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深入解析Java的Spring框架中的混合事务与bean的区分

时间:2020-03-24 13:23     来源/作者:cdai

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

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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

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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

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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

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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

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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
 

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<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

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  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。

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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

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<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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