类	说明
BaseMapper.java	crud方法。（这是一个接口）
ServiceImpl.java	对BaseMapper.java的封装，实现IService接口。有：list、saveOrUpdate、count等方法。可以直接用，或者新建类来继承此类。
IService.java	这是一个接口，一般与ServiceImpl.java结合使用，例如：产品服务： //接口 public interface IProductService extends IService<Product>{ } //实现 public class ProductServiceImpl extends ServiceImpl<ProductMapper, Product> implements IProductService{ } 此时，就可以直接调用IService的接口来使用ServiceImpl的实现方法了。
Wrappers.java	提供创建Wrapper、LambdaWrapper的静态方法。
ChainWrappers.java	提供创建ChainLambdaWrapper的静态方法。

IService.java

Iservice.java所有方法

Mybatis-Plus--使用_java

IService.java方法说明

方法	说明
getBaseMapper	获得对应的baseMapper。从而可以去调用BaseMapper方法。当然，实际上IService的方法已完全包含BaseMapper的方法，无需这样做。
getOne	1个参数时，若有多个值则报错。 2个参数时，若第二个参数设为false，则有多个值时不报错，取第一个。
lambdaQuery	会返回一个LambdaQueryChainWrapper实例，用法见Wrapper专题。本处示例： List<User> list = userService.lambdaQuery().eq(User::getAge, 18).list();
lambdaUpdate	会返回一个LambdaUpdateChainWrapper实例，用法见Wrapper专题。可更新/删除。本处示例： boolean update = userService.lambdaUpdate().eq(User::getAge, 18).set(User::getAge, 31).update(); boolean remove = userService.lambdaUpdate().eq(User::getAge, 18).remove();
list	无参数可查询所有
saveBatch	若参数为null，则返回false
saveOrUpdate	通过id来判断。若存在此id则更新，没有则插入。

参数Wrapper

可以跟LambdaQueryWrapper，例如：remove(new LambdaQueryWrapper<User>().eq(User::getName, "Tony"));

不能跟LambdaQueryChainWrapper，例如：remove(lambdaQuery().eq(User::getName, "Tony"))，否则报错：

org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.builder.BuilderException: Error evaluating expression 'ew.entity != null'. Cause: org.apache.ibatis.ognl.OgnlException: entity [com.baomidou.mybatisplus.core.exceptions.MybatisPlusException: can not use this method for "getEntity"]

返回值为list

例如：list();若一项都没有，则会返回空的List，需要用list.isEmpty()判断，只用list == null判断是不够的

BaseMapper.java

BaseMapper.java所有方法

Mybatis-Plus--使用_java_02

项目实例

源码地址

https://gitee.com/shapeless/demo_MybtisPlus/tree/master/MultiTable_Page

项目结构

Mybatis-Plus--使用_java_03

config

package com.example.conf;

import com.baomidou.mybatisplus.extension.plugins.PaginationInterceptor;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.transaction.annotation.EnableTransactionManagement;

@Configuration
@MapperScan("com.example.**.mapper")
public class MybatisPlusConfig {

    /**
     * 分页插件
     */
    @Bean
    public PaginationInterceptor paginationInterceptor() {
        return new PaginationInterceptor();
    }
}

entity/vo

package com.example.entity;

import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;

import java.util.Date;

@Data
@TableName("t_question")
public class Question {
    // 问答主键id
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;

    // 学生外键id
    @TableField("student_id")
    private Integer studentId;

    // 问题内容
    private String content;

    // 问题悬赏的积分
    private Integer value;
}

package com.example.entity;

import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import lombok.Data;

@Data
@TableName("t_student")
public class Student {
    // 学生主键id
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;

    // 学生名称
    private String name;

    // 学生积分数
    private Integer points;
}

package com.example.vo;

import lombok.Data;

import java.io.Serializable;

@Data
public class QuestionStudentVO {
    // 问答主键id
    private Integer id;

    // 学生外键id
    private Integer studentId;

    private String name;

    // 问题内容
    private String content;

    // 问题悬赏的积分
    private Integer value;
}

mapper

package com.example.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.example.entity.Question;
import com.example.vo.QuestionStudentVO;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface QuestionMapper extends BaseMapper<Question> {
    @Select("SELECT t_question.*,t_student.`name` FROM t_question,t_student WHERE t_question.student_id=t_student.id")
    List<QuestionStudentVO> getQuestionStudent();

