延遲加載:
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,只有當前臺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:只有當前臺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-開啓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爲參數傳遞給從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.此處是隻發出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.當前臺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());//當前臺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.此處是隻發出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()+",");////此處是隻發出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()+",");//此處是隻發出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()+",");//此處是隻發出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: