spring 1.1.3 JDBC编程式事务管理探析(二):如何实现TransactionTemplate.execute的回调接口TransactionCal

 一、TransactionCallback接口

Object doInTransaction(TransactionStatus status)

TransactionTemplate已经搭好了事务框架，我们只需将多条SQL语句放在doInTransaction中执行就可以了。但是SQL语句通过什么方法来执行呢?

二、方法一:获取Connection来执行

获取Connection又有两种方法

1.从TransactionStatus中索得,因为TransactionStatus含有Transaction对象,后者保存了Connection的句柄

private
 
static
 Connection getConnection(TransactionStatus ts) {
  JdbcTransactionObjectSupport transObj = (JdbcTransactionObjectSupport)((DefaultTransactionStatus)ts).getTransaction();
  ConnectionHolder holder = transObj.getConnectionHolder();
  Connection con = holder.getConnection();
  return
 con;
}

2.从TransactionSynchronizationManager的线程变量中获取

private
 Connection getConnection() {
  DataSourceTransactionManager manager = (DataSourceTransactionManager)transactionTemplate.getTransactionManager();
  DataSource ds = manager.getDataSource();
  return
 DataSourceUtils.getConnection(ds);
//
}

这里贴出DataSourceUtils.getConnection(ds)的部分代码:

public
 
static
 Connection getConnection(DataSource dataSource, 
boolean
 allowSynchronization)
  throws
 CannotGetJdbcConnectionException {
  try
 {
    return
 doGetConnection(dataSource, allowSynchronization);
  }
  catch
 (SQLException ex) {
    throw
 
new
 CannotGetJdbcConnectionException(
"Could not get JDBC connection"
, ex);
  }
}

protected
 
static
 Connection doGetConnection(DataSource dataSource, 
boolean
 allowSynchronization)
  throws
 SQLException {
  ...
  ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(dataSource);
  if
 (conHolder != 
null
) {
    return
 conHolder.getConnection();
  }
  ...
}

获取了Connection之后,就可以按照常规的方法来操作SQL语句了

public
 
void
 addEmployee1(Employee employee) 
throws
 SQLException {
  transactionTemplate.execute(new
 TransactionCallback() {
    public
 Object doInTransaction(TransactionStatus ts) {
      try
 {
        //Connection con = getConnection(ts);
        Connection con = getConnection();
        Statement st = con.createStatement();
        st.executeUpdate("INSERT INTO Employee (username,age) VALUES('lizi', 22)"
);
        st.executeUpdate("INSERT INTO Employee (username,age) VALUES('lijun',55)"
);
        st.close();

      } catch
 (Exception ex) {
        System.out.println("更新失败"
);
        ts.setRollbackOnly();
      }
      return
 
null
;
    }
  });
}

三、方法二:通过JdbcTemplate来操作
1.创建JdbcTemplate:

DataSourceTransactionManager manager = (DataSourceTransactionManager)transactionTemplate.getTransactionManager();
DataSource ds = manager.getDataSource();
JdbcTemplate jdbcTemplate = new
 JdbcTemplate(ds);

2.创建之后,执行:

public
 
void
 addEmployee(Employee employee) 
throws
 SQLException {
  transactionTemplate.execute(new
 TransactionCallback() {
    public
 Object doInTransaction(TransactionStatus ts) {
      try
 {
        jdbcTemplate.update("INSERT INTO Employee (username,age) VALUES(?, ?)"
,
          new
 Object[]{
"lizi"
,
new
 Integer(
22
)});
        jdbcTemplate.update("INSERT INTO Employee (username,age) VALUES(?, ?)"
,
          new
 Object[]{
"lijun"
,
new
 Integer(
55
)});
        System.out.println("更新成功"
);

      } catch
 (Exception ex) {
        System.out.println("更新失败"
);
        ts.setRollbackOnly();
      }
      return
 
null
;
    }
  });
}

3.问题

• jdbcTemplate的Connection肯定是从DataSource获取的,但这个Connection是否和TransactionTemplate的事务框架中的已建立的连接是同一个?
• jdbcTemplate自已的框架也是try{..}catch{..}finally{..}，每一个jdbcTemplate.update方法都在这个框架中,会不会执行完后将所用的连接关闭呢?如果关闭,后续方法如何采用同一个连接?

4.jdbcTemplate的连接来源:

Connection con = DataSourceUtils.getConnection(getDataSource());

是通过DataSourceUtils的getConnection方法来获取连接的，也就是TransactionSynchronizationManager的线程变量,与事务框架中的连接是同一个.

5.jdbcTemplate的finally会把连接关闭吗?

finally中的代码是:

DataSourceUtils.closeConnectionIfNecessary(con, getDataSource());

看看DataSourceUtils

public
 
static
 
void
 closeConnectionIfNecessary(Connection con, DataSource dataSource) {
  try
 {
    doCloseConnectionIfNecessary(con, dataSource);
  }
  catch
 (SQLException ex) {
    logger.error("Could not close JDBC connection"
, ex);
  }
}

protected
 
static
 
void
 doCloseConnectionIfNecessary(Connection con, DataSource ds) 
throws
 SQLException {
  ...
  ConnectionHolder conHolder = (ConnectionHolder) TransactionSynchronizationManager.getResource(ds);
  if
 (conHolder != 
null
 && con == conHolder.getConnection()) {
    //如果是事务中的连接,不需要关闭
    return
;
  }

  //非事务连接则关闭
  if
 (!(ds 
instanceof
 SmartDataSource) || ((SmartDataSource) ds).shouldClose(con)) {
    logger.debug("Closing JDBC connection"
);
    con.close();
  }
}