lazy策略原理:只有在使用查询sql返回的数据是才真正发出sql语句到数据库,否则不发出(主要用在多表的联合查询)
1.一对一延迟加载:
假设数据库中有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;
假设要查询某个人的姓名和身份证号码:
原理:在查询姓名时,实际本没有查询出身份证号码的信息,只有当前台使用身份证号时才发出对card的查询,需要查询出身份证号码是采取查询的一种策略;
实现实例:
实现步骤:
1-导入mybatis 的依赖jar包
2-添加log4j文件 (可查看内存中实际执行的程序)
1-原理:只有当前台使用身份证号时才发出对card的查询,否则只发出person信息的查询
2-开启lazy:在conf.xml
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<settings>
<setting name= "lazyloadingenabled" value= "true" />
<setting name= "aggressivelazyloading" value= "false" />
</settings>
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3.实现:
(1)在mapper.xml映射文件中:
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<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>
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1-select:指定关联的查询语句
2-column:指定主语句中的哪个字段的值作为参数传递给从sql语句
(2)在mapper接口中定义方法:
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public person selectpersonandcardlazybypid( int pid);
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(3)使用junit测试结果:
1.此处是只发出person信息的查询;
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@test
public void testselectpersonandcardlazybypid(){ //lazy策略一对1
person p=pm.selectpersonandcardlazybypid( 1 );
//system.out.println(p);
system.out.println(p.getpname()+ "," );
//system.out.println(p.getpname()+","+p.getcard().getcnum());
}
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结果执行的查询语句:
2.当前台使用身份证号时才发出对card的查询
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@test
public void testselectpersonandcardlazybypid(){ //lazy策略一对1
person p=pm.selectpersonandcardlazybypid( 1 );
//system.out.println(p);
system.out.println(p.getpname()+ "," );
system.out.println(p.getpname()+ "," +p.getcard().getcnum()); //当前台使用身份证号时才发出对card的查询
}
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结果执行的查询语句:
2.一对多延迟加载:
实现实例:
假设数据库中有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
假设要查询某个人的姓名和住址,身份证号码:
(1)mapper.xml映射文件:
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<!-- lazy策略一对多 -->
<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>
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(2)mapper接口定义方法:
1.此处是只发出person信息的查询;
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@test
public void testselectpersonandcardandadderlazybypid(){ //lazy策略一对多
person p=pm.selectpersonandcardandadderlazybypid( 1 );
system.out.println(p.getpname()+ "," ); ////此处是只发出person信息的查询
}
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结果执行的查询语句:
2.此处是发出person信息和身份信息的查询;
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@test
public void testselectpersonandcardandadderlazybypid(){ //lazy策略一对多
person p=pm.selectpersonandcardandadderlazybypid( 1 );
system.out.println(p.getpname()+ "," ); //此处是只发出person信息的查询;
system.out.println(p.getpname()+ "," +p.getcard().getcnum()); //此处是发出person信息和身份信息的查询;
}
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结果执行的查询语句:
3.此处发出person信息和身份信息,地址信息的查询;
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@test
public void testselectpersonandcardandadderlazybypid(){ //lazy策略一对多
person p=pm.selectpersonandcardandadderlazybypid( 1 );
system.out.println(p.getpname()+ "," ); //此处是只发出person信息的查询;
system.out.println(p.getpname()+ "," +p.getcard().getcnum()); //此处是发出person信息和身份信息的查询;
//system.out.println(p.getpname()+","+p.getcard().getcnum());
for (adder adder : p.getadder()) { ////此处发出person信息和身份信息,地址信息的查询;
system.out.println(adder.getashi());
}
}
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结果执行的查询语句:
总结
以上所述是小编给大家介绍的mybatis中延迟加载lazy策略的方法,希望对大家有所帮助,如果大家有任何疑问欢迎给我留言,小编会及时回复大家的!
原文链接:https://blog.csdn.net/m0_37905429/article/details/77074312