ibatis的selectKey学习

背景：
      在使用ibatis插入数据进数据库的时候，会用到一些sequence的数据，有些情况下，在插入完成之后还需要将sequence的值返回，然后才能进行下一步的操作。
      使用ibatis的selectKey就可以得到sequence的值，同时也会将值返回。不过对于不同的数据库有不同的操作方式。
      对于oracle：
      <insert id="insertUser" parameterClass="ibatis.User">
           <selectKey resultClass="long" keyProperty="id">
              select SEQ_USER_ID.nextval as id from dual
          </selectKey>
           insert into user
          (id,name,password)
          values
          (#id#,#name#,#password#)
      </insert>
       该句话执行完之后，传进来的参数User对象DO里的id字段就会被赋值成sequence的值。

     对于mysql
     <insert id="insertUser" parameterClass="ibatis.User">
          insert into user
          (name,password)
          values
          (#name#,#password#)
           <selectKey resultClass="long" keyProperty="id">  
             SELECT LAST_INSERT_ID() AS ID  
        </selectKey>  
      </insert>
       将selectKey放在insert之后，通过LAST_INSERT_ID()  获得刚插入的自动增长的id的值。


附上ibatis源码：

//-- Basic Methods  
  /** 
   * Call an insert statement by ID 
   * 
   * @param sessionScope - the session 
   * @param id      - the statement ID 
   * @param param   - the parameter object 
   * @return - the generated key (or null) 
   * @throws SQLException - if the insert fails 
   */  
  public Object insert(SessionScope sessionScope, String id, Object param) throws SQLException {  
    Object generatedKey = null;  
  
    MappedStatement ms = getMappedStatement(id);  
    Transaction trans = getTransaction(sessionScope);  
    boolean autoStart = trans == null;  
  
    try {  
       //开始事务  
      trans = autoStartTransaction(sessionScope, autoStart, trans);  
  
      SelectKeyStatement selectKeyStatement = null;  
      if (ms instanceof InsertStatement) {  
        selectKeyStatement = ((InsertStatement) ms).getSelectKeyStatement();  
      }  
  
      // Here we get the old value for the key property. We'll want it later if for some reason the  
      // insert fails.  
      Object oldKeyValue = null;  
      String keyProperty = null;  
      boolean resetKeyValueOnFailure = false;  
      if (selectKeyStatement != null && !selectKeyStatement.isRunAfterSQL()) {  
        keyProperty = selectKeyStatement.getKeyProperty();  
        oldKeyValue = PROBE.getObject(param, keyProperty);  
        generatedKey = executeSelectKey(sessionScope, trans, ms, param);  
        resetKeyValueOnFailure = true;  
      }  
  
      StatementScope statementScope = beginStatementScope(sessionScope, ms);  
      try {  
        ms.executeUpdate(statementScope, trans, param);  
      }catch (SQLException e){  
        // uh-oh, the insert failed, so if we set the reset flag earlier, we'll put the old value  
        // back...  
        if(resetKeyValueOnFailure) PROBE.setObject(param, keyProperty, oldKeyValue);  
        // ...and still throw the exception.  
        throw e;  
      } finally {  
        endStatementScope(statementScope);  
      }  
  
      if (selectKeyStatement != null && selectKeyStatement.isRunAfterSQL()) {  
        generatedKey = executeSelectKey(sessionScope, trans, ms, param);  
      }  
      //提交事务  
      autoCommitTransaction(sessionScope, autoStart);  
    } finally {  
    //结束事务  
      autoEndTransaction(sessionScope, autoStart);  
    }  
  
    return generatedKey;  
  }  

转载地址：http://aliahhqcheng.iteye.com/blog/1415885