Mybatis-Plus入门篇

「爱情、让人受尽委屈。」 2022-04-01 11:18 329阅读 0赞

# 前言：Mybatis-Plus的强大之处在于结合JPA、Mybatis两者特性，通俗点来说就是结合了Hibernate和Mybatis两者的特性，达到快速开发的目的 #

### **优缺点对比：Hibernate不够灵活，但是不用编写SQL，Mybatis够灵活，但是要编写大量的SQL语句。只有充分两者的长处，才能提高开发效率。（ps:JPA的一种实现就是Hibernate，这里说法不做区分）** ###

持久层开发一般选用的策略：

（1）**Mybatis+逆向工程**（若字段改变，就需要重新生成逆向工程代码，也不够灵活）

（2）**Mybatis+JPA** (这种比较常用，Mybatis操作联表以及复杂的SQL，JPA操作单表)

（3）**Mybatis-Plus**：一站式开发

下面就以Mybatis-Plus的开发模式给大家做出一些讲解（基于spring boot）：

**项目目录结构：**

entity:带有注解的实体类

pojo:最简单的实体类

mapper:DAO层

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODgwMjA2MQ_size_16_color_FFFFFF_t_70][]

**department表结构**

![20181230161749196.png][]

**employee表结构（dep\_id是外键）**

![20181230161815184.png][]

**首先添加Mybatis-Plus依赖**

<dependency>
    	<groupId>com.baomidou</groupId>
    	<artifactId>mybatis-plus-boot-starter</artifactId>
    	<version>2.3.3</version>
    </dependency>

**application.properties配置文件相关配置（这里最好不要去掉****serverTimezone=GMT****，有可能报会错）**

#配置mysql数据库
    spring.datasource.url= jdbc:mysql://localhost:3306/mybatisplus?serverTimezone=GMT
    spring.datasource.driver-class-name=com.mysql.jdbc.Driver
    spring.datasource.username=root
    spring.datasource.password=123456
    #配置mybatis-plus的配置文件以及映射文件
    mybatis-plus.config-location=classpath:/mybatis/mybatis-config.xml
    mybatis-plus.mapper-locations=classpath:/mybatis/mapper/*.xml
    #显示sql语句
    logging.level.cn.zdxh.mp.mapper=debug

**Employee实体类**

@TableName：指定表名，如果表名和实体类一致，可以省略（可选）

@TableId：指定主键名，以及主键生成策略，这里是自动增长（必选）

@TableField：指定字段名，同理，一致可省略（可选）

ps：mybatis-plus2.3以后，自动开启驼峰命名法

//表名
    @TableName("employee")
    public class Employee {
        //主键，以及生成策略
        @TableId(value = "id",type = IdType.AUTO)
        private Integer id;
        //员工姓名
        @TableField //可省略
        private String empName;
        //员工性别
        private String empSex;
        //员工电话
        private String empPhone;
        //员工对应部门的外键id
        private Integer depId;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getEmpName() {
            return empName;
        }
    
        public void setEmpName(String empName) {
            this.empName = empName;
        }
    
        public String getEmpSex() {
            return empSex;
        }
    
        public void setEmpSex(String empSex) {
            this.empSex = empSex;
        }
    
        public String getEmpPhone() {
            return empPhone;
        }
    
        public void setEmpPhone(String empPhone) {
            this.empPhone = empPhone;
        }
    
        public Integer getDepId() {
            return depId;
        }
    
        public void setDepId(Integer depId) {
            this.depId = depId;
        }
    
        @Override
        public String toString() {
            return "Employee{" +
                    "id=" + id +
                    ", empName='" + empName + '\'' +
                    ", empSex='" + empSex + '\'' +
                    ", empPhone='" + empPhone + '\'' +
                    ", depId=" + depId +
                    '}';
        }
    }

**Department实体类**

//表名
    @TableName("department")
    public class Department {
        //主键，以及生成策略
        @TableId(value = "id",type = IdType.AUTO)
        private Integer id;
        //部门名称
        private String departmentName;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getDepartmentName() {
            return departmentName;
        }
    
        public void setDepartmentName(String departmentName) {
            this.departmentName = departmentName;
        }
    
        @Override
        public String toString() {
            return "Department{" +
                    "id=" + id +
                    ", departmentName='" + departmentName + '\'' +
                    '}';
        }
    }

