Mybatis事务（一）事务管理方式(JDBC和MANAGED)

Mybatis管理事务是分为两种方式:

(1)使用JDBC的事务管理机制,就是利用java.sql.Connection对象完成对事务的提交

(2)使用MANAGED的事务管理机制，这种机制mybatis自身不会去实现事务管理，而是让程序的容器（JBOSS,WebLogic）来实现对事务的管理

在Mybatis的配置文件中可以配置事务管理方式如下：

 xml version="1.0" encoding="UTF-8"?>
  >
  <configuration>
      <environments default="development">
           <environment id="development">

               <transactionManager type="JDBC" />

               <dataSource type="POOLED">
                   <property name="driver" value="com.mysql.jdbc.Driver" />
                  <property name="url" value="jdbc:mysql://localhost:3306/mybatis" />
                  <property name="username" value="root" />
                  <property name="password" value="XDP" />
              dataSource>
          environment>
      environments>
  configuration>

Mybatis提供了一个事务接口Transaction，以及两个实现类jdbcTransaction和ManagedTransaction，当spring与Mybatis一起使用时，spring提供了一个实现类SpringManagedTransaction
Transaction接口：提供的抽象方法有获取数据库连接getConnection，提交事务commit，回滚事务rollback和关闭连接close，源码如下：

 //事务接口
 ublic interface Transaction {
  /**
   * Retrieve inner database connection
   * @return DataBase connection
   * @throws SQLException
   */
   //获得数据库连接
  Connection getConnection() throws SQLException;
  /**
   * 提交
   * Commit inner database connection.
   * @throws SQLException
   */
  void commit() throws SQLException;
  /**
   * 回滚
   * Rollback inner database connection.
   * @throws SQLException
   */
  void rollback() throws SQLException;
  /**
   * 关闭连接
   * Close inner database connection.
   * @throws SQLException
   */
  void close() throws SQLException;

1)JdbcTransaction实现类：Transaction的实现类，通过使用jdbc提供的方式来管理事务，通过Connection提供的事务管理方法来进行事务管理，源码如下：

 public class JdbcTransaction implements Transaction {

   private static final Log log = LogFactory.getLog(JdbcTransaction.class);

   /* 连接**/
   protected Connection connection;

   /* 数据源**/
   protected DataSource dataSource;

   /* 事务等级**/
   protected TransactionIsolationLevel level;

   /* 事务提交**/
   protected boolean autoCommmit;

   public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
     dataSource = ds;
     level = desiredLevel;
     autoCommmit = desiredAutoCommit;
   }

   public JdbcTransaction(Connection connection) {
     this.connection = connection;
   }

   @Override
   public Connection getConnection() throws SQLException {
     if (connection == null) {
       openConnection();
     }
     //返回连接
     return connection;
   }

   @Override
   public void commit() throws SQLException {
     if (connection != null && !connection.getAutoCommit()) {
       if (log.isDebugEnabled()) {
         log.debug("Committing JDBC Connection [" + connection + "]");
       }
       //连接提交
       connection.commit();
     }
   }

   @Override
   public void rollback() throws SQLException {
     if (connection != null && !connection.getAutoCommit()) {
       if (log.isDebugEnabled()) {
         log.debug("Rolling back JDBC Connection [" + connection + "]");
       }
       //连接回滚
       connection.rollback();
     }
   }

   @Override
   public void close() throws SQLException {
     if (connection != null) {
       resetAutoCommit();
       if (log.isDebugEnabled()) {
         log.debug("Closing JDBC Connection [" + connection + "]");
       }
       //关闭连接
       connection.close();
     }
   }

   protected void setDesiredAutoCommit(boolean desiredAutoCommit) {
     try {
       //事务提交状态不一致时修改
       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. select操作没有commit和rollback事务
         // Some databases start transactions with select statements 一些数据库在select操作是会开启事务
         // 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) {
       if (log.isDebugEnabled()) {
         log.debug("Error resetting autocommit to true "
           + "before closing the connection.  Cause: " + e);
       }
     }
   }
   //打开连接
   protected void openConnection() throws SQLException {
     if (log.isDebugEnabled()) {
       log.debug("Opening JDBC Connection");
     }
     //从数据源中获得连接
     connection = dataSource.getConnection();
     if (level != null) {
       connection.setTransactionIsolation(level.getLevel());
     }
     setDesiredAutoCommit(autoCommmit);
   }

