前言

Mybatis算是对数据库操作的利器了。但是在处理分页的时候，Mybatis并没有什么特别的方法，一般需要自己去写limit子句实现，成本较高。好在有国内开发者写了一个PageHelper插件，可以帮助我们快速实现分页查询。

官网地址

首先创建一个Maven项目

导入相关依赖：

<!-- 依赖列表-->
    <dependencies>
<!-- mysql驱动-->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.25</version>
        </dependency>
<!-- 单元测试-->
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13</version>
            <scope>test</scope>
        </dependency>
<!-- pageHelper依赖-->
        <dependency>
            <groupId>com.github.pagehelper</groupId>
            <artifactId>pagehelper</artifactId>
            <version>5.2.1</version>
        </dependency>
<!-- mybatis依赖-->
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.7</version>
        </dependency>
<!-- log4j日志依赖-->
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
    </dependencies>

记住导入PageHelper依赖：

<dependency>
	<groupId>com.github.pagehelper</groupId>
	<artifactId>pagehelper</artifactId>
	<version>5.2.1</version>
</dependency>

数据库中创建一张表

设置Mybatis配置文件

在resources目录下,创建一个sqlMapConfig.xml文件,用于做一些全局的mybatis基本的配置,例如数据库连接信息,事务,mapper映射文件路径,日志实现,类的别名,也包含了分页插件的基本配置。

<configuration>
<!--使用外部属性配置文件 resource:指定类路径下的某个属性配置文件-->
	<properties resource="db.properties" />
	<settings>
        <!--日志的实现是LOG4J-->
		<setting name="logImpl" value="LOG4J" />
	</settings>
	 <!--声明别名-->
	<typeAliases>
		<package name="com.ren.domain" />
	</typeAliases>
    <!--mybatis的插件配置-->
	<plugins>
		<!-- com.github.pagehelper为PageHelper类所在包名 -->
		<plugin interceptor="com.github.pagehelper.PageInterceptor">
			<!-- 表示使用mysql的分页方法 -->
			<property name="helperDialect" value="mysql" />
			<!-- 表示当页码长度为0 的时候,就不进行分页查询 -->
			<property name="pageSizeZero" value="true"/>
		</plugin>
	</plugins>
	
	<!-- 配置连接数据库四要素 -->
	<environments default="dev">
		<environment id="dev">
			<transactionManager type="JDBC" />
			<dataSource type="POOLED">
				<property name="driver" value="${driverClassName}" />
				<property name="url" value="${url}" />
				<property name="username" value="${username}" />
				<property name="password" value="${password}" />
			</dataSource>
		</environment>
	</environments>
 
	<mappers>
		<mapper resource="com/ren/mapper/StudentMapper.xml" />
	</mappers>
	
</configuration>

编写pojo实体类和mapper接口和mapper映射文件

Student实体类

package pojo;

public class Student {
    private Integer id;

    private String name;

    private String email;

    private Integer age;

    public Student() {

    }

    public Student(Integer id, String name, String email, Integer age) {
        this.id = id;
        this.name = name;
        this.email = email;
        this.age = age;
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                ", age=" + age +
                '}';
    }
}

mapper接口

package mapper;

import pojo.Student;

import java.util.List;

public interface StudentMapper {
    //查询所有学生
    List<Student> selectAll();
}

Mapper映射文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="mapper.StudentMapper">
    <select id="selectAll" resultType="pojo.Student">
        select * from student order by id
    </select>
</mapper>

在sqlMapConfig文件中指定Mapper文件位置

<!-- mapper映射文件位置-->
<mappers>
	<package name="mapper"/>
</mappers>

编写MyBatisUtils

package utils;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

/** * 工具类：创建SqlSession对象 */
public class MyBatisUtil {

    private static SqlSessionFactory factory  = null;

    static {

        String config="sqlMapConfig.xml";
        try {
           InputStream inputStream  =Resources.getResourceAsStream(config);
           factory  = new SqlSessionFactoryBuilder().build(inputStream);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    //创建方法，获取SqlSession对象
    public static SqlSession getSqlSession(){
        SqlSession session = null;
        if( factory != null ){
            session =factory.openSession();// openSession(true);
        }
        return session;
    }

}

创建测试类

import com.github.pagehelper.PageHelper;
import mapper.StudentMapper;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;
import pojo.Student;
import utils.MyBatisUtil;

import java.util.List;

public class pageHelper_Test {
    @Test
    public void PageHelperTest(){
        //获取sqlSession
        SqlSession sqlSession = MyBatisUtil.getSqlSession();
        //获取mapper代理
        StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
        //调用PageHelper
        PageHelper.startPage(2,2);
        //查询所有学生
        List<Student> studentList = mapper.selectAll();
        //遍历结果List集合
        studentList.forEach(student -> System.out.println(student));
        //关闭sqlSession对象
        sqlSession.close();
    }
}

查看结果

可以看到sql语句,在mapper文件中并没有写limit,pageHelper已经帮我们加上了limit关键字完成了查询.OK了