MyBatis 读取操作 - 菜鸟教程

读取操作

在上一章中，我们讨论了如何通过执行CREATE操作使用MyBatis将值插入到STUDENT表中。本章介绍如何使用MyBatis读取表中的数据。我们在MySQL中有以下STUDENT表-

在MySQL数据库中创建STUDENT表，如下所示-


CREATE TABLE details.student(
   ID int(10) NOT NULL AUTO_INCREMENT,
   NAME varchar(100) NOT NULL,
   BRANCH varchar(255) NOT NULL,
   PERCENTAGE int(3) NOT NULL,
   PHONE int(11) NOT NULL,
   EMAIL varchar(255) NOT NULL,
   PRIMARY KEY (`ID`)
);

假设此表有两个记录为


+----+----------+--------+------------+-----------+--------------------+
| ID |  NAME    | BRANCH | PERCENTAGE |   PHONE   |      EMAIL         |
+----+----------+--------+------------+-----------+--------------------+
| 1  | Mohammad |   It   |     80     | 984803322 | Mohammad@gmail.com |
| 2  | shyam    |   It   |     75     | 984800000 | shyam@gmail.com    |
+----+----------+--------+------------+-----------+--------------------+

STUDENT POJO 类

为了执行读取操作，我们将Student.java中的Student类修改为-


public class Student {
   private int id;
   private String name;
   private String branch;
   private int percentage;
   private int phone;
   private String email;

   public Student(int id, String name, String branch, int percentage, int phone, String email) {
      super();
      this.id = id;
      this.name = name;
      this.branch = branch;
      this.percentage = percentage;
      this.phone = phone;
      this.email = email;
   }
        
   public Student() {}

   public int getId() {
      return id;
   }
                
   public String getName() {
      return name;
   }
        
   public int getPhone() {
      return phone;
   }
        
   public String getEmail() {
      return email;
   }
   
   public String getBranch() {
      return branch;
   }

   public int getPercentage() {
      return percentage;
   }    
        
}

Student.xml文件

要使用MyBatis定义SQL映射语句，我们将在Student.xml文件中添加<select>标记，并且在该标记定义内，我们将定义一个“id” ，它将在mybatisRead.java文件中用于在数据库上执行SQL SELECT查询。在读取记录时，我们可以一次获取所有记录，也可以使用where子句获取特定记录。在下面给出的XML中，您可以观察这两个查询。要检索特定记录，我们需要一个唯一的键来表示该记录。因此，我们还定义了Student类型的结果映射“id”（唯一键），以将select查询的结果与Student类的变量进行映射。


<?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 = "Student">    

   <resultMap id = "result" type = "Student">
      <result property = "id" column = "ID"/>         
   </resultMap>
        
   <select id = "getAll" resultMap = "result">
      SELECT * FROM STUDENT; 
   </select>
    
   <select id = "getById" parameterType = "int" resultMap = "result">
      SELECT * FROM STUDENT WHERE ID = #{id};
   </select>
        
</mapper>

获取所有记录

该文件具有应用程序级逻辑，可从Student表中读取所有记录。创建并保存mybatisRead_ALL.java文件，如下所示-


import java.io.IOException;
import java.io.Reader;
import java.util.List;

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

public class mybatisRead_ALL { 

   public static void main(String args[]) throws IOException{

      Reader reader = Resources.getResourceAsReader("SqlMapConfig.xml");
      SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);               
      SqlSession session = sqlSessionFactory.openSession();
      
      //select contact all contacts             
      List<Student> student = session.selectList("Student.getAll");
          
      for(Student st : student ){         
         System.out.println(st.getId());
         System.out.println(st.getName());
         System.out.println(st.getBranch());
         System.out.println(st.getPercentage());         
         System.out.println(st.getEmail());        
         System.out.println(st.getPhone());   
      }  
                
      System.out.println("Records Read Successfully ");          
      session.commit();   
      session.close();                  
   }
}

编译与执行：

如上所示创建Student.xml。
如本教程的MYBATIS-配置XML章节中所示，创建SqlMapConfig.xml 。
如上所示创建Student.java并进行编译。
如上所示创建mybatisRead_ALL.java并进行编译。
执行mybatisRead_ALL二进制文件以运行程序。

您将获得学生表的所有记录为-


++++++++++++++ details of the student who's id is  :1 +++++++++++++++++++
1
Mohammad
It
80
Mohammad@gmail.com
984803322
++++++++++++++ details of the student who's id is  :2
+++++++++++++++++++
2
shyam
It
75
shyam@gmail.com
984800000
Records Read Successfully

获取特定记录

复制并保存以下名为mybatisRead_byID的程序-


import java.io.IOException;
import java.io.Reader;

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

public class mybatisRead_byID { 

   public static void main(String args[]) throws IOException{
      
      int i = 1;
      Reader reader = Resources.getResourceAsReader("SqlMapConfig.xml");
      SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);               
      SqlSession session = sqlSessionFactory.openSession();      
          
      //select a particular student  by  id     
      Student student = (Student) session.selectOne("Student.getById", 2); 
          
      //Print the student details
      System.out.println(student.getId());
      System.out.println(student.getName());
      System.out.println(student.getBranch());
      System.out.println(student.getPercentage());      
      System.out.println(student.getEmail());      
      System.out.println(student.getPhone());
                
      session.commit();
      session.close();
                        
   }
   
}

编译与执行：

如上所示创建Student.xml。
如本教程的MYBATIS-配置XML章节中所示，创建SqlMapConfig.xml 。
如上所示创建Student.java并进行编译。
如上所示创建mybatisRead_byID.java并进行编译。
执行mybatisRead_byID二进制文件以运行程序。

您将得到以下结果，并且将从Student表中读取一条记录，如下所示：


2
shyam
It
75
shyam@gmail.com
984800000