransactionActive());
}
//设置是否保存自动提交
public void setMustRestoreAutoCommit(boolean mustRestoreAutoCommit) {
this.mustRestoreAutoCommit = mustRestoreAutoCommit;
}
public boolean isMustRestoreAutoCommit() {
return this.mustRestoreAutoCommit;
}
//设置数据库连接在操作失败是，是否只回滚处理
public void setRollbackOnly() {
getConnectionHolder().setRollbackOnly();
}
public boolean isRollbackOnly() {
return getConnectionHolder().isRollbackOnly();
}
}
}

通过上述对数据源事务处理器的源码分析，我们看到，事务的提交、回滚等操作是通过直接调用数据库连接Connection的提交和回滚方法实现的，由于自动事务提交对应用程序性能影响很大，因此在进行事务提交时，我们首先禁止数据库连接的自动事务提交，事务提供操作通过手动实现。

3.HibernateTransactionManager事务处理器的实现：
相对于数据源的事务处理器来说，Hibernate的事务处理器相对要复杂一些，它是通过对Hibernate的会话Session的管理来完成事务处理实现的。Hibernate事务处理器的事务处理相关源码如下：

public class HibernateTransactionManager extends AbstractPlatformTransactionManager
implements ResourceTransactionManager, BeanFactoryAware, InitializingBean {
……
//获取Hibernate事务
protected Object doGetTransaction() {
//创建Hibernate事务对象
HibernateTransactionObject txObject = new HibernateTransactionObject();
//根据是否允许嵌套事务设置事务对象是否允许保存点
txObject.setSavepointAllowed(isNestedTransactionAllowed());
//从线程中获取SessionHolder,SessionHolder是在事务开始时与线程绑定的。
SessionHolder sessionHolder =
(SessionHolder) TransactionSynchronizationManager.getResource(getSessionFactory());
//如果获取到的SessionHolder不为null
if (sessionHolder != null) {
if (logger.isDebugEnabled()) {
logger.debug("Found thread-bound Session [" +
SessionFactoryUtils.toString(sessionHolder.getSession()) + "] for Hibernate transaction");
}
//把获取到的SessionHolder设置到Hibernate事务对象中
txObject.setSessionHolder(sessionHolder);
}
//如果当前Hibernate事务处理器有被管理的Hibernate Session
else if (this.hibernateManagedSession) {
try {
//获取当前的Hibernate Session
Session session = getSessionFactory().getCurrentSession();
if (logger.isDebugEnabled()) {
logger.debug("Found Hibernate-managed Session [" +
SessionFactoryUtils.toString(session) + "] for Spring-managed transaction");
}
//设置Hibernate事务对象已经存在指定的Session
txObject.setExistingSession(session);
}
catch (HibernateException ex) {
throw new DataAccessResourceFailureException(
"Could not obtain Hibernate-managed Session for Spring-managed transaction", ex);
}
}
//如果获取到的数据源不为null
if (getDataSource() != null) {
//将获取到的数据源和当前线程绑定
ConnectionHolder conHolder = (ConnectionHolder)
TransactionSynchronizationManager.getResource(getDataSource());
txObject.setConnectionHolder(conHolder);
}
return txObject;
}
//是否已存在事务
protected boolean isExistingTransaction(Object transaction) {
HibernateTransactionObject txObject = (HibernateTransactionObject) transaction;
//根据事务对象是否存在Spring管理的事务，或者通过判断是否存在Hibernate //Session或者事务对象中有被Hibernate管理的事务
return (txObject.hasSpringManagedTransaction() ||
(this.hibernateManagedSession && txObject.hasHibernateManagedTransaction()));
}
//处理事务开始
protected void doBegin(Object transaction, TransactionDefinition definition) {
//获取事务对象
HibernateTransactionObject txObject = (HibernateTransactionObject) transaction;
//如果事务对象有ConnectionHolder，且事务对象的数据库连接不是事务同步的
if (txObject.hasConnectionHolder() && !txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
throw new IllegalTransactionStateException(
"Pre-bound JDBC Connection found! HibernateTransactionManager does not support " +
"running within DataSourceTransactionManager if told to manage the DataSource itself. " +
"It is recommended to use a single HibernateTransactionManager for all transactions " +
"on a single DataSource, no matter whether Hibernate or JDBC access.");
}
Session session = null;
try {
//如果事务对象的SessionHolder为null，或者事务对象Hibernate
//Session是事务同步的
if (txObject.getSessionHolder() == null || txObject.getSessionHolder().isSynchronizedWithTransaction()) {
//获取Hibernate事务处理器中的实体拦截器
Interceptor entityInterceptor = getEntityInterceptor();
//获取Hibernate Session，如果实体拦截器不为null，则打开指定
//实体拦截器的Session，如果实体拦截器为null，则打开新Session
Session newSession = (entityInterceptor != null ?