**EmployeeCustomer实体类（普通的pojo，没做注解） **

public class EmployeeCustomer {
        //主键
        private Integer id;
        //员工姓名
        private String empName;
        //员工性别
        private String empSex;
        //员工电话
        private String empPhone;
        //一个员工对应一个部门
        private Department department;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getEmpName() {
            return empName;
        }
    
        public void setEmpName(String empName) {
            this.empName = empName;
        }
    
        public String getEmpSex() {
            return empSex;
        }
    
        public void setEmpSex(String empSex) {
            this.empSex = empSex;
        }
    
        public String getEmpPhone() {
            return empPhone;
        }
    
        public void setEmpPhone(String empPhone) {
            this.empPhone = empPhone;
        }
    
        public Department getDepartment() {
            return department;
        }
    
        public void setDepartment(Department department) {
            this.department = department;
        }
    
        @Override
        public String toString() {
            return "EmployeeCustomer{" +
                    "id=" + id +
                    ", empName='" + empName + '\'' +
                    ", empSex='" + empSex + '\'' +
                    ", empPhone='" + empPhone + '\'' +
                    ", department=" + department +
                    '}';
        }
    }

**EmployeeMapper接口**

**只需要继承一个标记接口BaseMapper<T>，并指定目的对象的泛型，即可进行简单增删改查**

public interface EmployeeMapper extends BaseMapper<Employee> {
        //通过员工id查询员工的信息，以及所在部门的信息
        public EmployeeCustomer selectEmployeeDetailById(Integer id);
    }

**DepartmentMapper接口**

public interface DepartmentMapper  extends BaseMapper<Department>{
    }

**来看看BaseMapper接口究竟为我们准备了哪些通用方法**

多达20个，足以满足一般的增删改查

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODgwMjA2MQ_size_16_color_FFFFFF_t_70 1][]

**其中在EmployeeMapper.xml中自定义了联表查询的方法**

<?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="cn.zdxh.mp.mapper.EmployeeMapper">
        <resultMap id="selectEmpDet" type="cn.zdxh.mp.pojo.EmployeeCustomer">
            <id property="id" column="id"/>
            <result property="empName" column="emp_name"/>
            <result property="empSex" column="emp_sex"/>
            <result property="empPhone" column="emp_phone"/>
            <association property="department" javaType="cn.zdxh.mp.entity.Department">
                <id property="id" column="id"/>
                <result property="departmentName" column="department_name"/>
            </association>
        </resultMap>
        <select id="selectEmployeeDetailById" parameterType="integer" resultMap="selectEmpDet">
            SELECT
              e.id,
              e.emp_name,
              e.emp_sex,
              e.emp_phone,
              d.id,
              d.department_name
            FROM employee e
            LEFT JOIN department d
            ON e.dep_id=d.id
            WHERE e.id=#{id}
        </select>
    </mapper>

**最后不要忘记了添加mapper包扫描**

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODgwMjA2MQ_size_16_color_FFFFFF_t_70 2][]

** 下面就可以用测试类进行测试了**

**测试xml中自定义的方法：**

@Test
    	public void testXmlSelect() {
    		EmployeeCustomer employeeCustomer = employeeMapper.selectEmployeeDetailById(1);
    		System.out.println(employeeCustomer);
    	}

**测试BaseMapper中提供的方法：**

@Test
    	public void testBaseSelect(){
    		Employee employee = employeeMapper.selectById(1);
    		System.out.println(employee);
    	}
    
    	@Test
    	public void testBaseInsert(){
    		Department department=new Department();
    		department.setDepartmentName("市场部");
    		departmentMapper.insert(department);
    	}

以上测试方法统统没问题

**不仅如此，还可以进行有条件的查询、分页查询**

**条件查询：EntityWrapper条件构造器**

@Test
    	public void testBaseSelect2(){
    		List<Employee> employees = employeeMapper.selectList(
    				new EntityWrapper<Employee>()
    						.eq("emp_name", "ning")//注意是列名
    		);
    		System.out.println(employees);
    	}

条件构造器有相当多的构造方法可以调用，具体使用请参照官方文档

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl8zODgwMjA2MQ_size_16_color_FFFFFF_t_70 3][]

**分页查询同理：Page<>对象**

**注意：默认的分页查询是逻辑分页，若要进行物理分页，还要添加分页插件PaginationInterceptoer才行**

**总结：Mybatis-Plus真的好用**

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[20181230161749196.png]: /images/20220401/f7ae840b99c140cd9150fbb8d8e77635.png
[20181230161815184.png]: /images/20220401/c3efb752ab1c41d7ac18ec4478ae3c96.png
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