MyBatis源碼淺析-創(chuàng)新互聯(lián)

什么是MyBatis

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MyBatis是支持定制化SQL、存儲過程以及高級映射的優(yōu)秀的持久層框架。MyBatis 避免了幾乎所有的 JDBC 代碼和手工設(shè)置參數(shù)以及抽取結(jié)果集。MyBatis 使用簡單的 XML 或注解來配置和映射基本體，將接口和 Java 的 POJOs(Plain Old Java Objects,普通的 Java對象)映射成數(shù)據(jù)庫中的記錄。

MyBatis簡單示例

雖然在使用MyBatis時一般都會使用XML文件，但是本文為了分析程序的簡單性，簡單的測試程序?qū)⒉话琗ML配置，該測試程序包含一個接口、一個啟動類：

public interface UserMapper { @Select("SELECT * FROM user WHERE id = #{id}") User selectUser(int id); }public class Test2 { public static void main(String[] args) { SqlSessionFactory sqlSessionFactory = initSqlSessionFactory(); SqlSession session = sqlSessionFactory.openSession(); try { User user = (User) session.selectOne( "org.mybatis.example.UserMapper.selectUser", 1); System.out.println(user.getUserAddress()); System.out.println(user.getUserName()); } finally { session.close(); } } private static SqlSessionFactory initSqlSessionFactory() { DataSource dataSource = new PooledDataSource("com.mysql.jdbc.Driver", "jdbc:mysql://127.0.0.1:3306/jdbc", "root", ""); TransactionFactory transactionFactory = new JdbcTransactionFactory(); Environment environment = new Environment("development", transactionFactory, dataSource); Configuration configuration = new Configuration(environment); configuration.addMapper(UserMapper.class); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder() .build(configuration); return sqlSessionFactory; } }

UserMapper是一個接口，我們在構(gòu)建sqlSessionFactory時通過configuration.addMapper(UserMapper.class)把該接口注冊進了sqlSessionFactory中。從上面的代碼中我們可以看出，要使用MyBatis，我們應(yīng)該經(jīng)過以下步驟：1、創(chuàng)建sqlSessionFactory（一次性操作）；2、用sqlSessionFactory對象構(gòu)造sqlSession對象；3、調(diào)用sqlSession的相應(yīng)方法；4、關(guān)閉sqlSession對象。

在main方法中，我們沒有配置sql，也沒有根據(jù)查詢結(jié)果拼接對象，只需在調(diào)用sqlSession方法時傳入一個命名空間以及方法參數(shù)參數(shù)即可，所有的操作都是面向?qū)ο蟮?。在UserMapper接口中，我們定制了自己的sql，MyBatis把書寫sql的權(quán)利給予了我們，方便我們進行sql優(yōu)化及sql排錯。

JDBC基礎(chǔ)回顧

直接使用JDBC是很痛苦的，JDBC連接數(shù)據(jù)庫包含以下幾個基本步驟：1、注冊驅(qū)動；2、建立連接(Connection)；3、創(chuàng)建SQL語句(Statement)；4、執(zhí)行語句；5、處理執(zhí)行結(jié)果(ResultSet)；6、釋放資源，示例代碼如下：

test() SQLException{ .forName(); Connection conn = DriverManager.getConnection(, , ); Statement st = conn.createStatement(); ResultSet rs = st.executeQuery(); (rs.()) { 　　　　　　User user = User(rs.getObject(), rs.getObject()); } rs.close(); st.close(); conn.close(); }

可以看到與直接使用JDBC相比，MyBatis為我們簡化了很多工作：

1、把創(chuàng)建連接相關(guān)工作抽象成一個sqlSessionFactory對象，一次創(chuàng)建多次使用；

2、把sql語句從業(yè)務(wù)層剝離，代碼邏輯更加清晰，增加可維護性；

3、自動完成結(jié)果集處理，不需要我們編寫重復(fù)代碼。

但是，我們應(yīng)該知道的是，框架雖然能夠幫助我們簡化工作，但是框架底層的代碼肯定還是最基礎(chǔ)的JDBC代碼，因為這是Java平臺連接數(shù)據(jù)庫的通用方法，今天我將分析一下MyBatis源碼，看看MyBatis是如何把這些基礎(chǔ)代碼封裝成一個框架的。

