Spring事务管理器的设计与实现

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public class DataSourceTransactionManager extends AbstractPlatformTransactionManager
		implements ResourceTransactionManager, InitializingBean {

    /** 持有 javax.sql.DataSource对象 */
    private DataSource dataSource;

    /**
     * 这里是产生 Transaction对象 的地方，为 Transaction 的创建提供服务，对数据库而言，
     * 事务工作是由 Connection 来完成的。这里把数据库的 Connection对象 放到了 ConnectionHolder 中，
     * 然后封装到一个 DataSourceTransactionObject对象 中，在这个封装过程中增加了许多为事务处理服务的
     * 控制数据
     */
    @Override
    protected Object doGetTransaction() {
        DataSourceTransactionObject txObject = new DataSourceTransactionObject();
        txObject.setSavepointAllowed(isNestedTransactionAllowed());
        // 获取与当前线程绑定的 数据库Connection，这个 Connection 在第一个事务开始
        // 的地方与线程绑定
        ConnectionHolder conHolder =
            (ConnectionHolder) TransactionSynchronizationManager.getResource(this.dataSource);
        txObject.setConnectionHolder(conHolder, false);
        return txObject;
    }

    /**
     * 判断是否存在活跃的事务，由 ConnectionHolder 的 transactionActive属性 来控制
     */
    @Override
    protected boolean isExistingTransaction(Object transaction) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
        return (txObject.getConnectionHolder() != null && txObject.getConnectionHolder().isTransactionActive());
    }

    /**
     * 这里是处理事务开始的地方，在这里设置隔离级别，但忽略超时
     */
    @Override
    protected void doBegin(Object transaction, TransactionDefinition definition) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) transaction;
        Connection con = null;

        try {
            if (txObject.getConnectionHolder() == null ||
                    txObject.getConnectionHolder().isSynchronizedWithTransaction()) {
                Connection newCon = this.dataSource.getConnection();
                if (logger.isDebugEnabled()) {
                    logger.debug("Acquired Connection [" + newCon + "] for JDBC transaction");
                }
                txObject.setConnectionHolder(new ConnectionHolder(newCon), true);
            }

            txObject.getConnectionHolder().setSynchronizedWithTransaction(true);
            con = txObject.getConnectionHolder().getConnection();

            Integer previousIsolationLevel = DataSourceUtils.prepareConnectionForTransaction(con, definition);
            txObject.setPreviousIsolationLevel(previousIsolationLevel);

            // 这里是 数据库Connection 完成事务处理的重要配置，需要把 autoCommit属性 关掉
            if (con.getAutoCommit()) {
                txObject.setMustRestoreAutoCommit(true);
                if (logger.isDebugEnabled()) {
                    logger.debug("Switching JDBC 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);
            }

            // 把当前的 数据库Connection 与线程绑定
            if (txObject.isNewConnectionHolder()) {
                TransactionSynchronizationManager.bindResource(getDataSource(), txObject.getConnectionHolder());
            }
        }

        catch (Throwable ex) {
            DataSourceUtils.releaseConnection(con, this.dataSource);
            throw new CannotCreateTransactionException("Could not open JDBC Connection for transaction", ex);
        }
    }

    /**
     * 事务提交的具体实现
     */
    @Override
    protected void doCommit(DefaultTransactionStatus status) {
        // 取得 Connection 以后，通过Connection 进行提交
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
        Connection con = txObject.getConnectionHolder().getConnection();
        if (status.isDebug()) {
            logger.debug("Committing JDBC transaction on Connection [" + con + "]");
        }
        try {
            con.commit();
        }
        catch (SQLException ex) {
            throw new TransactionSystemException("Could not commit JDBC transaction", ex);
        }
    }

    /**
     * 事务提交的具体实现，通过 Connection对象 的 rollback()方法 实现
     */
    @Override
    protected void doRollback(DefaultTransactionStatus status) {
        DataSourceTransactionObject txObject = (DataSourceTransactionObject) status.getTransaction();
        Connection con = txObject.getConnectionHolder().getConnection();
        if (status.isDebug()) {
            logger.debug("Rolling back JDBC transaction on Connection [" + con + "]");
        }
        try {
            con.rollback();
        }
        catch (SQLException ex) {
            throw new TransactionSystemException("Could not roll back JDBC transaction", ex);
        }
    }
}