IBatis: Discriminator Column Example Inheritance Mapping(Ibatis中的继承映射)

继承映射介绍-iBATIS supports inheritance hierarchies by using a special mapping called a discriminator. Using a discriminator you can determine the type of class to be instantiated based on a value in the database. The discriminator is a part of the ResultMap and works much like a switch statement.

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问题：对于Manager和Developer来说，他们都是Employee类的子类，且都有自己特殊的属性，Manager有managerId和info属性，developer有developerId和project属性。在使用IBATIS查询一批Employee类时，如何通过自动映射通过判断Employee_type取得他们各自的属性？

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?1、pojo beans

2、sqlMap

?3、test code
public class TestSqlMap {  private static SqlMapClient sqlMapClient = null;  @Before  public void loadFile() {    String resource = "sqlMap/sql-map-config.xml";    try {      Reader reader = Resources.getResourceAsReader(resource);      sqlMapClient = SqlMapClientBuilder.buildSqlMapClient(reader);      reader.close();    } catch (FileNotFoundException e) {      // TODO Auto-generated catch block      e.printStackTrace();    } catch (IOException e) {      // TODO Auto-generated catch block      e.printStackTrace();    }  }    @Test  public void testSql(){    try {      List<Employee> list =  sqlMapClient.queryForList("getAllEmployees",null);                for (Employee employee : list){        System.out.println(employee.toString());      }            assertEquals(5, list.size());    } catch (SQLException e) {      // TODO Auto-generated catch block      e.printStackTrace();    }  }
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4、test result：
Employee ID = 1 Name = Kate Manager ID = 1 Info = info Kate
Employee ID = 4 Name = James Manager ID = 2 Info = info James
Employee ID = 5 Name = Susan Developer ID = 3 Project = web
Employee ID = 2 Name = Josh Developer ID = 1 Project = web
Employee ID = 3 Name = Peter Developer ID = 2 Project = desktop
修改sqlMap，去掉managerMap的extends="resultEmployee"和developerMap的extends="resultEmployee"?:
Employee ID = 0 Name = null Manager ID = 1 Info = info Kate
Employee ID = 0 Name = null Manager ID = 2 Info = info James
Employee ID = 0 Name = null Developer ID = 3 Project = web
Employee ID = 0 Name = null Developer ID = 1 Project = web
Employee ID = 0 Name = null Developer ID = 2 Project = desktop
5、在以上的例子中，子映射（sub map）对应这标准的结果映射，如果discriminator 找到了一个值去匹配subMap ，则父结果集的映射将失效，参加第四点的修改后的输出，除非子映射明确的扩展了父映射（extends）。
如果discriminator 找不到值去匹配subMap，则使用父结果映射:
Employee ID = 1 Name = Kate
Employee ID = 4 Name = James
Employee ID = 5 Name = Susan
Employee ID = 2 Name = Josh
Employee ID = 3 Name = Peter
    1 楼    learnworld    2012-04-08              不错，很清晰！