MyBatis調(diào)用流程

我們最終調(diào)用的是sqlSession對象上的方法，所以我們先跟蹤sqlSession的創(chuàng)建方法：sqlSessionFactory.openSession()，最終這個方法會調(diào)用到DefaultSqlSessionFactory的以下方法：

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) { Transaction tx = null; try { final Environment environment = configuration.getEnvironment(); final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment); tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit); final Executor executor = configuration.newExecutor(tx, execType); return new DefaultSqlSession(configuration, executor, autoCommit); } catch (Exception e) { closeTransaction(tx); // may have fetched a connection so lets call close() throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }

最終返回的對象是一個DefaultSqlSession對象，在調(diào)試模式下， 我們看到autoCommit為false，executor為CachingExecutor類型，在CachingExecutor里面有屬性delegate，其類型為simpleExecutor：

MyBatis源碼淺析

現(xiàn)在，我們跟進DefaultSqlSession的selectOne()方法，查看該方法的調(diào)用流程，selectOne()方法又會調(diào)用selectList()方法：

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) { try { MappedStatement ms = configuration.getMappedStatement(statement); List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER); return result; } catch (Exception e) { throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e); } finally { ErrorContext.instance().reset(); } }

可以看到要得到查詢結(jié)果，最終還是要調(diào)用executor上的query方法，這里的executor是CachingExecutor實例，跟進程序得到如下代碼：

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException { BoundSql boundSql = ms.getBoundSql(parameterObject); CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql); return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); } public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { Cache cache = ms.getCache(); if (cache != null) { flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, parameterObject, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); if (list == null) { list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); tcm.putObject(cache, key, list); // issue #578. Query must be not synchronized to prevent deadlocks } return list; } } return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }

MyBatis框架首先生成了一個boundSql和CacheKey，在boundSql中包含有我們傳入的sql語句：

MyBatis源碼淺析

生成boundSql和CacheKey后會調(diào)用一個重載函數(shù)，在重載函數(shù)中，我們會檢測是否有緩存，這個緩存是MyBatis的二級緩存，我們沒有配置，那么直接調(diào)用最后一句delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql)，前面說過這個delagate其實就是simpleExecutor，跟進去查看一下：

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId()); if (closed) throw new ExecutorException("Executor was closed."); if (queryStack == 0 && ms.isFlushCacheRequired()) { clearLocalCache(); } List<E> list; try { queryStack++; list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } if (queryStack == 0) { for (DeferredLoad deferredLoad : deferredLoads) { deferredLoad.load(); } deferredLoads.clear(); // issue #601 if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) { clearLocalCache(); // issue #482 } } return list; }

關(guān)鍵代碼是以下三行：

list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); }

首先嘗試從localCache中根據(jù)key得到List，這里的localCache是MyBatis的一級緩存，如果得不到則調(diào)用queryFromDatabase()從數(shù)據(jù)庫中查詢：

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try { list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { localCache.removeObject(key); } localCache.putObject(key, list); if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }

其中關(guān)鍵代碼是調(diào)用doQuery()代碼，SimpleExecutor的doQuery()方法如下：

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException { Statement stmt = null; try { Configuration configuration = ms.getConfiguration(); StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql); stmt = prepareStatement(handler, ms.getStatementLog()); return handler.<E>query(stmt, resultHandler); } finally { closeStatement(stmt); } }

調(diào)用了prepareStatement方法，該方法如下：

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException { Statement stmt; Connection connection = getConnection(statementLog); stmt = handler.prepare(connection); handler.parameterize(stmt); return stmt; }

終于，我們看到熟悉的代碼了，首先得到Connection，然后從Connection中得到Statement，同時在調(diào)試模式下我們看到，我們的sql語句已經(jīng)被設(shè)置到stmt中了：

MyBatis源碼淺析

現(xiàn)在Statement對象有了，sql也設(shè)置進去了，就只差執(zhí)行以及對象映射了，繼續(xù)跟進代碼，我們會跟蹤到org.apache.ibatis.executor.statement.

