iBATIS一對多/多對多N+1問題解決方案

對於iBATIS一對多/多對多的問題，傳統的辦法是在一對多/多對多關聯的屬性上再做一次子查詢，這個解決辦法很簡單易懂，但是有個缺點，會導致N+1 selects，導致查詢的性能瓶頸，更好的解決辦法是sql做一個表連接，然後主表的resultMap配置上加“groupBy='...'”屬性，這樣一次查詢就搞定，避免了N+1問題，下面請看代碼：

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE sqlMap PUBLIC "-//iBATIS.com//DTD SQL Map 2.0//EN"

"http://www.ibatis.com/dtd/sql-map-2.dtd">
<sqlMap namespace="sys">
    <typeAlias alias="menu" type="com.cuishen.sqlmaptest.vo.Menu" />
    <resultMap id="sub-menu-map" class="menu">
        <result property="id" column="subid" />
        <result property="name" column="subname" />
        <result property="url" column="suburl" />
        <result property="layer" column="sublayer" />
        <result property="grade" column="subgrade" />
        <result property="position" column="subposition" />
        <result property="parentId" column="subparentid" />
    </resultMap>
    <resultMap id="menu-map" class="menu" groupBy="id">
        <result property="id" column="id" />
        <result property="name" column="name" />
        <result property="url" column="url" />
        <result property="layer" column="layer" />
        <result property="grade" column="grade" />
        <result property="position" column="position" />
        <result property="parentId" column="parentid" />
        <result property="subMenus" resultMap="sys.sub-menu-map" />
    </resultMap>

    <select id="getTopMenu" parameterClass="java.util.HashMap" resultMap="sys.menu-map">
    select
    t.id as id,t.name as name,t.url as url,t.layer as layer,t.grade as grade,t.position as position,
    t.parentid as parentid,s.id as subid,s.name as subname,s.url as suburl,s.layer as sublayer,
    s.grade as subgrade,s.position as subposition,s.parentid as subparentid
    from menu t left join menu s
    on t.id = s.parentid
    where t.layer=1 order by t.position, s.position
    </select>
</sqlMap>

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE sqlMap PUBLIC "-//iBATIS.com//DTD SQL Map 2.0//EN" 

"http://www.ibatis.com/dtd/sql-map-2.dtd">
<sqlMap namespace="sys">
	<typeAlias alias="menu" type="com.cuishen.sqlmaptest.vo.Menu" />
	<resultMap id="sub-menu-map" class="menu">
		<result property="id" column="subid" />
		<result property="name" column="subname" />
		<result property="url" column="suburl" />
		<result property="layer" column="sublayer" />
		<result property="grade" column="subgrade" />
		<result property="position" column="subposition" />
		<result property="parentId" column="subparentid" />
	</resultMap>
	<resultMap id="menu-map" class="menu" groupBy="id">
		<result property="id" column="id" />
		<result property="name" column="name" />
		<result property="url" column="url" />
		<result property="layer" column="layer" />
		<result property="grade" column="grade" />
		<result property="position" column="position" />
		<result property="parentId" column="parentid" />
		<result property="subMenus" resultMap="sys.sub-menu-map" />
	</resultMap>

	<select id="getTopMenu" parameterClass="java.util.HashMap" resultMap="sys.menu-map">
	select 
	t.id as id,t.name as name,t.url as url,t.layer as layer,t.grade as grade,t.position as position,
	t.parentid as parentid,s.id as subid,s.name as subname,s.url as suburl,s.layer as sublayer,
	s.grade as subgrade,s.position as subposition,s.parentid as subparentid
	from menu t left join menu s
	on t.id = s.parentid
	where t.layer=1 order by t.position, s.position
	</select>
</sqlMap>

com.cuishen.sqlmaptest.vo.Menu如下：

package com.cuishen.sqlmaptest.vo;

import java.util.List;

