深入解析Java的Spring框架中的混合事务与bean的区分

混合事务
在ORM框架的事务管理器的事务内，使用JdbcTemplate执行SQL是不会纳入事务管理的。
下面进行源码分析，看为什么必须要在DataSourceTransactionManager的事务内使用JdbcTemplate。

1.开启事务
DataSourceTransactionManager

protected void doBegin(Object transaction,TransactionDefinition definition) {
DataSourceTransactionObjecttxObject = (DataSourceTransactionObject) transaction;
Connection con = null;
try {
if(txObject.getConnectionHolder() == null ||
txObject.getConnectionHolder().isSynchronizedWithTransaction()){
ConnectionnewCon = this.dataSource.getConnection();
if(logger.isDebugEnabled()) {
logger.debug("AcquiredConnection [" + newCon + "] for JDBC transaction");
}
txObject.setConnectionHolder(newConnectionHolder(newCon), true);
}
txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
con =txObject.getConnectionHolder().getConnection();
IntegerpreviousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con,definition);
txObject.setPreviousIsolationLevel(previousIsolationLevel);
// Switch to manualcommit if necessary. This is very expensive in some JDBC drivers,
// so we don't wantto do it unnecessarily (for example if we've explicitly
// configured theconnection pool to set it already).
if(con.getAutoCommit()) {
txObject.setMustRestoreAutoCommit(true);
if(logger.isDebugEnabled()) {
logger.debug("SwitchingJDBC Connection [" + con + "] to manual commit");
}
con.setAutoCommit(false);
}
txObject.getConnectionHolder().setTransactionActive(true);
int timeout =determineTimeout(definition);
if (timeout !=TransactionDefinition.TIMEOUT_DEFAULT) {
txObject.getConnectionHolder().setTimeoutInSeconds(timeout);
}
// Bind the sessionholder to the thread.
if(txObject.isNewConnectionHolder()) {
TransactionSynchronizationManager.bindResource(getDataSource(),txObject.getConnectionHolder());
}
}
catch (Exception ex) {
DataSourceUtils.releaseConnection(con,this.dataSource);
throw newCannotCreateTransactionException("Could not open JDBC Connection fortransaction", ex);
}
}

doBegin()方法会以数据源名为Key，ConnectionHolder(保存着连接)为Value，将已经开启事务的数据库连接绑定到一个ThreadLocal变量上。

2.绑定连接

public static void bindResource(Objectkey, Object value) throws IllegalStateException {
Object actualKey =TransactionSynchronizationUtils.unwrapResourceIfNecessary(key);
Assert.notNull(value,"Value must not be null");
Map<Object, Object> map = resources.get();
// set ThreadLocal Map ifnone found
if (map == null) {
map = newHashMap<Object, Object>();
resources.set(map);
}
Object oldValue = map.put(actualKey, value);
// Transparently suppress aResourceHolder that was marked as void...
if (oldValue instanceofResourceHolder && ((ResourceHolder) oldValue).isVoid()) {
oldValue = null;
}
if (oldValue != null) {
throw newIllegalStateException("Already value [" + oldValue + "] for key[" +
actualKey+ "] bound to thread [" + Thread.currentThread().getName() +"]");
}
if (logger.isTraceEnabled()){
logger.trace("Boundvalue [" + value + "] for key [" + actualKey + "] to thread[" +
Thread.currentThread().getName()+ "]");
}
}

resources变量就是上面提到的ThreadLocal变量，这样后续JdbcTemplate就可以用DataSource作为Key，查找到这个数据库连接。

3.执行SQL
JdbcTemplate

public Objectexecute(PreparedStatementCreator psc, PreparedStatementCallback action)
throwsDataAccessException {
Assert.notNull(psc,"PreparedStatementCreator must not be null");
Assert.notNull(action,"Callback object must not be null");
if (logger.isDebugEnabled()){
String sql =getSql(psc);
logger.debug("Executingprepared SQL statement" + (sql != null ? " [" + sql +"]" : ""));
}
Connection con = DataSourceUtils.getConnection(getDataSource());
PreparedStatement ps = null;
try {
Connection conToUse= con;
if(this.nativeJdbcExtractor != null &&
this.nativeJdbcExtractor.isNativeConnectionNecessaryForNativePreparedStatements()){
conToUse =this.nativeJdbcExtractor.getNativeConnection(con);
}
ps =psc.createPreparedStatement(conToUse);
applyStatementSettings(ps);
PreparedStatementpsToUse = ps;
if(this.nativeJdbcExtractor != null) {
psToUse =this.nativeJdbcExtractor.getNativePreparedStatement(ps);
}
Object result =action.doInPreparedStatement(psToUse);
handleWarnings(ps);
return result;
}
catch (SQLException ex) {
// ReleaseConnection early, to avoid potential connection pool deadlock
// in the case whenthe exception translator hasn't been initialized yet.
if (psc instanceofParameterDisposer) {
((ParameterDisposer)psc).cleanupParameters();
}
String sql =getSql(psc);
psc = null;
JdbcUtils.closeStatement(ps);
ps = null;
DataSourceUtils.releaseConnection(con,getDataSource());
con = null;
throwgetExceptionTranslator().translate("PreparedStatementCallback", sql,ex);
}
finally {
if (psc instanceofParameterDisposer) {
((ParameterDisposer)psc).cleanupParameters();
}
JdbcUtils.closeStatement(ps);
DataSourceUtils.releaseConnection(con,getDataSource());
}
}

