MyBatis框架的一级缓存机制（SqlSession级别）

 代码：

User.java

package com.bean;

import java.io.Serializable;

public class User implements Serializable {
	private Integer id;
	private String name;
	private String sex;
	private Integer age;
	public User() {
		super();
		// TODO Auto-generated constructor stub
	}
	public User(String name, String sex, Integer age) {
		super();
		this.name = name;
		this.sex = sex;
		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 getSex() {
		return sex;
	}
	public void setSex(String sex) {
		this.sex = sex;
	}
	public Integer getAge() {
		return age;
	}
	public void setAge(Integer age) {
		this.age = age;
	}
	@Override
	public String toString() {
		// TODO Auto-generated method stub
		return "User [id=" + id + ", name=" + name + ", sex=" + sex + ", age=" + age + "]";
	}
	
}
           

FKSqlSessionFactory.java

package com.factory;

import java.io.InputStream;
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 FKSqlSessionFactory {
	
	private static SqlSessionFactory sqlSessionFactory = null;
	
	// 初始化创建SqlSessionFactory对象
	static{
		try {
			// 读取mybatis-config.xml文件
			InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
			sqlSessionFactory = new SqlSessionFactoryBuilder()
					.build(inputStream);
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
	
	// 获取SqlSession对象的静态方法
	public static SqlSession getSqlSession(){
		return sqlSessionFactory.openSession();
	}

	// 获取SqlSessionFactory的静态方法
	public static SqlSessionFactory getSqlSessionFactory() {
		return sqlSessionFactory;
	}
}
           

UserMapper.java

package com.mapper;

import java.util.List;

import com.bean.User;

public interface UserMapper {
	// 根据id查询User
	User selectUserById(Integer id);
	
	// 查询所有User
	List<User> selectAllUser();
		
	// 根据id删除User
	void deleteUserById(Integer id);
}
           

UserMapper.xml

<?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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="com.mapper.UserMapper">
  <!-- 根据id查询User -->
  <select id="selectUserById" parameterType="int" resultType="com.bean.User">
       select * from tb_user3 where id = #{id}
  </select>
  
  <!-- 查询所有User -->
  <select id="selectAllUser" resultType="com.bean.User">
  	select * from tb_user3
  </select>
  
  <!-- 根据id删除User -->
  <delete id="deleteUserById" parameterType="int" >
  	 delete from tb_user3 where id = #{id}
  </delete>
</mapper>
           

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
  <!--  XML 配置文件包含对 MyBatis 系统的核心设置 -->
<configuration>
	<properties resource="db.properties"/>
	<!-- 指定 MyBatis 所用日志的具体实现 -->
	<settings>
		<setting name="logImpl" value="LOG4J"/>
	</settings>
	<!-- 设置别名 -->
	<typeAliases>
		<typeAlias  alias="user" type="com.bean.User"/>
	</typeAliases>
	<environments default="mysql">
	<!-- 环境配置，即连接的数据库。 -->
    <environment id="mysql">
    <!--  指定事务管理类型，type="JDBC"指直接简单使用了JDBC的提交和回滚设置 -->
      <transactionManager type="JDBC"/>
      <!--  dataSource指数据源配置，POOLED是JDBC连接对象的数据源连接池的实现。 -->
      <dataSource type="POOLED">
        <property name="driver" value="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${password}"/>
      </dataSource>
    </environment>
  </environments>
  <!-- mappers告诉了MyBatis去哪里找持久化类的映射文件 -->
  <mappers>
  	<mapper resource="com/mapper/UserMapper.xml"/>
  </mappers>
</configuration>
           

TestOneLevelCache.java

package com.test;

import org.apache.ibatis.session.SqlSession;

import com.bean.User;
import com.factory.FKSqlSessionFactory;
import com.mapper.UserMapper;

public class TestOneLevelCache {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
        TestOneLevelCache test = new TestOneLevelCache();
      //test.testCache1();
      //test.testCache2();
        test.testCache3();
	}

	/*
	 * 一级缓存: 也就Session级的缓存(默认开启)
	 */
	public void testCache1 (){
		// 使用工厂类获得SqlSession对象
		SqlSession session=FKSqlSessionFactory.getSqlSession();
		// 获得UserMapping对象
		UserMapper um=session.getMapper(UserMapper.class);
		// 查询id为1的User对象，会执行select语句
		User user=um.selectUserById(1);
		System.out.println(user);
		// 再次查询id为1的User对象,因为是同一个SqlSession,所以会从之前的一级缓存中查找数据
		User user2=um.selectUserById(1);
		System.out.println(user2);
		session.close();
	}
	
	public void testCache2 (){
		// 使用工厂类获得SqlSession对象
		SqlSession session = FKSqlSessionFactory.getSqlSession();
		// 获得UserMapping对象
		UserMapper um = session.getMapper(UserMapper.class);
		// 查询id为1的User对象，会执行select语句
		User user = um.selectUserById(1);
		System.out.println(user);
		// 执行delete操作
		um.deleteUserById(4);
		session.commit();
		// 再次查询id为1的User对象,因为DML操作会清空SqlSession缓存,所以会再次执行select语句
		User user2 = um.selectUserById(1);
		System.out.println(user2);
		session.close();
	}
	
	public void testCache3 (){
		// 使用工厂类获得SqlSession对象
		SqlSession session = FKSqlSessionFactory.getSqlSession();
		// 获得UserMapping对象
		UserMapper um = session.getMapper(UserMapper.class);
		// 查询id为1的User对象，会执行select语句
		User user = um.selectUserById(1);
		System.out.println(user);
		// 关闭一级缓存
		session.close();
		// 再次访问,需要再次获取一级缓存，然后才能查找数据，否则会抛出异常
		session = FKSqlSessionFactory.getSqlSession();
		// 再次获得UserMapping对象
		um = session.getMapper(UserMapper.class);
		// 再次访问，因为现在使用的是一个新的SqlSession对象,所以会再次执行select语句
		User user2 = um.selectUserById(1);
		System.out.println(user2);
		// 关闭SqlSession对象
		session.close();
	}
}
           

截图：