ibatis中selectKey的用法与源码

最近用到插入oracle数据库时，主键id是sequnce自动生成，具体写法如下
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>
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>

上面是两种数据库的实现。

下面我们看看源码是如何做的
//-- 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;
      //对于oracle适用于执行insert之前执行selectKey获取id值
      if (selectKeyStatement != null && !selectKeyStatement.isRunAfterSQL()) {
        keyProperty = selectKeyStatement.getKeyProperty();
        oldKeyValue = PROBE.getObject(param, keyProperty);
        //sequnce获取id的值
        generatedKey = executeSelectKey(sessionScope, trans, ms, param);
        resetKeyValueOnFailure = true;
      }

      StatementScope statementScope = beginStatementScope(sessionScope, ms);
      try {
        //执行sql语句
        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);
      }
      //对于mysql适用于执行完insert后，获取id值
      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;
  }