General mapper learning notes

Keywords: Database Mybatis JDBC Attribute

General Mapper notes

Build development environment (spring boot)

Database table

Import dependency


To configure

  port: 8088
#jdbc configuration
    driver-class-name: com.mysql.jdbc.Driver
    url: jdbc:mysql://localhost:3306/mybatis?useUnicode=true&characterEncoding=utf8
    username: root
    password: 123456

Entity class

import javax.persistence.*;

public class Employee {

   private Integer empId;//emp_id
   private String empName;//emp_name
   private Double empSalary;//emp_salary_apple
   private Integer empAge;//emp_age
   public Employee() {

   public Employee(Integer empId, String empName, Double empSalary, Integer empAge) {
      this.empId = empId;
      this.empName = empName;
      this.empSalary = empSalary;
      this.empAge = empAge;

   public String toString() {
      return "Employee [empId=" + empId + ", empName=" + empName + ", empSalary=" + empSalary + ", empAge=" + empAge
            + "]";

   public Integer getEmpId() {
      return empId;

   public void setEmpId(Integer empId) {
      this.empId = empId;

   public String getEmpName() {
      return empName;

   public void setEmpName(String empName) {
      this.empName = empName;

   public Double getEmpSalary() {
      return empSalary;

   public void setEmpSalary(Double empSalary) {
      this.empSalary = empSalary;

   public Integer getEmpAge() {
      return empAge;

   public void setEmpAge(Integer empAge) {
      this.empAge = empAge;


dao interface


import org.springframework.stereotype.Repository;
import tk.mybatis.mapper.common.Mapper;

public interface EmpLoyeeDao extends Mapper<Employee> {



import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

public class EmployeeService {
    private EmpLoyeeDao empLoyeeDao;

    public Employee getOne(Employee employeeQueryCondition) {
        return empLoyeeDao.selectOne(employeeQueryCondition);


Test class

package mapper.test;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;

@SpringBootTest(classes = Application.class)
public class Test1 {
    private  EmployeeService employeeService;

    public void testSelectOne()
        //1. Create entity class to encapsulate query criteria
        Employee employee = new Employee(null,"bob",null,null);
        //2. Execute query
        Employee result = employeeService.getOne(employee);

Common annotations

@Table annotation

Function: establish correspondence between entity class and database table.

Default rule: the first letter of entity class name is lowercase as the table name. Employee class → employee table.

Usage: specify the Table name of the target database Table in the name attribute of the @ Table annotation

@Column annotation

Function: establish correspondence between entity class fields and database table fields.

Default rule:

  • Entity field: hump naming
  • Database table fields: use "" to distinguish words

Usage: specify the field name of the target field in the name attribute of the @ Column annotation
@Id annotation
When general Mapper executes the xxxByPrimaryKey(key) method, there are two situations.
Case 1: the @ Id annotation is not used to explicitly specify the primary key field

The reason why the above WHERE clause is generated is that the general Mapper takes all the fields in the entity class together as the union primary key.

Case 2: use @ Id primary key to explicitly mark entity class fields corresponding to primary key fields in database tables.

@Generated value annotation

Function: let the general Mapper write back the primary key value automatically generated by the database to the entity class object after the insert operation.

Add primary key usage:

@Transient primary key

Used to mark entity class fields that do not correspond to database table fields.

common method

selectOne method

General Mapper automatically generates SQL statements for us

Entity class encapsulates the rules of WHERE clause generated by query condition

  • Generate a WHERE clause with a non empty value
  • Use "=" for comparison in conditional expressions

An entity class result must be returned. If there are multiple results, an exception will be thrown

xxxByPrimaryKey method

You need to use the @ Id primary key to clearly mark the entity class fields corresponding to the database table primary key fields, otherwise the general Mapper will use all entity class fields as the union primary key.

xxxSelective method

If the non primary key field is null, it will not be added to the SQL statement.

QBC query


Criteria is the plural form of criteria. Rules, standards, rules. Equivalent to query condition in SQL statement.
QBC query is a modular encapsulation of query conditions through Java objects.

Code example

public void testSelectByExample()
    //Objective: where (EMP < salary >? And EMP < age >) or (EMP < salary <? And EMP < age >?)
    //1. Create Example object
    Example example = new Example(Employee.class);

    //i. Set sorting information

    //ii. Set "de duplication"

    //iii. set select field


    //2. Create Criteria object through Example object
    Example.Criteria criteria01 = example.createCriteria();
    Example.Criteria criteria02 = example.createCriteria();

    //3. Set query Criteria in two Criteria objects
    //Property parameter: property name of entity class
    //Value parameter: attribute value of entity class
    criteria01.andGreaterThan("empSalary", 3000)
            .andLessThan("empAge", 25);

    criteria02.andLessThan("empSalary", 5000)
            .andGreaterThan("empAge", 30);

    //4. Use OR keyword to assemble two Criteria objects

    //5. Execute query
    List<Employee> empList = employeeService.getEmpListByExample(example);

    for (Employee employee : empList) {

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Posted by LightningSt on Wed, 12 Feb 2020 22:45:27 -0800