Understand the basics of Mybatis

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##mybatis

mybatis-config.xml detailed configuration (when configuring, you must delete the redundant attributes and cannot have Chinese characters, otherwise an error will be reported!)

<?xml  version="1.0" encoding="UTF-8" ?>nbsp;configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd"><!--configuration核心配置  配置文件的根元素 --><configuration>
    <!-- 属性：定义配置外在化 -->
    <properties></properties>
    <!-- 设置：定义mybatis的一些全局性设置 -->
    <settings>
        <!-- 具体的参数名和参数值 -->
        <setting></setting>
    </settings>
    <!-- 类型名称：为一些类定义别名 -->
    <typealiases>
        <!-- 实体类少 建议 第一种取别名方式-->
        <typealias></typealias>
        <!--实体类多 建议  第二种取别名方式
        默认情况下用这种方式 别名为类名 首字母最好小写
        -->
        <package></package>
    </typealiases>
    <!-- 类型处理器：定义Java类型与数据库中的数据类型之间的转换关系 -->
    <typehandlers></typehandlers>
    <!-- 对象工厂 -->
    <objectfactory></objectfactory>
    <!-- 插件：mybatis的插件,插件可以修改mybatis的内部运行规则 -->
    <plugins>
        <plugin></plugin>
    </plugins>
    <!-- 环境：配置mybatis的环境 -->
    <environments>
        <!-- 环境变量：可以配置多个环境变量，比如使用多数据源时，就需要配置多个环境变量 -->
        <environment>
            <!-- 事务管理器 -->
            <transactionmanager></transactionmanager>
            <!-- 数据源 配置连接我的数据库-->
            <datasource>
                <property></property>
                <property></property>
                <property></property>
                <property></property>
            </datasource>
        </environment>
    </environments>
    <!-- 数据库厂商标识 -->
    <databaseidprovider></databaseidprovider>
    <!-- 映射器：指定映射文件或者映射类 -->
    <mappers>
        <mapper></mapper>
    </mappers></configuration>

Paging

Reduce the amount of data access

limt implements paging
sql statement: select * from table name limt 0,5;

0: The starting position of the data
• 5: Data length

The first type: using Mybatis 1 interface

  List<user> getUserByLimit(Map<string> map);</string></user>

2mapeer.xml

   <select>
        select *
        from mybatis.user
        limit ${starIndex},${pageSize}    </select>

2-1 Result set mapping

<resultmap>
        <result></result>
    </resultmap>

3 Test

 @Test
    public void getUserByLimitTest() {
        SqlSession sqlSession = MyBatisUtils.getSqlSession ();
        UserMapper mapper = sqlSession.getMapper (UserMapper.class);
        HashMap hashMap = new HashMap<string> ();
        hashMap.put ("starIndex", 1);
        hashMap.put ("pageSize", 2);
        List userByLimit = mapper.getUserByLimit (hashMap);
        for (Object o : userByLimit) {
            System.out.println (o);
        }

        sqlSession.close ();
    }</string>

Second: Use RowBounds method 1. Interface
List getUserList();
2. Implement the interface

<select>
        select *
        from mybatis.user    </select>

3. Test:

 /**
     * 测试使用RowBounds实现分页
     */@Test
    public void getUserByLimitRowBoundsTest() {
        SqlSession sqlSession = MyBatisUtils.getSqlSession ();
        RowBounds rowBounds = new RowBounds (0, 2);
        List<user> userList = sqlSession.selectList ("com.kuang.w.dao.UserMapper.getUserList", null, rowBounds);
        for (User user : userList) {
            System.out.println (user);
        }
        //关闭
        sqlSession.close ();
    }</user>

Third method: Use Mybatis’ paging plug-in pageHeIper

sql Many-to-one processing

Database:

pojo The teacher-table table in the database corresponds to the entity class Teacher

package com.kuang.w.pojo;

import lombok.Data;

/**
 * @author W
 */
@Data
public class Teacher {
    private int tId;
    private String tName;

}

The user table in the database corresponds to the entity class Student

package com.kuang.w.pojo;import lombok.Data;/**
 * @author W
 */@Datapublic class Student {
    private int id;
    private int tid;
    private String name;
    private String password;
    private Teacher teacher;}

1.Interface

   List<student> getStudentList();</student>

2.xml configuration implementation interface

  <!-- 多对一查询
    1 子查询 mysql 通过一个表里是数据   与另一个表的一个数据相的情况下 查询另一个的数据 一起显示
  -->
    <select>
        select *
        from mybatis.user;    </select>
    <resultmap>
        <!--  复杂属性 对象用 ：association 集合用：collection-->
        <!--column 数据库中的字段 property 实体类中的属性-->
        <result></result>
        <result></result>
        <result></result>
        <!--javaType	一个 Java 类的全限定名
        ，或一个类型别名（关于内置的类型别名，可以参考上面的表格）。
        如果你映射到一个 JavaBean，MyBatis 通常可以推断类型。
        然而，如果你映射到的是 HashMap，
        那么你应该明确地指定 javaType 来保证行为与期望的相一致。-->
        <association></association>
    </resultmap>
    <select>
        select *
        from mybatis.teacher_table
        where tid = #{id};    </select>