4.获得连接
DataSourceUtils

public static Connection doGetConnection(DataSourcedataSource) throws SQLException {
Assert.notNull(dataSource,"No DataSource specified");
ConnectionHolder conHolder = (ConnectionHolder)TransactionSynchronizationManager.getResource(dataSource);
if (conHolder != null&& (conHolder.hasConnection() ||conHolder.isSynchronizedWithTransaction())) {
conHolder.requested();
if(!conHolder.hasConnection()) {
logger.debug("Fetchingresumed JDBC Connection from DataSource");
conHolder.setConnection(dataSource.getConnection());
}
returnconHolder.getConnection();
}
// Else we either got noholder or an empty thread-bound holder here.
logger.debug("FetchingJDBC Connection from DataSource");
Connection con =dataSource.getConnection();
if (TransactionSynchronizationManager.isSynchronizationActive()){
logger.debug("Registeringtransaction synchronization for JDBC Connection");
// Use sameConnection for further JDBC actions within the transaction.
// Thread-boundobject will get removed by synchronization at transaction completion.
ConnectionHolderholderToUse = conHolder;
if (holderToUse ==null) {
holderToUse= new ConnectionHolder(con);
}
else {
holderToUse.setConnection(con);
}
holderToUse.requested();
TransactionSynchronizationManager.registerSynchronization(
newConnectionSynchronization(holderToUse, dataSource));
holderToUse.setSynchronizedWithTransaction(true);
if (holderToUse !=conHolder) {
TransactionSynchronizationManager.bindResource(dataSource,holderToUse);
}
}
return con;
}

由此可见，DataSourceUtils也是通过TransactionSynchronizationManager获得连接的。所以只要JdbcTemplate与DataSourceTransactionManager有相同的DataSource，就一定能得到相同的数据库连接，自然就能正确地提交、回滚事务。
 
再以Hibernate为例来说明开篇提到的问题，看看为什么ORM框架的事务管理器不能管理JdbcTemplate。

5 ORM事务管理器
HibernateTransactionManager

if(txObject.isNewSessionHolder()) {
TransactionSynchronizationManager.bindResource(getSessionFactory(),txObject.getSessionHolder());
}

因为ORM框架都不是直接将DataSource注入到TransactionManager中使用的，而是像上面Hibernate事务管理器一样，使用自己的SessionFactory等对象来操作DataSource。所以尽管可能SessionFactory和JdbcTemplate底层都是一样的数据源，但因为在TransactionSynchronizationManager中绑定时使用了不同的Key(一个是sessionFactory名，一个是dataSource名)，所以JdbcTemplate执行时是拿不到ORM事务管理器开启事务的那个数据库连接的。

bean的区分
一个公共工程中的Spring配置文件，可能会被多个工程引用。因为每个工程可能只需要公共工程中的一部分Bean，所以这些工程的Spring容器启动时，需要区分开哪些Bean要创建出来。
1.应用实例
以Apache开源框架Jetspeed中的一段配置为例：page-manager.xml
 

<bean name="xmlPageManager"class="org.apache.jetspeed.page.psml.CastorXmlPageManager"init-method="init" destroy-method="destroy">
<meta key="j2:cat" value="xmlPageManager orpageSerializer" />
<constructor-arg index="0">
<ref bean="IdGenerator"/>
</constructor-arg>
<constructor-arg index="1">
<refbean="xmlDocumentHandlerFactory" />
</constructor-arg>
……
</bean>
<bean id="dbPageManager"class="org.apache.jetspeed.page.impl.DatabasePageManager"init-method="init" destroy-method="destroy">
<meta key="j2:cat" value="dbPageManager orpageSerializer" />
<!-- OJB configuration file resourcepath -->
<constructor-arg index="0">
<value>JETSPEED-INF/ojb/page-manager-repository.xml</value>
</constructor-arg>
<!-- fragment id generator -->
<constructor-arg index="1">
<ref bean="IdGenerator"/>
</constructor-arg>
……
</bean>

2.Bean过滤器
JetspeedBeanDefinitionFilter在Spring容器解析每个Bean定义时，会取出上面Bean配置中j2:cat对应的值，例如dbPageManageror pageSerializer。然后将这部分作为正则表达式与当前的Key(从配置文件中读出)进行匹配。只有匹配上的Bean，才会被Spring容器创建出来。
 
