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                # Mybatis之事務管理 ### 簡介 Mybatis的事務管理分為兩種JdbcTransaction,ManagedTransaction。其中JdbcTransaction僅僅是對數據庫連接Connection的一個包裝、內部管理數據庫事務還是調用Connection的提交、回滾等事務操作方法。ManagedTransaction更直接、什么也沒有做。直接將事務交給外部容器管理。 ### Mybatis事務管理相關類結構圖 類概覽: ![類概覽](https://box.kancloud.cn/2016-08-08_57a85803ca136.jpg "") 類UML圖(典型的簡單工廠模式來創建Transaction): ![類UML圖](https://box.kancloud.cn/2016-08-08_57a85803dc606.jpg "") - Transaction 封裝事務管理方法的接口 - TransactionFactory 抽象事務工廠生產方法 - JdbcTransactionFactory實現TransactionFactory、用于生產JdbcTransaction的工廠類 - ManagedTransactionFactory實現TransactionFactory、用于生產ManagedTransaction的工廠類 - JdbcTransaction實現Transaction、只是對事務進行了一層包裝、實際調用數據庫連接Connection的事務管理方法 - ManagedTransaction 實現Transaction沒有對數據庫連接做任何事務處理、交由外部容器管理 ### 源碼事務 ### 事務配置 Mybatis中關于事務的配置是通過`<transaction type="xx"/>`來指定的。配置如下: ~~~ <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </dataSource> </environment> </environments> ~~~ - type為”JDBC”時、使用JdbcTransaction管理事務。 - type為”managed”時、使用ManagedTransaction管理事務(也就是交由外部容器管理) [Mybatis深入之初始化過程 ](http://blog.csdn.net/crave_shy/article/details/46013493 "Mybatis深入之初始化過程")中知道配置文件如何解析的、其中關于事務方面的解析: ~~~ private void environmentsElement(XNode context) throws Exception { //只關注事務部分... TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); ... } ~~~ ~~~ private TransactionFactory transactionManagerElement(XNode context) throws Exception { if (context != null) { String type = context.getStringAttribute("type"); Properties props = context.getChildrenAsProperties(); TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance(); factory.setProperties(props); return factory; } throw new BuilderException("Environment declaration requires a TransactionFactory."); } ~~~ ~~~ typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class); typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class); ~~~ - 重點在于根據type類型判斷實例化何種TransactionFactory - 前面已經知道Mybatis兩種事務配置的方式、這里使用的jdbc類型的事務 - 上一篇分析DataSource實例化過程中有一段是關于根據DataSource的type來獲取何種Factory的、這里原理同樣 - 通過TypeAliasRegistry根據type=’JDBC’來獲取TransactionFactory實現類JdbcTransactionFactory 關鍵在于JdbcTransactionFactory通過newInstance()使用無參構造函數時做了什么工作 ~~~ public class JdbcTransactionFactory implements TransactionFactory { public void setProperties(Properties props) { } public Transaction newTransaction(Connection conn) { return new JdbcTransaction(conn); } public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) { return new JdbcTransaction(ds, level, autoCommit); } } ~~~ - JdbcTransactionFactory默認無參構造方法被調用 - setProperties沒有做任何實質性處理 - 對比ManagedTransactionFactory不再貼代碼 下面就是獲取具有事務特性的數據庫連接了 JdbcTransaction: ~~~ public Transaction newTransaction(Connection conn) { return new JdbcTransaction(conn); } ~~~ ManagedTransaction: ~~~ public Transaction newTransaction(Connection conn) { return new ManagedTransaction(conn, closeConnection); } ~~~ - 兩者都是通過Connection來創建具體的實例 JdbcTransaction: ~~~ public class JdbcTransaction implements Transaction { private static final Log log = LogFactory.getLog(JdbcTransaction.class); protected Connection connection; protected DataSource dataSource; protected TransactionIsolationLevel level; protected boolean autoCommmit; public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) { dataSource = ds; level = desiredLevel; autoCommmit = desiredAutoCommit; } public JdbcTransaction(Connection connection) { this.connection = connection; } public Connection getConnection() throws SQLException { if (connection == null) { openConnection(); } return connection; } public void commit() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Committing JDBC Connection [" + connection + "]"); } connection.commit(); } } public void rollback() throws SQLException { if (connection != null && !connection.getAutoCommit()) { if (log.isDebugEnabled()) { log.debug("Rolling back JDBC Connection [" + connection + "]"); } connection.rollback(); } } public void close() throws SQLException { if (connection != null) { resetAutoCommit(); if (log.isDebugEnabled()) { log.debug("Closing JDBC Connection [" + connection + "]"); } connection.close(); } } protected void setDesiredAutoCommit(boolean desiredAutoCommit) { try { if (connection.getAutoCommit() != desiredAutoCommit) { if (log.isDebugEnabled()) { log.debug("Setting autocommit to " + desiredAutoCommit + " on JDBC Connection [" + connection + "]"); } connection.setAutoCommit(desiredAutoCommit); } } catch (SQLException e) { // Only a very poorly implemented driver would fail here, // and there's not much we can do about that. throw new TransactionException("Error configuring AutoCommit. " + "Your driver may not support getAutoCommit() or setAutoCommit(). " + "Requested setting: " + desiredAutoCommit + ". Cause: " + e, e); } } protected void resetAutoCommit() { try { if (!connection.getAutoCommit()) { // MyBatis does not call commit/rollback on a connection if just selects were performed. // Some databases start transactions with select statements // and they mandate a commit/rollback before closing the connection. // A workaround is setting the autocommit to true before closing the connection. // Sybase throws an exception here. if (log.isDebugEnabled()) { log.debug("Resetting autocommit to true on JDBC Connection [" + connection + "]"); } connection.setAutoCommit(true); } } catch (SQLException e) { log.debug("Error resetting autocommit to true " + "before closing the connection. Cause: " + e); } } 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); } } ~~~ - 從源碼中可知、JdbcTransaction如何管理事務的、如前面所說調用DataSource事務操作方法。 - 并且對select不進行事務控制 - 當使用DataSource創建數據庫連接時、數據庫的事務隔離級別使用DataSource默認的事務隔離級別 - 如需指定事務的隔離級別、必須手動創建JdbcTransaction(調用另一個構造函數) - 關于事務隔離級別會在補充中有 ManagedTransaction: ~~~ public class ManagedTransaction implements Transaction { private static final Log log = LogFactory.getLog(ManagedTransaction.class); private DataSource dataSource; private TransactionIsolationLevel level; private Connection connection; private boolean closeConnection; public ManagedTransaction(Connection connection, boolean closeConnection) { this.connection = connection; this.closeConnection = closeConnection; } public ManagedTransaction(DataSource ds, TransactionIsolationLevel level, boolean closeConnection) { this.dataSource = ds; this.level = level; this.closeConnection = closeConnection; } public Connection getConnection() throws SQLException { if (this.connection == null) { openConnection(); } return this.connection; } public void commit() throws SQLException { // Does nothing } public void rollback() throws SQLException { // Does nothing } public void close() throws SQLException { if (this.closeConnection && this.connection != null) { if (log.isDebugEnabled()) { log.debug("Closing JDBC Connection [" + this.connection + "]"); } this.connection.close(); } } protected void openConnection() throws SQLException { if (log.isDebugEnabled()) { log.debug("Opening JDBC Connection"); } this.connection = this.dataSource.getConnection(); if (this.level != null) { this.connection.setTransactionIsolation(this.level.getLevel()); } } } ~~~ - 重點看一下`commit()``rollback()`方法,沒有方法體。驗證前面其關于事務的管理方式 到這里事務暫時告一段落、一般在使用時會與spring結合、將數據庫連接、事務管理都交由spring管理。 ### 補充 數據庫隔離級別: 先對不同隔離級別涉及到的名詞解釋: ? 臟讀: 對于兩個事物 T1, T2, T1 讀取了已經被 T2 更新但還沒有被提交的字段. 之后, 若 T2 回滾, T1讀取的內容就是臨時且無效的. ? 不可重復讀: 對于兩個事物 T1, T2, T1 讀取了一個字段, 然后 T2 更新了該字段. 之后, T1再次讀取同一個字段, 值就不同了. ? 幻讀: 對于兩個事物 T1, T2, T1 從一個表中讀取了一個字段, 然后 T2 在該表中插入了一些新的行. 之后, 如果 T1 再次讀取同一個表, 就會多出幾 具體的隔離級別定義: READ UNCOMMITTED(讀未提交數據) 允許事務讀取未被其他事務提交的變更,臟讀、不可重復讀和幻讀的問題都會出現 READ COMMITED(讀已提交數據) 只允許事務讀取已經被其他事務提交的變更,可以避免臟讀,但不可重復讀和幻讀問題仍然會出現 REPEATABLE READ(可重復讀) 確保事務可以多次從一個字段中讀取相同的值,在這個事務持續期間,禁止其他事務對這個字段進行更新,可以避免臟讀和不可重復讀,但幻讀的問題依然存在 SERIALIZABLE(串行化) 確保事務可以從一個表中讀取相同的行,在這個事務持續期間,禁止其他事務對該表執行插入、更新和刪除操作,所有并發問題都可以避免,但性能十分低 Oracle 支持的 2 種事務隔離級別:READ COMMITED, SERIALIZABLE. Oracle 默認的事務隔離級別為: READ COMMITED Mysql 支持 4 中事務隔離級別. Mysql 默認的事務隔離級別為: REPEATABLE READ 更多內容:[Mybatis 目錄](http://blog.csdn.net/crave_shy/article/details/45825599)
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