10.4 MyBatis事务管理 10.4.3事务的配置创建和使用

1.事务的配置

我们在使用MyBatis时,一般会在MyBatis的根配置文件mybatis-config.xml中定义类似如下的信息:

<environment id="mysql">
    <transactionManager type="JDBC"/>
    <dataSource type="pooled">
        <property
            name="driver"
            value="${driver}"/>
        <property
            name="url"
            value="${url}"/>
        <property
            name="username"
            value="${username}"/>
        <property
            name="password"
            value="${password}"/>
    </dataSource>
</environment>

environment标签定义了连接某个数据库的信息,其子标签transactionManager的type属性决定我们用什么类型的事务管理机制。

如何查看mybatis源码

mybatis的源码放在github上了,所以到github上即可查看mybatis的源码.地址为:
https://github.com/mybatis/mybatis-3/tree/master/src/main/java/org/apache/ibatis

2.事务工厂的创建

MyBatis事务的创建是交给org.apache.ibatis.transaction.TransactionFactory事务工厂来完成的。如果我们将transactionManager标签的type属性配置为JDBC,那么,在MyBatis初始化解析environment标签时,会根据type="JDBC“创建一个JdbcTransactionFactory工厂.

• 如果type="JDBC",则MyBatis会创建一个JdbcTransactionFactory的实例;
• 如果type="MANAGED",则MyBatis会创建一个MangedTransactionFactory的实例。

3.事务工厂TransactionFactory

通过事务工厂TransactionFactory很容易获取到Transaction对象实例。我们以JdbcTransaction为例,看一下JdbcTransactionFactory是怎样生成JdbcTransaction的。
JdbcTransaction源代码如下:
如下

public class JdbcTransactionFactory implements TransactionFactory {
  @Override
  public void setProperties(Properties props) {
  }
  //根据给定的数据库连接Connection创建Transaction
  @Override
  public Transaction newTransaction(Connection conn) {
    return new JdbcTransaction(conn);
  }
  //根据DateSource,隔离级别,是否自动提交创建Transacion
  @Override
  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit);
  }
}

如上代码所示,JdbcTransactionFactory会创建JDBC类型的Transaction,即JdbcTransaction。类似地,ManagedTransactionFactory也会创建ManagedTransaction。下面我们分别深入解析JdbcTransaction和ManagedTransaction,看它们到底是怎样实现事务管理的.

4.JdbcTransaction

JdbcTransaction可直接使用JDBC的提交和回滚事务管理机制。它依赖于从dataSource中取得的连接connection来管理transaction的作用域,connection对象的获取被延迟到调用getConnection()方法时。如果将autocomt设置为on,开启状态的话,则它会忽略commit和rollback.
也就是说,JdbcTransaction是使用java.sql.Connection上的commit和rollback功能来完成事务操作的,JdbcTransaction只是相当于对java.sql.Connection事务处理进行了再次封装,Transaction的事务管理都是通过java.sql.Connection实现的JdbcTransaction的源代码实现如下:

public class JdbcTransaction implements Transaction {
  private static final Log log = LogFactory.getLog(JdbcTransaction.class);
  // 数据库连接
  protected Connection connection;
  // 数据源
  protected DataSource dataSource;
  // 隔离级别
  protected TransactionIsolationLevel level;
  // 是否自动提交
  protected boolean autoCommit;
  public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
    dataSource = ds;
    level = desiredLevel;
    autoCommit = desiredAutoCommit;
  }
  public JdbcTransaction(Connection connection) {
    this.connection = connection;
  }
  @Override
  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }
  //使用Connection的commit()
  @Override
  public void commit() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Committing JDBC Connection [" + connection + "]");
      }
      connection.commit();
    }
  }
  //使用Connection的rollback();
  @Override
  public void rollback() throws SQLException {
    if (connection != null && !connection.getAutoCommit()) {
      if (log.isDebugEnabled()) {
        log.debug("Rolling back JDBC Connection [" + connection + "]");
      }
      connection.rollback();
    }
  }
  //使用Connection的close()
  @Override
  public void close() throws SQLException {
    if (connection != null) {
      resetAutoCommit();
      if (log.isDebugEnabled()) {
        log.debug("Closing JDBC Connection [" + connection + "]");
      }
      connection.close();
    }
  }
  // 省略部分代码.
}

从JdbcTransaction类的源代码可以看出,JdbcTransaction就是使用java.sql.Connection上的commit、rollback功能来完成事务操作的.

5.ManagedTransaction

ManagedTransaction让容器来管理事务Transaction的整个生命周期,意思就是说,使用ManagedTransaction的commit和rollback功能不会对事务有任何的影响,它什么都不会做,它将事务管理的权力移交给了容器。ManagedTransaction的源代码如下:

public class ManagedTransaction implements Transaction {
  private static final Log log = LogFactory.getLog(ManagedTransaction.class);
  private DataSource dataSource;
  private TransactionIsolationLevel level;
  private Connection connection;
  private final 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) {
      openConnection();
    }
    return this.connection;
  }
  @Override
  public void commit() throws SQLException {
    // Does nothing
  }
  @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");
    }
    this.connection = this.dataSource.getConnection();
    if (this.level != null) {
      this.connection.setTransactionIsolation(this.level.getLevel());
    }
  }
  @Override
  public Integer getTimeout() throws SQLException {
    return null;
  }
}

从ManagedTransaction类的源代码可以看出,提交和回滚时它什么都没有做,也就是说,当使用ManagedTransaction时MyBatis的事务是交给容器来操作管理的.