深入分析Spring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
TransactionSynchronizationManager
===============================================================================
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。
