MyBatis框架的二级缓存机制（mapper级别）

古城微笑少年丶 2022-06-18 04:22 222阅读 0赞

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">
    
    <mapper namespace="com.mapper.UserMapper">
    
      <cache eviction="LRU" flushInterval="600000" size="512" readOnly="true"/>
    
      
      
      <select id="selectUserById" parameterType="int" resultType="com.bean.User">
           select * from tb_user3 where id = #{id}
      </select>
      
      
      <select id="selectAllUser" resultType="com.bean.User">
      	select * from tb_user3
      </select>
      
      
      <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">
    
    <configuration>
    	<properties resource="db.properties"/>
    	
    	<settings>
    		<setting name="logImpl" value="LOG4J"/>
    		
    		<setting name="cacheEnabled" value="true"/>
    	</settings>
    	
    	<typeAliases>
    		<typeAlias  alias="user" type="com.bean.User"/>
    	</typeAliases>
    	
      <environments default="mysql">
    	
        <environment id="mysql">
        
          <transactionManager type="JDBC"/>
          
          <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>
      	<mapper resource="com/mapper/UserMapper.xml"/>
      </mappers>
    </configuration>

TestTwoLevelCache.java

package com.test;
    
    import org.apache.ibatis.session.SqlSession;
    
    import com.bean.User;
    import com.factory.FKSqlSessionFactory;
    import com.mapper.UserMapper;
    
    public class TestTwoLevelCache {
    
    	public static void main(String[] args) {
    		// TODO Auto-generated method stub
           TestTwoLevelCache test = new TestTwoLevelCache();
           //test.testCache1();
           test.testCache2();
    	}
    
    	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);
    		// 执行delete操作
    		um.deleteUserById(5);
    		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 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);
    		// 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：

![Center][]

总结：使用缓存可以最大程度地减轻数据查询的压力，提高数据库性能。

[Center]: /images/20220618/1284b64a6d964ca4a1f59c5123bbede2.png