 <!--2 多表联查-->
    <select>
        select u.id       uid,
               u.name     uname,
               u.password upassword,
               u.tid      utid,
               t.tname
        from mybatis.user u,
             mybatis.teacher_table t
        where t.tid = u.tid;    </select>
     <!-- 映射-->
    <resultmap>
        <result></result>
        <result></result>
        <result></result>
        <result></result>
        <association>
            <result></result>
        </association>
    </resultmap>

mybatis-config.xm configuration

<?xml  version="1.0" encoding="UTF8" ?>nbsp;configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration>
    <properties></properties>
    <settings>
        <setting></setting>
    </settings>
    <typealiases>
        <typealias></typealias>
        <typealias></typealias>
    </typealiases>

    <environments>
        <environment>
            <transactionmanager></transactionmanager>
            <datasource>
                <property></property>
                <property></property>
                <property></property>
                <property></property>
            </datasource>
        </environment>
    </environments>
    <mappers>
        <!--   <mapper resource="com/kuang/w/dao/TeacherMapper.xml"></mapper>
           <mapper resource="com/kuang/w/dao/StudentMapper.xml"></mapper>-->
        <mapper></mapper>
        <mapper></mapper>
    </mappers></configuration>

3 Test

 @Test
    public void getStudentListTest() {
        SqlSession sqlSession = MyBatisUtils.getSqlSession ();
        StudentMapper mapper = sqlSession.getMapper (StudentMapper.class);


        List<student> studentList = mapper.getStudentList ();
        for (Student student : studentList) {
            System.out.println (student);
        }

        sqlSession.commit ();
        sqlSession.close ();
    }</student>

sql one-to-many processing

The entity class corresponding to the data table structure remains unchanged

First way: Multi-table joint query

1 interface

    List<teacher> getTeacher(int tid);</teacher>

2.1 xml implementation interface

  <select>
        select t.tid, t.tname, u.id, u.name, u.password
        from mybatis.user u,
             mybatis.teacher_table t
        where t.tid = u.tid
          and t.tid = #{tid};    </select>

2.2 Mapping configuration

<resultmap>
        <result></result>
        <result></result>
        <!--  复杂属性 对象用 ：association 集合用：collection-->
        <collection>
            <!--javaType 指定属性类型 一个 Java 类的全限定名-->
            <result></result>
            <result></result>
            <result></result>
            <result></result>
        </collection>
    </resultmap>

3Test

 /*测试一对多*/
    @Test
    public void getTeacherTest2() {
        SqlSession sqlSession = MyBatisUtils.getSqlSession ();
        TeacherMapper mapper = sqlSession.getMapper (TeacherMapper.class);
        List<teacher> teacher = mapper.getTeacher (1);
        for (Teacher teacher1 : teacher) {
            System.out.println (teacher1);
        }

		//提交事务   架子  这里可以不要
        sqlSession.commit ();
        // 关闭
        sqlSession.close ();
    }</teacher>

Result

com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit4 com.kuang.w.dao.myTest,getTeacherTest2
Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.PooledDataSource forcefully closed/removed all connections.PooledDataSource forcefully closed/removed all connections.PooledDataSource forcefully closed/removed all connections.PooledDataSource forcefully closed/removed all connections.Opening JDBC Connection
Created connection 164974746.Setting autocommit to false on JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@9d5509a]==>  Preparing: select t.tid, t.tname, u.id, u.name, u.password from mybatis.user u, mybatis.teacher_table t where t.tid = u.tid and t.tid = ?; ==> Parameters: 1(Integer)Second way: Subquery
 1Interface
    List<teacher> getTeacher(int tid);</teacher>
2 Implement interface
 <!--第二种方式： 子查询-->
    <select>
        select *
        from mybatis.teacher_table
        where tid = #{tid};    </select>
    <resultmap>
        <!--  复杂属性 对象用 ：association 集合用：collection
        我们需要单独处理对象: association 集合: collection
        javaType=""指定属性的类型!
        集合中的泛型信息，我们使用ofType 获取
        -->
        <result></result>
        <result></result>
        <collection>
        </collection>
    </resultmap>
    <select>
        select *
        from mybatis.user
        where tid = #{tid};    </select>
3Test the same as above
. . . . 
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