Mybatis源码分析之（八）Mybatis事务管理机制

事务指作为单个逻辑工作单元执行的一系列操作，要么完全地执行，要么完全地不执行。对于数据库操作来说是必不可少的。

在Mybatis框架中有二种事务实现，一种是JdbcTransaction,另一种是ManagedTransaction。他们的配置都是通过environment中的transactionManager 标签来配置的。加载过程在
Mybatis源码分析之（二）根据配置文件创建SqlSessionFactory（Configuration的创建过程）中，欢迎大家移步去看看。

    <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>

JdbcTransaction

public class JdbcTransaction implements Transaction {

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

  //与数据库的连接
  protected Connection connection;
  //数据源信息
  protected DataSource dataSource;
  //表示事务级别
  protected TransactionIsolationLevel level;
  //是否自动提交
  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();
    }
    return connection;
  }

  @Override
  public void commit() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Committing JDBC Connection [" + connection + "]");
      }
      //最终使用的是connection的commit
      connection.commit();
    }
  }

  @Override
  public void rollback() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Rolling back JDBC Connection [" + connection + "]");
      }
      //最终使用的是connection的rollback
      connection.rollback();
    }
  }

  @Override
  public void close() throws SQLException {
    if (connection != null) {
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      //最终使用的是connection的close
      connection.close();
    }
  }

}

也就是说JdbcTransaction其实只是在jdbc的connection上面封装了一下，实际使用的其实还是jdbc的事务。、

ManagedTransaction

public class ManagedTransaction implements Transaction {

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

   //数据源信息
  private DataSource dataSource;
  //表示事务级别
  private TransactionIsolationLevel level;
  //与数据库的连接
  private Connection connection;
  //关闭链接的标志符
  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;
  }

  @Override
  public Connection getConnection() throws SQLException {
    if (this.connection == null) {
    //打开链接
      openConnection();
    }
    return this.connection;
  }
//commit不做任何处理
  @Override
  public void commit() throws SQLException {
    // Does nothing
  }

//rollback不做任何处理
  @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 + "]");
      }
      this.connection.close();
    }
  }
  protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    //通过dataSource获取链接
    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
      this.connection.setTransactionIsolation(this.level.getLevel());
    }
  }
}

Mybatis不再控制会话的事务，而是让容器来管理transaction的整个生命周期 。
因为ManagedTransaction的commit和rollback没有任何实现，若你是单用了Mybatis一个框架，那么使用ManagedTransaction作为事务实现，那么当我们无法将一个数据库操作过程，作为一个事务提交上去。因为ManagedTransaction里commit实现为空。