MyBatis之多表之间的联系与缓存

多表之间的联系：

注：本文章的演示都是在上篇文章项目的基础上进行的，所以有的代码变动不大的就不再重复贴了，以免太过冗余。点明一点：每写一个映射文件都要配置在mybatis-config.xml 的 中

一对多：查询哪些人有哪些车

Demo3.java

 package cn.hncu.demo;

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

 import org.apache.ibatis.session.SqlSession;
 import org.junit.Test;

 import cn.hncu.domain.Card;
 import cn.hncu.domain.Person;
 import cn.hncu.domain.Role;
 import cn.hncu.domain.User;
 import cn.hncu.utils.SqlSessionUtils;

 //演示表与表之间的关系：一对一，一对多和多对多
 public class Demo3 {

     //一对多：一个人(person)多辆车(car)
     @Test
     public void test1() throws SQLException{
         //查询哪些人有哪些车-----innor join
         SqlSession s=SqlSessionUtils.getSqlSession();
         List persons=s.selectList("persons.person1");
         s.close();
         for(Person person:persons)
         System.out.println("person: "+person);
     }

Person.java

 package cn.hncu.domain;

 import java.util.ArrayList;
 import java.util.List;

 //一方
 public class Person {
     private String id;
     private String name;

     //建一个集合表示多方
     private List cars=new ArrayList();

     //为了实现表之间关系中的“一对一”，在此必须添加一个对方的值对象
     private Card card;
     public String getId() {
         return id;
     }
     public void setId(String id) {
         this.id = id;
     }
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public List getCars() {
         return cars;
     }
     public void setCars(List cars) {
         this.cars = cars;
     }

     public Card getCard() {
         return card;
     }
     public void setCard(Card card) {
         this.card = card;
     }
     @Override
     public String toString() {
         return "Person [id=" + id + ", name=" + name + ", cars=" + cars + "]";
     }

 }

Car.java

 package cn.hncu.domain;

 //多方
 public class Car {
     private String id;
     private String name;
     private Double price;

     //声明一个值对象表示一方
     private Person person;

     public String getId() {
         return id;
     }
     public void setId(String id) {
         this.id = id;
     }
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public Double getPrice() {
         return price;
     }
     public void setPrice(Double price) {
         this.price = price;
     }
     public Person getPerson() {
         return person;
     }
     public void setPerson(Person person) {
         this.person = person;
     }
     @Override
     public String toString() {
         return "Car [id=" + id + ", name=" + name + ", price=" + price + "]";
     }

 }

Person.xml

 xml version="1.0" encoding="UTF-8" ?>

 "http://mybatis.org/dtd/mybatis-3-mapper.dtd">


 <mapper namespace="persons">



     <resultMap type="cn.hncu.domain.Person" id="ps">
         <id property="id" column="pid"/>
         <result property="name" column="pname"/>


         <collection property="cars" javaType="cn.hncu.domain.Car">
             <id property="id" column="cid"/>
             <result property="name" column="cname" javaType="string" jdbcType="VARCHAR"/>
             <result property="price" column="cprice" javaType="_double" jdbcType="NUMERIC"/>
         collection>

     resultMap>
     <select id="person1" resultMap="ps">
         select p.pid as pid,p.pname as pname,c.id as cid,c.name as cname,c.price as cprice
         from person p inner join car c on p.pid =c.pid
     select>

数据库查询结果：

 

控制台查询结果：

 

一对多：查询每个人的车辆信息-----left join

 @Test
     public void test2() throws SQLException{
         //查询每个人的车辆信息-----left join
         SqlSession s=SqlSessionUtils.getSqlSession();
         List persons=s.selectList("persons.person2");
         s.close();
         for(Person person:persons)
             System.out.println("person: "+person);
     }

 

     <select id="person2" resultMap="ps2">
         select pid,pname from person
     select>
     <resultMap type="cn.hncu.domain.Person" id="ps2">
         <id property="id" column="pid"/>
         <result property="name" column="pname"/>
         <collection property="cars" column="pid" select="cars1">collection>
     resultMap>
     <select id="cars1" resultType="cn.hncu.domain.Car" parameterType="string">
         select * from car where pid=#{value}
     select>

数据库：

控制台：

 

一对一：一个人对应一张身份证

 @Test
     public void test3() throws SQLException{
         //查询每个人的车辆信息-----left join
         SqlSession s=SqlSessionUtils.getSqlSession();
         List cards=s.selectList("persons.card1");
         s.close();
         for(Card card:cards)
             System.out.println("card: "+card);
     }

 

     <select id="card1" resultMap="c1">
         select c.card_id as id,c.card_gov as gov,p.pid as pid,p.pname as pname
         from cards c inner join person p on c.pid=p.pid
     select>
     <resultMap type="cn.hncu.domain.Card" id="c1">