getSessionFactory().openSession(entityInterceptor) : getSessionFactory().openSession());
if (logger.isDebugEnabled()) {
logger.debug("Opened new Session [" + SessionFactoryUtils.toString(newSession) +
"] for Hibernate transaction");
}
//将获取的Hibernate Session设置到事务对象中
txObject.setSession(newSession);
}
//如果Hibernate事务处理器中的SessionHolder不为null，则
//获取SessionHolder中已有的Hibernate Session
session = txObject.getSessionHolder().getSession();
//允许为JDBC连接改变事务设置
if (this.prepareConnection && isSameConnectionForEntireSession(session)) {
if (logger.isDebugEnabled()) {
logger.debug(
"Preparing JDBC Connection of Hibernate Session [" + SessionFactoryUtils.toString(session) + "]");
}
//获取Session连接
Connection con = session.connection();
//获取事务的隔离级别
Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
//设置事务对象的事务隔离级别	txObject.setPreviousIsolationLevel(previousIsolationLevel);
}
//不允许为JDBC连接改成事务设置
else {
//如果事务隔离级别不是默认事务隔离级别
if (definition.getIsolationLevel() != TransactionDefinition.ISOLATION_DEFAULT) {
throw new InvalidIsolationLevelException(
"HibernateTransactionManager is not allowed to support custom isolation levels: " +
"make sure that its 'prepareConnection' flag is on (the default) and that the " +
"Hibernate connection release mode is set to 'on_close' (SpringTransactionFactory's default). " +
"Make sure that your LocalSessionFactoryBean actually uses SpringTransactionFactory: Your " +
"Hibernate properties should *not* include a 'hibernate.transaction.factory_class' property!");
}
if (logger.isDebugEnabled()) {
logger.debug(
"Not preparing JDBC Connection of Hibernate Session [" + SessionFactoryUtils.toString(session) + "]");
}
}
//如果事务是只读，且事务对象是新的Hibernate Session
if (definition.isReadOnly() && txObject.isNewSession()) {
//设置Hibernate Session刷新模式为手动
session.setFlushMode(FlushMode.MANUAL);
}
//如果事务是非只读的，且事务对象不是新Hibernate Session
if (!definition.isReadOnly() && !txObject.isNewSession()) {
//或者Hibernate的刷新模式
FlushMode flushMode = session.getFlushMode();
//设置Session的刷新模式
if (flushMode.lessThan(FlushMode.COMMIT)) {
session.setFlushMode(FlushMode.AUTO);
//为事务对象设置刷新模式	txObject.getSessionHolder().setPreviousFlushMode(flushMode);
}
}
Transaction hibTx;
//获取事务超时时长
int timeout = determineTimeout(definition);
//如果事务配置的超时时长不是事务默认超时时长
if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
//获取Hibernate Session事务
hibTx = session.getTransaction();
//为事务对象设置超时时长
hibTx.setTimeout(timeout);
//开启事务
hibTx.begin();
}
//如果事务配置的超时时长是默认超时时长
else {
//通过Hibernate Session直接开启事务
hibTx = session.beginTransaction();
}
//把事务设置到事务对象的SessionHolder中，并且线程绑定
txObject.getSessionHolder().setTransaction(hibTx);
//如果数据源不为null，即设置了数据源
if (getDataSource() != null) {
//使用Hibernate Session打开数据库连接
Connection con = session.connection();
//创建ConnectionHolder
ConnectionHolder conHolder = new ConnectionHolder(con);
//设置超时时长
if (timeout != TransactionDefinition.TIMEOUT_DEFAULT) {
conHolder.setTimeoutInSeconds(timeout);
}
if (logger.isDebugEnabled()) {
logger.debug("Exposing Hibernate transaction as JDBC transaction [" + con + "]");
}
//将数据源和JDBC ConnectionHolder绑定到当前线程	TransactionSynchronizationManager.bindResource(getDataSource(), conHolder);
//将创建的JDBC ConnectionHolder设置到事务对象中
txObject.setConnectionHolder(conHolder);
}
//如果事务对象中的SessionHolder是新的
if (txObject.isNewSessionHolder()) {
//当SessionHolder和当前线程绑定起来	TransactionSynchronizationManager.bindResource(getSessionFactory(), txObject.getSessionHolder());
}
//设置事务对象中的SessionHolder是事务同步的	txObject.getSessionHolder().setSynchronizedWithTransaction(true);
}
//事务开启过程中异常处理
catch (Exception ex) {
if (txObject.isNewSession()) {
try {
//如果Session的事务上激活的，回滚Session的事务
if (session.getTransaction().isActive()) {
session.getTransaction().rollback();
}
}
catch (Throwable ex2) {
logger.debug("Could not rollback Session after failed transaction begin", ex);
