spring源码学习系列4.1-spring实现对ibatis的事务管理

事务由spring管理,可以理解为由spring管理数据库连接。ibatis执行sql时,需保证获取的数据库连接与spring管理的数据库连接是同一个

这篇文章主要从ibatis的角度分析,ibatis如何获取spring中管理的数据库连接（这里最主要的是用到了ThreadLocal技术）


应用配置:

<bean id="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">
		<property name="dataSource" ref="dataSource" />
		<property name="configLocation">
			<value>classpath:context/dao/SqlMapConfig.xml</value>
		</property>
	</bean>

	<bean id="sqlMapClientTemplate" class="org.springframework.orm.ibatis.SqlMapClientTemplate">
		<property name="sqlMapClient" ref="sqlMapClient"></property>
	</bean>

	<bean id="userDAO" class="com.byron.template.dao.impl.UserDAOImpl">
		<property name="sqlMapClientTemplate" ref="sqlMapClientTemplate"></property>
	</bean>

程序调用(以insert为例):

public class UserDAOImpl extends SqlMapClientDaoSupport implements UserDAO {  

  
    public int insertUser(User user) {  
        int id = (Integer) getSqlMapClientTemplate().insert("insertUser", user);  
        return id;  
    }  
}

spring-orm#sqlMapClientTemplate#insert

public Object insert(final String statementName, final Object parameterObject)
			throws DataAccessException {

		return execute(new SqlMapClientCallback<Object>() {
			public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {
				return executor.insert(statementName, parameterObject);
			}
		});
	}

spring-orm#sqlMapClientTemplate#execute

public <T> T execute(SqlMapClientCallback<T> action) throws DataAccessException {
		Assert.notNull(action, "Callback object must not be null");
		Assert.notNull(this.sqlMapClient, "No SqlMapClient specified");

		SqlMapSession session = this.sqlMapClient.openSession();
		if (logger.isDebugEnabled()) {
			logger.debug("Opened SqlMapSession [" + session + "] for iBATIS operation");
		}
		Connection ibatisCon = null;

		try {
			Connection springCon = null;
			DataSource dataSource = getDataSource();
			boolean transactionAware = (dataSource instanceof TransactionAwareDataSourceProxy);

			// Obtain JDBC Connection to operate on...
			try {
				ibatisCon = session.getCurrentConnection();
				if (ibatisCon == null) {
					springCon = (transactionAware ?
							dataSource.getConnection() : DataSourceUtils.doGetConnection(dataSource));
					session.setUserConnection(springCon);
					if (logger.isDebugEnabled()) {
						logger.debug("Obtained JDBC Connection [" + springCon + "] for iBATIS operation");
					}
				}
				else {
					if (logger.isDebugEnabled()) {
						logger.debug("Reusing JDBC Connection [" + ibatisCon + "] for iBATIS operation");
					}
				}
			}
			catch (SQLException ex) {
				throw new CannotGetJdbcConnectionException("Could not get JDBC Connection", ex);
			}

			// Execute given callback...
			try {
				return action.doInSqlMapClient(session);
			}
			catch (SQLException ex) {
				throw getExceptionTranslator().translate("SqlMapClient operation", null, ex);
			}
			finally {
				try {
					if (springCon != null) {
						if (transactionAware) {
							springCon.close();
						}
						else {
							DataSourceUtils.doReleaseConnection(springCon, dataSource);
						}
					}
				}
				catch (Throwable ex) {
					logger.debug("Could not close JDBC Connection", ex);
				}
			}

			// Processing finished - potentially session still to be closed.
		}
		finally {
			// Only close SqlMapSession if we know we've actually opened it
			// at the present level.
			if (ibatisCon == null) {
				session.close();
			}
		}
	}

关键点:

springCon = (transactionAware ?  
                            dataSource.getConnection() : DataSourceUtils.doGetConnection(dataSource));

SqlMapClientFactoryBean#afterPropertiesSet

public void afterPropertiesSet() throws Exception {
		if (this.lobHandler != null) {
			// Make given LobHandler available for SqlMapClient configuration.
			// Do early because because mapping resource might refer to custom types.
			configTimeLobHandlerHolder.set(this.lobHandler);
		}

		try {
			this.sqlMapClient = buildSqlMapClient(this.configLocations, this.mappingLocations, this.sqlMapClientProperties);

			// Tell the SqlMapClient to use the given DataSource, if any.
			if (this.dataSource != null) {
				TransactionConfig transactionConfig = (TransactionConfig) this.transactionConfigClass.newInstance();
				DataSource dataSourceToUse = this.dataSource;
				if (this.useTransactionAwareDataSource && !(this.dataSource instanceof TransactionAwareDataSourceProxy)) {
					dataSourceToUse = new TransactionAwareDataSourceProxy(this.dataSource);
				}
				transactionConfig.setDataSource(dataSourceToUse);
				transactionConfig.initialize(this.transactionConfigProperties);
				applyTransactionConfig(this.sqlMapClient, transactionConfig);
			}
		}

		finally {
			if (this.lobHandler != null) {
				// Reset LobHandler holder.
				configTimeLobHandlerHolder.remove();
			}
		}
	}

设置DataSource类型属性时，会为其生成代理类。该代理类并不是通过spring aop实现的代理类，只是实现了DataSource接口的静态代理类。

ibatis通过代理的DataSource的getConnection获取数据库连接


TransactionAwareDataSourceProxy是连接ibatis与spring的关键。ibatis获取数据库连接都是通过这个spring-jdbc中的类

TransactionAwareDataSourceProxy#getConnection

@Override
	public Connection getConnection() throws SQLException {
		DataSource ds = getTargetDataSource();
		Assert.state(ds != null, "'targetDataSource' is required");
		return getTransactionAwareConnectionProxy(ds);
	}

TransactionAwareDataSourceProxy#getTransactionAwareConnectionProxy

protected Connection getTransactionAwareConnectionProxy(DataSource targetDataSource) {
		return (Connection) Proxy.newProxyInstance(
				ConnectionProxy.class.getClassLoader(),
				new Class[] {ConnectionProxy.class},
				new TransactionAwareInvocationHandler(targetDataSource));
	}

spring对TransactionAwareDataSourceProxy获取的数据库连接也进行了代理,生成代理类的方式为Proxy+InvocationHandler

TransactionAwareInvocationHandler#invoke

if (this.target == null) {
				if (this.closed) {
					throw new SQLException("Connection handle already closed");
				}
				if (shouldObtainFixedConnection(this.targetDataSource)) {
					this.target = DataSourceUtils.doGetConnection(this.targetDataSource);
				}
			}
			Connection actualTarget = this.target;
			if (actualTarget == null) {
				actualTarget = DataSourceUtils.doGetConnection(this.targetDataSource);
			}

这里横切面主要为我们做的主要是获取同步的连接(线程内的)
DataSourceUtils#doGetConnection

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())) {
			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");
		Connection con = dataSource.getConnection();

		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;
	}

上面分析这些，ibatis本质上还是让TransactionSynchronizationManager获取数据库连接。至于中间的过程的实现，见仁见智

TransactionSynchronizationManager类中利用threadLocal技术保证了一个线程获取的数据库连接是一样的