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>