DataSourceUtils 管理参与事务性的JDBC连接
提供一些静态方法用于管理可能参与事务的JDBC连接。JDBCTemplate或JDBCDaoSupport自动使用它，而且在DataSourceTransactionManager或JTATransactionManager，hibernateTransactionManager中支持。DataSourceTransactionManager中 DataSourceTransactionManager.DataSourceTransactionObject在事务开始时存储连接，事务完结后使用它获取连接以回复只读，隔离级别及释放连接。在事务挂起和回复时只是解除或重新绑定Holder到当前线程。 成员变量 newConnectionHolder：ConnectionHolder是否为DataSourceTransactionManager doBegin创建。true说明由事务管理器创建，那么在doBegin方法中获取连接并将Holder绑定到线程，在事务完结后由doCleanupAfterCompletion方法接触Holder绑定并释放连接。 mustRestoreAutoCommit:在事务开始时保存连接自动提交模式，进入Spring事务管理将禁用连接自动提交模式，当事务完结后doCleanupAfterCompletion将回复Spring管理之前连接的自动提交模式(无论是禁用的还是启用的：虽然很多数据源提供了自动连接模式，但Spring要求必须禁用）。 方法 hasTransaction 由connectionHolder.isTransactionActive决定，事务开始时设置，完结后清除 setRollbackOnly 设置本地只读标志，AbstractPlatformTransactionManager中提到/** * DataSource transaction object, representing a ConnectionHolder. * Used as transaction object by DataSourceTransactionManager. */private static class DataSourceTransactionObject extends JdbcTransactionObjectSupport {private boolean newConnectionHolder;private boolean mustRestoreAutoCommit;public void setConnectionHolder(ConnectionHolder connectionHolder, boolean newConnectionHolder) {super.setConnectionHolder(connectionHolder);this.newConnectionHolder = newConnectionHolder;}public boolean isNewConnectionHolder() {return this.newConnectionHolder;}public boolean hasTransaction() {return (getConnectionHolder() != null && getConnectionHolder().isTransactionActive());}public void setMustRestoreAutoCommit(boolean mustRestoreAutoCommit) {this.mustRestoreAutoCommit = mustRestoreAutoCommit;}public boolean isMustRestoreAutoCommit() {return this.mustRestoreAutoCommit;}public void setRollbackOnly() {getConnectionHolder().setRollbackOnly();}public boolean isRollbackOnly() {return getConnectionHolder().isRollbackOnly();}} DataSourceUtils.ConnectionSynchronization主要用于JTATransactionManager的资源挂起，回复，资源清理；也支持其它应用的资源清理操作。 方法 suspend 事务挂起时调用，除了解绑资源更重要的是尽可能早的释放连接，DataSourceUtils可重新获取新的连接并绑定到Holder beforeCompletion 事务提交或回滚之前调用，解绑资源和尽可能早的释放连接，当连接被其他应用释放掉DataSourceUtils.releaseConnection（JTA规范规定事务处理完成后，必须关闭所有资源） afterCompletion 事务提交或回滚之后调用，在JTATransactionManager支持下，对于参与性事务该方法被注册到J2EE事务管理器上由JTA事务完成后执行，有可能与在一个不同于当前线程的线程中执行，那Holder便不可解除。所以我们必须将连接置空。而且在事务完成之后将事务状态：同步只读事务超时引用全部清空。 /** * Callback for resource cleanup at the end of a non-native JDBC transaction * (e.g. when participating in a JtaTransactionManager transaction). * @see org.springframework.transaction.jta.JtaTransactionManager */private static class ConnectionSynchronization extends TransactionSynchronizationAdapter {private final ConnectionHolder connectionHolder;private final DataSource dataSource;private int order;private boolean holderActive = true;public ConnectionSynchronization(ConnectionHolder connectionHolder, DataSource dataSource) {this.connectionHolder = connectionHolder;this.dataSource = dataSource;this.order = getConnectionSynchronizationOrder(dataSource);}@Overridepublic int getOrder() {return this.order;}@Overridepublic void suspend() {if (this.holderActive) {TransactionSynchronizationManager.unbindResource(this.dataSource);if (this.connectionHolder.hasConnection() && !this.connectionHolder.isOpen()) {// Release Connection on suspend if the application doesn't keep// a handle to it anymore. We will fetch a fresh Connection if the// application accesses the ConnectionHolder again after resume,// assuming that it will participate in the same