PreparedStatementHandler類的executor方法：

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException { PreparedStatement ps = (PreparedStatement) statement; ps.execute(); return resultSetHandler.<E> handleResultSets(ps); }

在這里，調(diào)用了ps.execute()方法執(zhí)行sql，接下來調(diào)用的resultSetHandler.<E> handleResultSets(ps)方法明顯是對結(jié)果集進行封裝，我就不繼續(xù)跟進了。

MyBatis的數(shù)據(jù)庫連接池

上面一部分介紹了MyBatis執(zhí)行的整體流程，這一部分打算討論一個具體話題：MyBatis的數(shù)據(jù)庫連接池。

我們知道，每次連接數(shù)據(jù)庫時都創(chuàng)建Connection是十分耗費性能的，所以我們在寫JDBC代碼時，一般都會使用數(shù)據(jù)庫連接池，把用過的Connection不是直接關(guān)閉，而是放入數(shù)據(jù)庫連接池中，方便下次復(fù)用，開源的數(shù)據(jù)庫連接池有DBCP、C3P0等，MyBatis也實現(xiàn)了自己的數(shù)據(jù)庫連接池，在這一節(jié)我將探索一下MyBatis實現(xiàn)的數(shù)據(jù)庫連接池源碼。

跟進上一節(jié)的getConnection()方法，我們最終會進入JdbcTransaction的getConnection()方法，getConnection()方法又會調(diào)用openConnection()方法，而openConnection()又將調(diào)用dataSource的getConnection()方法：

public Connection getConnection() throws SQLException { if (connection == null) { openConnection(); } return connection; }protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } connection = dataSource.getConnection(); if (level != null) { connection.setTransactionIsolation(level.getLevel()); } setDesiredAutoCommit(autoCommmit); }

這里的dataSource是PooledDataSource類型，跟進查看源碼如下：

public Connection getConnection() throws SQLException { return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection(); } private PooledConnection popConnection(String username, String password) throws SQLException { //暫不分析 }

可以看到，在這里我們返回的對象其實已經(jīng)不是原生的Connection對象了，而是一個動態(tài)代理對象，是PooledConnection的一個屬性，所有對對Connection對象的操作都將被PooledConnection攔截，我們可以查看PooledConnection的定義如下：

