Ibatis之3个不常用的Query方法

1.queryForObject

  /**
   * Executes a mapped SQL SELECT statement that returns data to populate
   * the supplied result object.
   * <p/>
   * The parameter object is generally used to supply the input
   * data for the WHERE clause parameter(s) of the SELECT statement.
   *
   * @param id              The name of the statement to execute.
   * @param parameterObject The parameter object (e.g. JavaBean, Map, XML etc.).
   * @param resultObject    The result object instance that should be populated with result data.
   * @return The single result object as supplied by the resultObject parameter, populated with the result set data,
   *         or null if no result was found
   * @throws java.sql.SQLException If more than one result was found, or if any other error occurs.
   */
  Object queryForObject(String id, Object parameterObject, Object resultObject) throws SQLException;

当查询对象是一个重量级对象、创建过程比较复杂时或者查询对象没有默认的构造方法时，通过该方法，可以在外部先构建好查询对象，然后传给Ibatis，Ibatis此时不会创建新对象，而是调用传入对象的set方法进行赋值。

2.queryForList

  /**
   * Executes a mapped SQL SELECT statement that returns data to populate
   * a number of result objects within a certain range.
   * <p/>
   * This overload assumes no parameter is needed.
   *
   * @param id              The name of the statement to execute.
   * @param skip            The number of results to ignore.
   * @param max             The maximum number of results to return.
   * @return A List of result objects.
   * @throws java.sql.SQLException If an error occurs.
   */
  List queryForList(String id, int skip, int max) throws SQLException;

利用这个方法可以实现分页功能，如（skip=0,max=10）返回前10条数据，(skip=10,max=10)返回第10-20条数据，但这个方法的分页效率非常低，因为Ibatis是把所有的查询结果查询出来之后才进行筛选操作。数据量小的时候用用还可以，所以这个方法比较鸡肋。

3.queryForMap

/**
   * Executes a mapped SQL SELECT statement that returns data to populate
   * a number of result objects that will be keyed into a Map.
   * <p/>
   * The parameter object is generally used to supply the input
   * data for the WHERE clause parameter(s) of the SELECT statement.
   *
   * @param id              The name of the statement to execute.
   * @param parameterObject The parameter object (e.g. JavaBean, Map, XML etc.).
   * @param keyProp         The property to be used as the key in the Map.
   * @return A Map keyed by keyProp with values being the result object instance.
   * @throws java.sql.SQLException If an error occurs.
   */
  Map queryForMap(String id, Object parameterObject, String keyProp) throws SQLException;

网上有不少帖子说这个方法只能返回一条记录是不对的，还有说是把resultClass的所有属性放到一个map中返回来也是不对的。这个方法是对queryForList的一个补充，大部分情况下我们用的都是queryForList返回对象的列表，但有时候放到Map里用起来可能更方便，如果没有这个方法还得自己进行转换，同样的一个<select ...>配置，不用做任何更改即可以用queryForList访问也可以用queryForMap访问。