         <constructor>
             <idArg column="id" javaType="string" jdbcType="VARCHAR"/>
         constructor>
         <result property="gov" column="gov" javaType="string" jdbcType="VARCHAR"/>
         <association property="person"  javaType="cn.hncu.domain.Person">
             <result property="id" column="pid"/>
             <result property="name" column="pname"/>
             <collection property="cars" select="cars1" column="pid">collection>
         association>
     resultMap>
 mapper>

数据库：

 

控制台：

 

多对多：人---角色的关系

 //多对多：人---角色的关系    开发步骤：1.建数据库表  2.写值对象(体现表之间关系)  3.写调用的java代码(在业务流程不熟悉的情况下，可先从需求下手) 4.写映射文件
     @Test
     public void test4() throws SQLException{
         //查询哪些人有哪些角色-----inner join
         SqlSession s=SqlSessionUtils.getSqlSession();
         List users=s.selectList("roles.user");
         s.close();
         for(User user:users)
             System.out.println("user: "+user);
         for(User u:users){
             String name=u.getName();
             Set roles=u.getRoles();
             String r="";
             for(Role role:roles){
                 r+=role.getName()+",";
             }
             System.out.println(name+","+r);
         }
     }

Role.xml

 xml version="1.0" encoding="UTF-8" ?>

 "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 <mapper namespace="roles">

     <cache/>


     <select id="user" resultMap="us">
     select u.id as id , u.name as name , u.pwd as pwd , r.id as rid , r.name as rname
         from users u inner join userrole ur on u.id=ur.uid inner join roles r on ur.rid=r.id
     select>
     <resultMap type="cn.hncu.domain.User" id="us">
     <id property="id" column="id"/>
     <result property="name" column="name" javaType="string" jdbcType="VARCHAR"/>
     <result property="pwd" column="pwd"/>
     <collection property="roles" javaType="cn.hncu.domain.Role">
         <id property="id" column="rid"/>
         <result property="name" column="rname"/>
     collection>
     resultMap>

 

两个值对象：

User.java

 package cn.hncu.domain;

 import java.util.HashSet;
 import java.util.Set;

 public class User {
     private String id;
     private String name;
     private String pwd;

     //专为多对多建立一个  保存对方的集合--用List或Set都可以
     private Set roles=new HashSet();
     public String getId() {
         return id;
     }
     public void setId(String id) {
         this.id = id;
     }
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }
     public String getPwd() {
         return pwd;
     }
     public void setPwd(String pwd) {
         this.pwd = pwd;
     }

     public Set getRoles() {
         return roles;
     }
     public void setRoles(Set roles) {
         this.roles = roles;
     }
     @Override
     public String toString() {
         return "User [id=" + id + ", name=" + name + ", pwd=" + pwd
                 + ", roles=" + roles + "]";
     }

 }

Role.java

 package cn.hncu.domain;

 import java.io.Serializable;
 import java.util.ArrayList;
 import java.util.List;

 public class Role implements Serializable{
     private String id;
     private String name;

     //专为多对多建立一个保存对方的集合 -----用list或set 都可以
     private List users=new ArrayList();

     public String getId() {
         return id;
     }

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

     public String getName() {
         return name;
     }

     public void setName(String name) {
         this.name = name;
     }

     public List getUsers() {
         return users;
     }

     public void setUsers(List users) {
         this.users = users;
     }

     @Override
     public String toString() {
         return "Role [id=" + id + ", name=" + name + "]";
     }

 }

数据库：

 

控制台：

 

mybatis缓存技术演示

 //※※mybatis缓存技术演示：
     //※※注意：被放入缓存的值对象必须实现serizable接口
     @Test
     public void caseDemo() throws SQLException{

         //同一个session用的是一级缓存
         SqlSession ss=SqlSessionUtils.getSqlSession();
         System.out.println("ss: "+ss);
         Role role=ss.selectOne("roles.cacheDemo","R001");//查单
         System.out.println(role.hashCode());
         Role role2=ss.selectOne("roles.cacheDemo","R001");//查单
         System.out.println(role2.hashCode());
         ss.close();

         System.out.println("----------------------------");
         //第二个session
         SqlSession ss2=SqlSessionUtils.getSqlSession();
         System.out.println("ss2: "+ss2);
         Role role22=ss2.selectOne("roles.cacheDemo","R001");//查单
         System.out.println(role22.hashCode());
         Role role222=ss2.selectOne("roles.cacheDemo","R001");//查单
         System.out.println(role222.hashCode());

     }
 }

     <select id="cacheDemo" useCache="false" resultType="cn.hncu.domain.Role" parameterType="string">
         select * from roles where id=#{value}
     select>
 mapper>