class PooledConnection implements InvocationHandler { private static final String CLOSE = "close"; private static final Class<?>[] IFACES = new Class<?>[] { Connection.class }; private int hashCode = 0; private PooledDataSource dataSource; private Connection realConnection; private Connection proxyConnection; private long checkoutTimestamp; private long createdTimestamp; private long lastUsedTimestamp; private int connectionTypeCode; private boolean valid; public PooledConnection(Connection connection, PooledDataSource dataSource) { this.hashCode = connection.hashCode(); this.realConnection = connection; this.dataSource = dataSource; this.createdTimestamp = System.currentTimeMillis(); this.lastUsedTimestamp = System.currentTimeMillis(); this.valid = true; this.proxyConnection = (Connection) Proxy.newProxyInstance( Connection.class.getClassLoader(), IFACES, this); } public void invalidate() { valid = false; } public boolean isValid() { return valid && realConnection != null && dataSource.pingConnection(this); } public Connection getRealConnection() { return realConnection; } public Connection getProxyConnection() { return proxyConnection; } public int getRealHashCode() { if (realConnection == null) { return 0; } else { return realConnection.hashCode(); } } public int getConnectionTypeCode() { return connectionTypeCode; } public void setConnectionTypeCode(int connectionTypeCode) { this.connectionTypeCode = connectionTypeCode; } public long getCreatedTimestamp() { return createdTimestamp; } public void setCreatedTimestamp(long createdTimestamp) { this.createdTimestamp = createdTimestamp; } public long getLastUsedTimestamp() { return lastUsedTimestamp; } public void setLastUsedTimestamp(long lastUsedTimestamp) { this.lastUsedTimestamp = lastUsedTimestamp; } public long getTimeElapsedSinceLastUse() { return System.currentTimeMillis() - lastUsedTimestamp; } public long getAge() { return System.currentTimeMillis() - createdTimestamp; } public long getCheckoutTimestamp() { return checkoutTimestamp; } public void setCheckoutTimestamp(long timestamp) { this.checkoutTimestamp = timestamp; } public long getCheckoutTime() { return System.currentTimeMillis() - checkoutTimestamp; } public int hashCode() { return hashCode; } public boolean equals(Object obj) { if (obj instanceof PooledConnection) { return realConnection.hashCode() == (((PooledConnection) obj).realConnection .hashCode()); } else if (obj instanceof Connection) { return hashCode == obj.hashCode(); } else { return false; } } public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { String methodName = method.getName(); if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) { dataSource.pushConnection(this); return null; } else { try { if (!Object.class.equals(method.getDeclaringClass())) { checkConnection(); } return method.invoke(realConnection, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } } private void checkConnection() throws SQLException { if (!valid) { throw new SQLException( "Error accessing PooledConnection. Connection is invalid."); } } }

可以看到這個類暴露了很多接口檢測Connection狀態(tài)，例如連接是否有效，連接創(chuàng)建時間最近使用連接等：

MyBatis源碼淺析

這個類實現(xiàn)了InvocationHandler接口，最主要的一個方法如下：

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { String methodName = method.getName(); if (CLOSE.hashCode() == methodName.hashCode() && CLOSE.equals(methodName)) { dataSource.pushConnection(this); return null; } else { try { if (!Object.class.equals(method.getDeclaringClass())) { checkConnection(); } return method.invoke(realConnection, args); } catch (Throwable t) { throw ExceptionUtil.unwrapThrowable(t); } } }

可以看到，PooledConnection會攔截close方法，當(dāng)客戶端調(diào)用close()方法時，程序不會關(guān)閉Connection，而是會調(diào)用dataSource.pushConnection(this)方法，該方法的實現(xiàn)如下：

protected void pushConnection(PooledConnection conn) throws SQLException { synchronized (state) { state.activeConnections.remove(conn); if (conn.isValid()) { if (state.idleConnections.size() < poolMaximumIdleConnections && conn.getConnectionTypeCode() == expectedConnectionTypeCode) { state.accumulatedCheckoutTime += conn.getCheckoutTime(); if (!conn.getRealConnection().getAutoCommit()) { conn.getRealConnection().rollback(); } PooledConnection newConn = new PooledConnection(conn.getRealConnection(), this); state.idleConnections.add(newConn); newConn.setCreatedTimestamp(conn.getCreatedTimestamp()); newConn.setLastUsedTimestamp(conn.getLastUsedTimestamp()); conn.invalidate(); if (log.isDebugEnabled()) { log.debug("Returned connection " + newConn.getRealHashCode() + " to pool."); } state.notifyAll(); } else { state.accumulatedCheckoutTime += conn.getCheckoutTime(); if (!conn.getRealConnection().getAutoCommit()) { conn.getRealConnection().rollback(); } conn.getRealConnection().close(); if (log.isDebugEnabled()) { log.debug("Closed connection " + conn.getRealHashCode() + "."); } conn.invalidate(); } } else { if (log.isDebugEnabled()) { log.debug("A bad connection (" + conn.getRealHashCode() + ") attempted to return to the pool, discarding connection."); } state.badConnectionCount++; } } }