}
finally {
//关闭Session
SessionFactoryUtils.closeSession(session);
}
}
throw new CannotCreateTransactionException("Could not open Hibernate Session for transaction", ex);
}
}
//事务挂起
protected Object doSuspend(Object transaction) {
HibernateTransactionObject txObject = (HibernateTransactionObject) transaction;
//把当前的SessionHolder从线程中和事务对象中释放
txObject.setSessionHolder(null);
//解析SessionHolder和线程的绑定
SessionHolder sessionHolder =
(SessionHolder) TransactionSynchronizationManager.unbindResource(getSessionFactory());
txObject.setConnectionHolder(null);
ConnectionHolder connectionHolder = null;
//解除数据源和线程的绑定
if (getDataSource() != null) {
connectionHolder = (ConnectionHolder) TransactionSynchronizationManager.unbindResource(getDataSource());
}
return new SuspendedResourcesHolder(sessionHolder, connectionHolder);
}
//事务恢复
protected void doResume(Object transaction, Object suspendedResources) {
SuspendedResourcesHolder resourcesHolder = (SuspendedResourcesHolder) suspendedResources;
//如果事务管理器中有SessionFactory
if (TransactionSynchronizationManager.hasResource(getSessionFactory())) {
//解除SessionFactory和当前线程的绑定
TransactionSynchronizationManager.unbindResource(getSessionFactory());
}
//如果事务管理器中没有SessionFactory，则将Session和当前线程绑定	TransactionSynchronizationManager.bindResource(getSessionFactory(), resourcesHolder.getSessionHolder());
if (getDataSource() != null) {
TransactionSynchronizationManager.bindResource(getDataSource(), resourcesHolder.getConnectionHolder());
}
}
//准备提交
protected void prepareForCommit(DefaultTransactionStatus status) {
//如果事务配置为FlushBeforeCommit，并且是新事务
if (this.earlyFlushBeforeCommit && status.isNewTransaction()) {
//获取事务对象
HibernateTransactionObject txObject = (HibernateTransactionObject) status.getTransaction();
//回去事务对象中的Session
Session session = txObject.getSessionHolder().getSession();
//如果Session的刷新模式不低于COMMIT
if (!session.getFlushMode().lessThan(FlushMode.COMMIT)) {
logger.debug("Performing an early flush for Hibernate transaction");
try {
//刷新Session
session.flush();
}
catch (HibernateException ex) {
throw convertHibernateAccessException(ex);
}
finally {
//把Session的刷新模式设置为MANUAL
session.setFlushMode(FlushMode.MANUAL);
}
}
}
}
//提交处理
protected void doCommit(DefaultTransactionStatus status) {
//获取当前的Hibernate事务对象
HibernateTransactionObject txObject = (HibernateTransactionObject) status.getTransaction();
if (status.isDebug()) {
logger.debug("Committing Hibernate transaction on Session [" +SessionFactoryUtils.toString(txObject.getSessionHolder().getSession()) + "]");
}
try {
//通过Hibernate事务完成提交
txObject.getSessionHolder().getTransaction().commit();
}
catch (org.hibernate.TransactionException ex) {
throw new TransactionSystemException("Could not commit Hibernate transaction", ex);
}
catch (HibernateException ex) {
throw convertHibernateAccessException(ex);
}
}
//回滚处理
protected void doRollback(DefaultTransactionStatus status) {
//获取Hibernate事务对象
HibernateTransactionObject txObject = (HibernateTransactionObject) status.getTransaction();
if (status.isDebug()) {
logger.debug("Rolling back Hibernate transaction on Session ["+SessionFactoryUtils.toString(txObject.getSessionHolder().getSession()) + "]");
}
try {
//通过Hibernate事务执行回滚操作
txObject.getSessionHolder().getTransaction().rollback();
}
catch (org.hibernate.TransactionException ex) {
throw new TransactionSystemException("Could not roll back Hibernate transaction", ex);
}
catch (HibernateException ex) {
throw convertHibernateAccessException(ex);
}
finally {
if (!txObject.isNewSession() && !this.hibernateManagedSession) {
//清除事务对象中的Hibernate Session
txObject.getSessionHolder().getSession().clear();
}
}
}
……
}

通过上面对Hibernate事务处理器的分析，我们看到真正执行提交、回滚等事务操作的还是Hibernate
Transaction事务对象，这与单独直接使用Hibernate没有什么区别，只是Spring将其做了通用封装，更加方便使用。