 }

2)ManagedTransaction实现类：通过容器来进行事务管理，所有它对事务提交和回滚并不会做任何操作，源码如下：

 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;
   }
   //数据源，事务等级及是否关闭事务
   public ManagedTransaction(DataSource ds, TransactionIsolationLevel level, boolean closeConnection) {
     this.dataSource = ds;
     this.level = level;
     this.closeConnection = closeConnection;
   }

   @Override
   public Connection getConnection() throws SQLException {
     if (this.connection == null) {
       openConnection();
     }
     return this.connection;
   }
   //提交操作无效
   @Override
   public void commit() throws SQLException {
     // Does nothing
   }
   //回滚操作无效
   @Override
   public void rollback() throws SQLException {
     // Does nothing
   }

   @Override
   public void close() throws SQLException {
     if (this.closeConnection && this.connection != null) {
       if (log.isDebugEnabled()) {
         log.debug("Closing JDBC Connection [" + this.connection + "]");
       }
       //关闭连接
       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());
     }
   }

 }

注意：如果我们使用MyBatis构建本地程序，即不是WEB程序，若将type设置成"MANAGED"，那么，我们执行的任何update操作，即使我们最后执行了commit操作，数据也不会保留，不会对数据库造成任何影响。因为我们将MyBatis配置成了“MANAGED”，即MyBatis自己不管理事务，而我们又是运行的本地程序，没有事务管理功能，所以对数据库的update操作都是无效的。

3)SpringManagedTransaction实现类：它其实也是通过使用JDBC来进行事务管理的，当spring的事务管理有效时，不需要操作commit/rollback/close，spring事务管理会自动帮我们完成，源码如下：

 public class SpringManagedTransaction implements Transaction {

   private static final Log LOGGER = LogFactory.getLog(SpringManagedTransaction.class);

   private final DataSource dataSource;

   private Connection connection;

   private boolean isConnectionTransactional;

   private boolean autoCommit;

   //获得数据源
   public SpringManagedTransaction(DataSource dataSource) {
     notNull(dataSource, "No DataSource specified");
     this.dataSource = dataSource;
   }

   /**
    * {@inheritDoc}
    * 返回数据库连接
    */
   @Override
   public Connection getConnection() throws SQLException {
     if (this.connection == null) {
       openConnection();
     }
     return this.connection;
   }

   /**
    * Gets a connection from Spring transaction manager and discovers if this
    * {@code Transaction} should manage connection or let it to Spring.
    *
    * It also reads autocommit setting because when using Spring Transaction MyBatis
    * thinks that autocommit is always false and will always call commit/rollback
    * so we need to no-op that calls.
    *从spring的事务管理中获得一个连接
    */
   private void openConnection() throws SQLException {
     this.connection = DataSourceUtils.getConnection(this.dataSource);
     this.autoCommit = this.connection.getAutoCommit();
     this.isConnectionTransactional = DataSourceUtils.isConnectionTransactional(this.connection, this.dataSource);

     if (LOGGER.isDebugEnabled()) {
       LOGGER.debug(
           "JDBC Connection ["
               + this.connection
               + "] will"
               + (this.isConnectionTransactional ? " " : " not ")
               + "be managed by Spring");
     }
   }

   /**
    * {@inheritDoc}
    */
   @Override
   public void commit() throws SQLException {
     if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
       if (LOGGER.isDebugEnabled()) {
         LOGGER.debug("Committing JDBC Connection [" + this.connection + "]");
       }
       this.connection.commit();
     }
   }

   /**
    * {@inheritDoc}
    */
   @Override
   public void rollback() throws SQLException {
     if (this.connection != null && !this.isConnectionTransactional && !this.autoCommit) {
       if (LOGGER.isDebugEnabled()) {
         LOGGER.debug("Rolling back JDBC Connection [" + this.connection + "]");
       }
       this.connection.rollback();
     }
   }

   /**
    * {@inheritDoc}
    */
   @Override
   public void close() throws SQLException {
     DataSourceUtils.releaseConnection(this.connection, this.dataSource);
   }

 }

Mybatis的事务管理机制还是比较简单的，其并没有做过多的操作，只是封装一下方便别人调用而已。