一對多

public class Type {
	String id;
	String name;
	List<User> users;

	@Select("select * from user where typeid = #{typeid}")
	public List<User> findUserById(String typeid);
	
	@Results({
			@Result(property="id",column="id"),
			//users對映List<User> users，
 
[email protected]是呼叫關聯查詢方法，"id"是關聯查詢條件，FetchType.LAZY是延遲載入
			@Result(property="users",column="id", [email protected](select="hello.dao.MybatisDao.findUserById",fetchType=FetchType.LAZY))
	})
	@Select("select * from type where id=#{id}")
	public Type findTypeById(String id);

		Type type = mybatisDao.findTypeById("1");
		System.out.println("延遲載入");
		type.getUsers();

其它關聯

一對一：把上面的[email protected]換成[email protected]，其他原理是一樣的
多對多：把多個欄位對映成[email protected]，就是多對多了

JAVA註解的侷限性

xml無限層巢狀對映

這裡以三層巢狀為例，以實現前端的三級選單樹。這是一個tree表，pid是其上級選單的id。

public class Tree {
	String id;
	String name;
	List<Tree> child;

@Mapper
public interface TreeDao {
	@ResultMap("tree")
	@Select("SELECT p1.id,p1.name,p2.id id2,p2.name name2,p3.id id3,p3.name name3 "
			+ "FROM tree p1,tree p2,tree p3 WHERE p1.id=p2.pid AND p2.id=p3.pid")
	public List findTree();  

#指定xml對映檔案的路徑
mybatis.mapper-locations=classpath:hello/mapper/*

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!-- 對映dao -->
<mapper namespace="hello.dao.TreeDao">
  <!-- 結果集型別 -->
  <resultMap id="tree" type="hello.pojo.Tree">
    <!-- 對映欄位 -->
    <result column="id"  property="id" />
    <result column="name"  property="name" />
    <!-- 巢狀第二張表 -->
    <collection  property="child" ofType="hello.pojo.Tree" >  
        <id column="id2" property="id" />
	<result column="name2" property="name" />
	<!-- 巢狀第三張表 -->
	<collection  property="child" ofType="hello.pojo.Tree" >  
        	<id column="id3" property="id" />
		<result column="name3" property="name" />
        </collection>  	 
     </collection>  	
  </resultMap>
</mapper>

[
  {
    "id": "1",
    "name": "一級樹",
    "child": [
      {
        "id": "11",
        "name": "二級樹-1",
        "child": [
          {
            "id": "112",
            "name": "三級樹-1",
            "child": null
          },
          {
            "id": "113",
            "name": "三級樹-2",
            "child": null
          }
        ]
      }
    ]
  }
]

使用JAVA註解還是XML