JetspeedBeanDefinitionFilter

public boolean match(BeanDefinition bd)
{
String beanCategoriesExpression = (String)bd.getAttribute(CATEGORY_META_KEY);
boolean matched = true;
if (beanCategoriesExpression != null)
{
matched = ((matcher != null)&& matcher.match(beanCategoriesExpression));
}
return matched;
}
public void registerDynamicAlias(BeanDefinitionRegistry registry, String beanName,BeanDefinition bd)
{
String aliases =(String)bd.getAttribute(ALIAS_META_KEY);
if (aliases != null)
{
StringTokenizer st = newStringTokenizer(aliases, " ,");
while (st.hasMoreTokens())
{
String alias = st.nextToken();
if (!alias.equals(beanName))
{
registry.registerAlias(beanName, alias);
}
}
}
}

match()方法中的CATEGORY_META_KEY的值就是j2:cat，matcher类中保存的就是当前的Key，并负责将当前Key与每个Bean的进行正则表达式匹配。
registerDynamicAlias的作用是：在Bean匹配成功后，定制的Spring容器会调用此方法为Bean注册别名。详见下面1.3中的源码。

3.定制Spring容器
定制一个Spring容器，重写registerBeanDefinition()方法，在Spring注册Bean时进行拦截。

public class FilteringXmlWebApplicationContextextends XmlWebApplicationContext
{
private JetspeedBeanDefinitionFilterfilter;
publicFilteringXmlWebApplicationContext(JetspeedBeanDefinitionFilter filter, String[]configLocations, Properties initProperties, ServletContext servletContext)
{
this(filter, configLocations,initProperties, servletContext, null);
}
publicFilteringXmlWebApplicationContext(JetspeedBeanDefinitionFilter filter, String[]configLocations, Properties initProperties, ServletContext servletContext,ApplicationContext parent)
{
super();
if (parent != null)
{
this.setParent(parent);
}
if (initProperties != null)
{
PropertyPlaceholderConfigurer ppc =new PropertyPlaceholderConfigurer();
ppc.setIgnoreUnresolvablePlaceholders(true);
ppc.setSystemPropertiesMode(PropertyPlaceholderConfigurer.SYSTEM_PROPERTIES_MODE_FALLBACK);
ppc.setProperties(initProperties);
addBeanFactoryPostProcessor(ppc);
}
setConfigLocations(configLocations);
setServletContext(servletContext);
this.filter = filter;
}
protected DefaultListableBeanFactorycreateBeanFactory()
{
return new FilteringListableBeanFactory(filter,getInternalParentBeanFactory());
}
}
public classFilteringListableBeanFactory extends DefaultListableBeanFactory
{
private JetspeedBeanDefinitionFilterfilter;
public FilteringListableBeanFactory(JetspeedBeanDefinitionFilterfilter, BeanFactory parentBeanFactory)
{
super(parentBeanFactory);
this.filter = filter;
if (this.filter == null)
{
this.filter = newJetspeedBeanDefinitionFilter();
}
this.filter.init();
}
/**
* Override of the registerBeanDefinitionmethod to optionally filter out a BeanDefinition and
* if requested dynamically register anbean alias
*/
public void registerBeanDefinition(StringbeanName, BeanDefinition bd)
throws BeanDefinitionStoreException
{
if (filter.match(bd))
{
super.registerBeanDefinition(beanName, bd);
if (filter != null)
{
filter.registerDynamicAlias(this, beanName, bd);
}
}
}
}

4.为Bean起别名
使用BeanReferenceFactoryBean工厂Bean，将上面配置的两个Bean(xmlPageManager和dbPageManager)包装起来。将Key配成各自的，实现通过配置当前Key来切换两种实现。别名都配成一个，这样引用他们的Bean就直接引用这个别名就行了。例如下面的PageLayoutComponent。
 
page-manager.xml

<bean class="org.springframework.beans.factory.config.BeanReferenceFactoryBean">
<meta key="j2:cat"value="xmlPageManager" />
<meta key="j2:alias"value="org.apache.jetspeed.page.PageManager" />
<propertyname="targetBeanName" value="xmlPageManager" />
</bean>
<bean class="org.springframework.beans.factory.config.BeanReferenceFactoryBean">
<meta key="j2:cat"value="dbPageManager" />
<meta key="j2:alias"value="org.apache.jetspeed.page.PageManager" />
<propertyname="targetBeanName" value="dbPageManager" />
</bean>
<bean id="org.apache.jetspeed.layout.PageLayoutComponent"
class="org.apache.jetspeed.layout.impl.PageLayoutComponentImpl">
<meta key="j2:cat"value="default" />
<constructor-arg index="0">
<refbean="org.apache.jetspeed.page.PageManager" />
</constructor-arg>
<constructor-arg index="1">
<value>jetspeed-layouts::VelocityOneColumn</value>
</constructor-arg>
</bean>

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