可以看到，首先會把Connection從活躍列表中刪除，然后檢測空閑列表的長度有沒有達到大長度（默認(rèn)為5），若沒有達到，把Connection放入空閑鏈表，否則關(guān)閉連接。這里的state是一個PoolState對象，該對象定義如下：

public class PoolState { protected PooledDataSource dataSource; protected final List<PooledConnection> idleConnections = new ArrayList<PooledConnection>(); protected final List<PooledConnection> activeConnections = new ArrayList<PooledConnection>(); protected long requestCount = 0; protected long accumulatedRequestTime = 0; protected long accumulatedCheckoutTime = 0; protected long claimedOverdueConnectionCount = 0; protected long accumulatedCheckoutTimeOfOverdueConnections = 0; protected long accumulatedWaitTime = 0; protected long hadToWaitCount = 0; protected long badConnectionCount = 0; public PoolState(PooledDataSource dataSource) { this.dataSource = dataSource; } public synchronized long getRequestCount() { return requestCount; } public synchronized long getAverageRequestTime() { return requestCount == 0 ? 0 : accumulatedRequestTime / requestCount; } public synchronized long getAverageWaitTime() { return hadToWaitCount == 0 ? 0 : accumulatedWaitTime / hadToWaitCount; } public synchronized long getHadToWaitCount() { return hadToWaitCount; } public synchronized long getBadConnectionCount() { return badConnectionCount; } public synchronized long getClaimedOverdueConnectionCount() { return claimedOverdueConnectionCount; } public synchronized long getAverageOverdueCheckoutTime() { return claimedOverdueConnectionCount == 0 ? 0 : accumulatedCheckoutTimeOfOverdueConnections / claimedOverdueConnectionCount; } public synchronized long getAverageCheckoutTime() { return requestCount == 0 ? 0 : accumulatedCheckoutTime / requestCount; } public synchronized int getIdleConnectionCount() { return idleConnections.size(); } public synchronized int getActiveConnectionCount() { return activeConnections.size(); } }

可以看到最終我們的Connection對象是放在ArrayList中的，該類還提供一些接口返回連接池基本信息。

好了，現(xiàn)在我們可以回去看看PooledDataSource的popConnection方法了：

private PooledConnection popConnection(String username, String password) throws SQLException { boolean countedWait = false; PooledConnection conn = null; long t = System.currentTimeMillis(); int localBadConnectionCount = 0; while (conn == null) { synchronized (state) { if (state.idleConnections.size() > 0) { // Pool has available connection conn = state.idleConnections.remove(0); if (log.isDebugEnabled()) { log.debug("Checked out connection " + conn.getRealHashCode() + " from pool."); } } else { // Pool does not have available connection if (state.activeConnections.size() < poolMaximumActiveConnections) { // Can create new connection conn = new PooledConnection(dataSource.getConnection(), this); @SuppressWarnings("unused") //used in logging, if enabled Connection realConn = conn.getRealConnection(); if (log.isDebugEnabled()) { log.debug("Created connection " + conn.getRealHashCode() + "."); } } else { // Cannot create new connection PooledConnection oldestActiveConnection = state.activeConnections.get(0); long longestCheckoutTime = oldestActiveConnection.getCheckoutTime(); if (longestCheckoutTime > poolMaximumCheckoutTime) { // Can claim overdue connection state.claimedOverdueConnectionCount++; state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime; state.accumulatedCheckoutTime += longestCheckoutTime; state.activeConnections.remove(oldestActiveConnection); if (!oldestActiveConnection.getRealConnection().getAutoCommit()) { oldestActiveConnection.getRealConnection().rollback(); } conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this); oldestActiveConnection.invalidate(); if (log.isDebugEnabled()) { log.debug("Claimed overdue connection " + conn.getRealHashCode() + "."); } } else { // Must wait try { if (!countedWait) { state.hadToWaitCount++; countedWait = true; } if (log.isDebugEnabled()) { log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection."); } long wt = System.currentTimeMillis(); state.wait(poolTimeToWait); state.accumulatedWaitTime += System.currentTimeMillis() - wt; } catch (InterruptedException e) { break; } } } } if (conn != null) { if (conn.isValid()) { if (!conn.getRealConnection().getAutoCommit()) { conn.getRealConnection().rollback(); } conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password)); conn.setCheckoutTimestamp(System.currentTimeMillis()); conn.setLastUsedTimestamp(System.currentTimeMillis()); state.activeConnections.add(conn); state.requestCount++; state.accumulatedRequestTime += System.currentTimeMillis() - t; } else { if (log.isDebugEnabled()) { log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection."); } state.badConnectionCount++; localBadConnectionCount++; conn = null; if (localBadConnectionCount > (poolMaximumIdleConnections + 3)) { if (log.isDebugEnabled()) { log.debug("PooledDataSource: Could not get a good connection to the database."); } throw new SQLException("PooledDataSource: Could not get a good connection to the database."); } } } } } if (conn == null) { if (log.isDebugEnabled()) { log.debug("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection."); } throw new SQLException("PooledDataSource: Unknown severe error condition. The connection pool returned a null connection."); } return conn; }