/**
 * POJO - 菜單
 * @author cuishen
 */
public class Menu implements java.io.Serializable {
    private static final long serialVersionUID = 7172793340860021199L;

    private Long id;
    private String name;
    private String url;
    private Short layer;
    private Short grade;
    private Short position;
    private Long parentId;
    private List subMenus;

    public Menu() { }

    public Long getId() {
        return this.id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    //...
    //getter、setter方法
    //...

    public List getSubMenus() {
        return subMenus;
    }

    public void setSubMenus(List subMenus) {
        this.subMenus = subMenus;
    }
}

package com.cuishen.sqlmaptest.vo;

import java.util.List;

/**
 * POJO - 菜單
 * @author cuishen
 */
public class Menu implements java.io.Serializable {
	private static final long serialVersionUID = 7172793340860021199L;

	private Long id;
	private String name;
	private String url;
	private Short layer;
	private Short grade;
	private Short position;
	private Long parentId;
	private List subMenus;

	public Menu() { }

	public Long getId() {
		return this.id;
	}

	public void setId(Long id) {
		this.id = id;
	}

	//...
	//getter、setter方法
	//...

	public List getSubMenus() {
		return subMenus;
	}

	public void setSubMenus(List subMenus) {
		this.subMenus = subMenus;
	}
}

測試類如下：

package com.cuishen.sqlmaptest;

import java.sql.SQLException;
import java.util.List;

import com.ibatis.sqlmap.client.SqlMapClient;

import com.cuishen.sqlmaptest.vo.Menu;

/**
 * iBATIS 1:N & M:N（N+1 selects）解決方案
 * @author cuishen
 */
public class Test {
    public static void main(String args[]) throws SQLException {
        SqlMapClient sqlMapClient = SqlMapClientFactory.getSqlMapClient();
        List topMenus = sqlMapClient.queryForList("getTopMenu", null);
        for(int i = 0; i < topMenus.size(); i++) {
            Menu menu = (Menu)topMenus.get(i);
            List subMenus = (List)menu.getSubMenus();
            System.out.println("top menu name >> " + menu.getName());
            for(int j = 0; j < subMenus.size(); j++) {
                Menu subMenu = (Menu)subMenus.get(j);
                System.out.println(">> sub menu name >> " + subMenu.getName());
            }
        }
    }
}

package com.cuishen.sqlmaptest;

import java.sql.SQLException;
import java.util.List;

import com.ibatis.sqlmap.client.SqlMapClient;

import com.cuishen.sqlmaptest.vo.Menu;

/**
 * iBATIS 1:N & M:N（N+1 selects）解決方案
 * @author cuishen
 */
public class Test {
	public static void main(String args[]) throws SQLException {
		SqlMapClient sqlMapClient = SqlMapClientFactory.getSqlMapClient();
		List topMenus = sqlMapClient.queryForList("getTopMenu", null);
		for(int i = 0; i < topMenus.size(); i++) {
			Menu menu = (Menu)topMenus.get(i);
			List subMenus = (List)menu.getSubMenus();
			System.out.println("top menu name >> " + menu.getName());
			for(int j = 0; j < subMenus.size(); j++) {
				Menu subMenu = (Menu)subMenus.get(j);
				System.out.println(">> sub menu name >> " + subMenu.getName());				
			}
		}
	}
}

是不是很簡單，但是有三點需要注意：

1. 注意這個配置：<result property="subMenus" resultMap="sys.sub-menu-map" />，resultMap是sys.sub-menu-map，要加命名空間sys，而不是sub-menu-map，否則iBATIS可能會報錯！

2. 一對多/多對多映射的集合屬性subMenus必須用簡單的getter/setter，不能在getter/setter裏做其他處理，否則會導致iBATIS報錯！！

3. 在iBATIS2.3.0.677上測試通過，更老舊的版本可能不支持該解決方案！！


從iBATIS3.0開始該解決方案的配置改爲：

<resultMap id="blogResult" type="Blog">
    <id property=”id” column="blog_id" />
    <result property="title" column="blog_title"/>
    <collection property="posts" ofType="Post">
        <id property="id" column="post_id"/>
        <result property="subject" column="post_subject"/>
        <result property="body" column="post_body"/>
    </collection>
</resultMap>