不带缓存：

 

带缓存：

 

这里对缓存再个人总结一下：

 1.  缓存有一级和二级缓存。SqlSession自己带有一级缓存，所有同一个缓存得到的对象的hash地址是相同的，不同的session拿到的对象hash地址不同。

 2.  mybatis(hibernate也一样)有二级缓存。mytbatis中默认情况下是没有开启缓存的，当不同session查询时，都要去数据库中去查询，效率降低。

   当开启mybatis缓存时，session第一次查询时没有缓存，会到数据库中查询，并将查询结果放到mybatis二级缓存和本session缓存中，当再存查询时，会到本session缓存池中寻找，找到即返回结果。

    所以带缓存的图中，两次的hashCode值是一样的。    而当第二个session去查询时，到mybatis的二级缓存池中找到结果，就会把结果克隆一份结果返回，并且不把结果放进本session缓存池中，再次查询时，依然到mybatis的二级缓存池中中克隆结果。所以两次结果的hashCode值不同。

 3.  只有session到数据库中查询，才会把查询结果放进session的缓存池中。

 

贴出一下我用mysql建的表格的代码，以供日后参考：

 ALTER TABLE car
     ADD CONSTRAINT stud_fk FOREIGN KEY(pid) REFERENCES person(pid);
 INSERT INTO person(pid,pname) VALUES("P001","Jack");
 INSERT INTO person(pid,pname) VALUES("P002","Rose");
 INSERT INTO person(pid,pname) VALUES("P003","Tom");
 INSERT INTO car(id,NAME,price,pid) VALUES("C001","Benz",100,"P001");
 INSERT INTO car(id,NAME,price,pid) VALUES("C002","BMW",150,"P001");
 INSERT INTO car(id,NAME,price,pid) VALUES("C003","QQ",10,"P003");
 DROP TABLE person;
 DROP TABLE car;
 CREATE TABLE car(
 id VARCHAR(32) PRIMARY KEY,
 NAME VARCHAR(128),
 price NUMERIC,
 pid VARCHAR(32)
 );
 SELECT pid,pname FROM person


 SELECT p.pid AS pid,p.pname AS pname,c.id AS cid,c.name AS cname,c.price AS cprice
         FROM person p INNER JOIN car c ON p.pid =c.pid

 SELECT p.pid AS pid,p.pname AS pname,c.id AS cid,c.name AS cname,c.price AS cprice
         FROM person p LEFT JOIN car c ON p.pid =c.pid


 CREATE TABLE cards(
 card_id VARCHAR(32) PRIMARY KEY,
 card_gov VARCHAR(128),
 pid VARCHAR(32)
 )
 ALTER TABLE cards
     ADD CONSTRAINT fk_pid FOREIGN KEY (pid) REFERENCES person(pid);

 INSERT INTO cards(card_id,card_gov,pid) VALUES("C001","湖南长沙","P001");
 INSERT INTO cards(card_id,card_gov,pid) VALUES("C002","湖南益阳","P002");
 INSERT INTO cards(card_id,card_gov,pid) VALUES("C003","北京朝阳","P003");

 SELECT c.card_id AS id,c.card_gov AS gov,p.pid AS pid,p.pname AS pname
         FROM cards c INNER JOIN person p ON c.pid=p.pid

 CREATE TABLE roles(
 id VARCHAR(32) PRIMARY KEY,
 NAME VARCHAR(128)
 )
 INSERT INTO roles(id,NAME) VALUES('R001','教师');
 INSERT INTO roles(id,NAME) VALUES('R002','学生');
 INSERT INTO roles(id,NAME) VALUES('R003','后勤');
 INSERT INTO roles(id,NAME) VALUES('R004','暂无');
 CREATE TABLE userRole(
 uid VARCHAR(32),
 rid VARCHAR(32)
 )
 ALTER TABLE userrole
     ADD CONSTRAINT fk_uid FOREIGN KEY (uid) REFERENCES users(id);
 ALTER TABLE userrole
     ADD CONSTRAINT fk_rid FOREIGN KEY (rid) REFERENCES roles(id);

 INSERT INTO userrole(uid,rid) VALUES('1',"R002");
 INSERT INTO userrole(uid,rid) VALUES('1',"R003");
 INSERT INTO userrole(uid,rid) VALUES('2',"R002");
 INSERT INTO userrole(uid,rid) VALUES('3',"R004");
 INSERT INTO userrole(uid,rid) VALUES('4',"R001");

 SELECT u.id AS id , u.name AS NAME , u.pwd AS pwd , r.id AS rid , r.name AS rname
 FROM users u INNER JOIN userrole ur ON u.id=ur.uid INNER JOIN roles r ON ur.rid=r.id