可以看到獲取Connection一共分以下幾種情況：1、如果有空閑Connection，那么直接使用空閑Connection，否則2；2、如果活躍Connection沒有達到活躍Connection的上限，那么創(chuàng)建一個新Connection并返回，否則3；3、如果達到活躍上限，且被檢出的Connection檢出時間過長，那么把該Connection置為失效，新創(chuàng)建一個Connection，否則4；4、等待空閑Connection。

至此，我們就把MyBatis的數(shù)據(jù)庫連接池代碼整理了一遍，其中有兩個關(guān)鍵點：1、檢出的Connection其實不是原生Connection，而是一個代理對象；2、存放Connection的容器是ArrayList，Connection的檢出遵從先進先出原則。

這篇博客講的很好，可以mark一下：×××/technology

MyBatis的事務(wù)

首先回顧一下JDBC的事務(wù)知識。

JDBC可以操作Connection的setAutoCommit()方法，給它false參數(shù)，提示數(shù)據(jù)庫啟動事務(wù)，在下達一連串的SQL命令后，自行調(diào)用Connection的commit()方法，提示數(shù)據(jù)庫確認(rèn)(Commit)操作。如果中間發(fā)生錯誤，則調(diào)用rollback()，提示數(shù)據(jù)庫撤銷(ROLLBACK)所有執(zhí)行。同時，如果僅想要撤回某個SQL執(zhí)行點，則可以設(shè)置存儲點(SAVEPOINT)。一個示范的事務(wù)流程如下：

Connection conn = ...;Savepoint point = null;try { conn.setAutoCommit(false); Statement stmt = conn.createStatement(); stmt.executeUpdate("INSERT INTO ..."); ... point = conn.setSavepoint(); stmt.executeUpdate("INSERT INTO ..."); ... conn.commit(); } catch (SQLException e) { e.printStackTrace(); if (conn != null) { try { if (point == null) { conn.rollback(); } else { conn.rollback(point); conn.releaseSavepoint(point); } } catch (SQLException ex) { ex.printStackTrace(); } } } finally { ... if (conn != null) { try { conn.setAutoCommit(true); conn.close(); } catch (SQLException ex) { ex.printStackTrace(); } } }

在MyBatis調(diào)用流程一節(jié)就寫過，在調(diào)試模式下， 我們看到autoCommit為false， 所以每個sqlSession其實都是一個事務(wù)，這也是為什么每次做刪、改、查時都必須調(diào)用commit的原因.

源碼來源：×××/technology

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