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;
     }