lazy策略原理:只有在使用查詢sql返回的數據是才真正發出sql語句到數據庫,否則不發出(主要用在多表的聯合查詢)
1.一對一延遲加載:
假設數據庫中有person表和card表:其中person表中有字段pid,pname,page,psex,cid,card表中有字段cid,cnum;
假設要查詢某個人的姓名和身份證號碼:
原理:在查詢姓名時,實際本沒有查詢出身份證號碼的信息,只有當前台使用身份證號時才發出對card的查詢,需要查詢出身份證號碼是採取查詢的一種策略;
實現實例:
實現步驟:
1-導入mybatis 的依賴jar包
2-添加log4j文件(可查看內存中實際執行的程序)
1-原理:只有當前台使用身份證號時才發出對card的查詢,否則只發出person信息的查詢
2-開啟lazy:在conf.xml
<settings> <setting name="lazyLoadingEnabled" value="true"/> <setting name="aggressiveLazyLoading" value="false"/> </settings>
3.實現:
(1)在mapper.xml映射文件中:
<select id="findCid" parameterType="int" resultType="card"> select * from card where cid=#{value} </select> <resultMap type="person" id="p_c1"> <id column="pid" property="pid" /> <result column="pname" property="pname" /> <result column="page" property="page" /> <result column="psex" property="psex" /> <association property="card" javaType="card" select="findCid" column="cid"> </association> </resultMap> <select id="selectpersonAndCardLazyByPid" parameterType="int" resultMap="p_c1"> SELECT * FROM person where pid=#{value} </select> 1-select:指定關聯的查詢語句
2-column:指定主語句中的哪個字段的值作為參數傳遞給從sql語句
(2)在mapper接口中定義方法:
public Person selectpersonAndCardLazyByPid(int pid);
(3)使用junit測試結果:
1.此處是只發出person信息的查詢;
@Test public void testselectpersonAndCardLazyByPid(){//lazy策略一對1 Person p=pm.selectpersonAndCardLazyByPid(1); //System.out.println(p); System.out.println(p.getPname()+","); //System.out.println(p.getPname()+","+p.getCard().getCnum()); }結果執行的查詢語句:
2.當前台使用身份證號時才發出對card的查詢
@Test public void testselectpersonAndCardLazyByPid(){//lazy策略一對1 Person p=pm.selectpersonAndCardLazyByPid(1); //System.out.println(p); System.out.println(p.getPname()+","); System.out.println(p.getPname()+","+p.getCard().getCnum());//當前台使用身份證號時才發出對card的查詢}結果執行的查詢語句:
2.一對多延遲加載:
實現實例:
假設數據庫中有person表和card身份信息表,adder地址表:其中person表中有字段pid,pname,page,psex,cid,card表中有字段cid,cnum;adder表有字段aid,ashi,pid
假設要查詢某個人的姓名和住址,身份證號碼:
(1)mapper.xml映射文件:
<!-- lazy策略一對多--> <select id="fingCard_Adder" parameterType="int" resultType="adder"> select * from adder where pid=#{value} </select> <select id="findCid1" parameterType="int" resultType="card"> select * from card where cid=#{value} </select> <resultMap type="person" id="p_c1_a1"> <id column="pid" property="pid" /> <result column="pname" property="pname" /> <result column="page" property="page" /> <result column="psex" property="psex" /> <association property="card" javaType="card" select="findCid1" column="cid"> </association> <collection property="adder" ofType="Adder" select="fingCard_Adder" column="pid"> </collection> </resultMap> <select id="selectpersonAndCardAndAdderLazyByPid" parameterType="int" resultMap="p_c1_a1"> SELECT * FROM person where pid=#{value} </select>(2)mapper接口定義方法:
1.此處是只發出person信息的查詢;
@Test public void testselectpersonAndCardAndAdderLazyByPid(){//lazy策略一對多Person p=pm.selectpersonAndCardAndAdderLazyByPid(1); System.out.println(p.getPname()+",");////此處是只發出person信息的查詢}結果執行的查詢語句:
2.此處是發出person信息和身份信息的查詢;
@Test public void testselectpersonAndCardAndAdderLazyByPid(){//lazy策略一對多Person p=pm.selectpersonAndCardAndAdderLazyByPid(1); System.out.println(p.getPname()+",");//此處是只發出person信息的查詢; System.out.println(p.getPname()+","+p.getCard().getCnum());//此處是發出person信息和身份信息的查詢; }結果執行的查詢語句:
3.此處發出person信息和身份信息,地址信息的查詢;
@Test public void testselectpersonAndCardAndAdderLazyByPid(){//lazy策略一對多Person p=pm.selectpersonAndCardAndAdderLazyByPid(1); System.out.println(p.getPname()+",");//此處是只發出person信息的查詢; System.out.println(p.getPname()+","+p.getCard().getCnum());//此處是發出person信息和身份信息的查詢; //System.out.println(p.getPname()+","+p.getCard().getCnum()); for (Adder adder : p.getAdder()) {////此處發出person信息和身份信息,地址信息的查詢; System.out.println(adder.getAshi()); } }結果執行的查詢語句:
總結
以上所述是小編給大家介紹的mybatis中延遲加載Lazy策略的方法,希望對大家有所幫助,如果大家有任何疑問歡迎給我留言,小編會及時回復大家的!