transaction.releaseConnection(this.connectionHolder.getConnection(), this.dataSource);this.connectionHolder.setConnection(null);}}}@Overridepublic void resume() {if (this.holderActive) {TransactionSynchronizationManager.bindResource(this.dataSource, this.connectionHolder);}}@Overridepublic void beforeCompletion() {// Release Connection early if the holder is not open anymore// (that is, not used by another resource like a Hibernate Session// that has its own cleanup via transaction synchronization),// to avoid issues with strict JTA implementations that expect// the close call before transaction completion.if (!this.connectionHolder.isOpen()) {TransactionSynchronizationManager.unbindResource(this.dataSource);this.holderActive = false;if (this.connectionHolder.hasConnection()) {releaseConnection(this.connectionHolder.getConnection(), this.dataSource);}}}@Overridepublic void afterCompletion(int status) {// If we haven't closed the Connection in beforeCompletion,// close it now. The holder might have been used for other// cleanup in the meantime, for example by a Hibernate Session.if (this.holderActive) {// The thread-bound ConnectionHolder might not be available anymore,// since afterCompletion might get called from a different thread.TransactionSynchronizationManager.unbindResourceIfPossible(this.dataSource);this.holderActive = false;if (this.connectionHolder.hasConnection()) {releaseConnection(this.connectionHolder.getConnection(), this.dataSource);// Reset the ConnectionHolder: It might remain bound to the thread.this.connectionHolder.setConnection(null);}}this.connectionHolder.reset();}} JTATransactionManager支持下无论事务开始，挂起，回复，完成都与资源操作无关（这是JTA事务具备而本地事务不具备的优点)。在应用中可调用JdbcTemplate或JDBCDaoSupport或DataSourceUtils。在Spring同步激活下，初次调用DataSourceUtils.getConnection时，会注册DataSourceUtils.ConnectionSynchronization实例。hibernateTransactionManager支持下当检测到javax.sql.DataSource时会绑定新的ConnectionHolder,它的connection是由Session提供的因此他只是一个连接代理，没有权利也不必关心连接的释放。只有刚获取的连接才有资格由ConnectionSynchronization资源释放。其它应用与之类似。 方法 doGetConnection获取JDBC连接public static Connection doGetConnection(DataSource dataSource) throws SQLException {Assert.notNull(dataSource, "No DataSource specified");ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);if (conHolder != null && (conHolder.hasConnection() || conHolder.isSynchronizedWithTransaction())) {//如果Holder被绑定着，并且能使用连接或者Holder作为事务管理器开启事务或Spring同步所管理，这种情况可视为连接可重新获取。conHolder.requested();if (!conHolder.hasConnection()) {logger.debug("Fetching resumed JDBC Connection from DataSource");conHolder.setConnection(dataSource.getConnection());}return conHolder.getConnection();}// Else we either got no holder or an empty thread-bound holder here.logger.debug("Fetching JDBC Connection from DataSource"); //获取新的连接Connection con = dataSource.getConnection(); //spring同步激活了，可用同步实例管理资源if (TransactionSynchronizationManager.isSynchronizationActive()) {logger.debug("Registering transaction synchronization for JDBC Connection");// Use same Connection for further JDBC actions within the transaction.// Thread-bound object will get removed by synchronization at transaction completion.ConnectionHolder holderToUse = conHolder;if (holderToUse == null) {holderToUse = new ConnectionHolder(con);}else {holderToUse.setConnection(con);}holderToUse.requested();TransactionSynchronizationManager.registerSynchronization(new ConnectionSynchronization(holderToUse, dataSource));holderToUse.setSynchronizedWithTransaction(true);if (holderToUse != conHolder) {TransactionSynchronizationManager.bindResource(dataSource, holderToUse);}}return con;}