    @Select("SELECT t_question.*,t_student.`name` FROM t_question,t_student WHERE t_question.student_id=t_student.id")
    List<QuestionStudentVO> getQuestionStudentPage (Page<QuestionStudentVO> page);
}

package com.example.mapper;

import com.baomidou.mybatisplus.core.mapper.BaseMapper;
import com.example.entity.Student;

@Repository
public interface StudentMapper extends BaseMapper<Student> {
}

service

Question

package com.example.service;

import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.baomidou.mybatisplus.extension.service.IService;
import com.example.entity.Question;
import com.example.vo.QuestionStudentVO;

import java.util.List;

public interface QuestionService extends IService<Question> {
    Page<QuestionStudentVO> getQuestionStudentPage (Page<QuestionStudentVO> page);

    List<QuestionStudentVO> getQuestionStudent();
}

package com.example.service.impl;

import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import com.example.entity.Question;
import com.example.mapper.QuestionMapper;
import com.example.service.QuestionService;
import com.example.vo.QuestionStudentVO;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class QuestionServiceImpl extends ServiceImpl<QuestionMapper, Question> implements QuestionService {
    //下边的两处this.baseMapper都可以用自动注入的QuestionMapper替换
    //即：
    //@Autowired
    //private QuestionMapper questionMapper;

    @Override
    public List<QuestionStudentVO> getQuestionStudent() {
        return this.baseMapper.getQuestionStudent();
    }

    @Override
    public Page<QuestionStudentVO> getQuestionStudentPage (Page<QuestionStudentVO> page) {
        return page.setRecords(this.baseMapper.getQuestionStudentPage (page));
    }
}

Student

package com.example.service;

import com.baomidou.mybatisplus.extension.service.IService;
import com.example.entity.Student;

public interface StudentService extends IService<Student> {
    public Student getRandomStudent();

    public int getCountOfStudents();
}

package com.example.service.impl;

import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import com.example.entity.Student;
import com.example.mapper.StudentMapper;
import com.example.service.StudentService;
import org.springframework.stereotype.Service;

@Service
public class StudentServiceImpl extends ServiceImpl<StudentMapper, Student> implements StudentService {
    //下边的两处this.baseMapper都可以用自动注入的StudentMapper替换
    //即：
    //@Autowired
    //private StudentMapper studentMapper;

    @Override
    public Student getRandomStudent() {
        return this.baseMapper.selectOne(null);
    }

    @Override
    public int getCountOfStudents() {
        return count(null);
    }
}

controller

package com.example.controller;

import com.baomidou.mybatisplus.extension.plugins.pagination.Page;
import com.example.entity.Question;
import com.example.entity.Student;
import com.example.service.QuestionService;
import com.example.service.StudentService;
import com.example.vo.QuestionStudentVO;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.HashMap;
import java.util.List;
import java.util.Map;

@RestController
@RequestMapping("/common")
public class CommonController {

    @Autowired
    QuestionService questionService;

    @Autowired
    StudentService studentService;

    @GetMapping("/getRandomStudent")
    public Student getRundomStudent() {
        Student student = studentService.getRandomStudent();
        return student;
    }

    @GetMapping("/getStudentCount")
    public Integer getmStudentCount() {
        Integer integer = studentService.getCountOfStudents();
        return integer;
    }

    @GetMapping("/getAllQuestionWithStudent")
    public Map<String, Object> getAllQuestionWithStudent() {
        Map<String, Object> map = new HashMap<>();
        List<QuestionStudentVO> questionStudent = questionService.getQuestionStudent();
        if (questionStudent.size() == 0) {
            map.put("code", 400);
        } else {
            map.put("code", 200);
            map.put("data", questionStudent);
        }
        return map;
    }

    @GetMapping("/getAllQuestionWithStudentByPage/{page}/{size}")
    public Map<String, Object> getAllQuestionWithStudentByPage(@PathVariable Integer page, @PathVariable Integer size) {
        Map<String, Object> map = new HashMap<>();
        Page<QuestionStudentVO> questionStudent = questionService.getQuestionStudentPage (new Page<>(page, size));
        if (questionStudent.getRecords().size() == 0) {
            map.put("code", 400);
        } else {
            map.put("code", 200);
            map.put("data", questionStudent);
        }
        return map;
    }
}