五、mybatis两级缓存

  一级缓存：（本地缓存） sqlSession级别的缓存。

• 　一级缓存是一直开启的。
•   与数据库同一次会话期间查询到的数据会放在本地缓存中。
•   以后如果需要获取相同的数据，直接从缓存中拿，没必要再去查数据库。

验证实例(实时使用上一篇的目录结构)：

接口文件EmployeeMapper.java：

Employee getEmpById(int id);

映射文件EmployeeMapper.xml

<select id="getEmpById" parameterType="Integer" resultType="entity.Employee">
        select LAST_NAME AS
        lastName,gender as gender,email as email from
        tbl_employee where id =#{asdsdfsdf}
    </select>

junit测试类：

@Test
    public void test02() {
        String resource = "mybatis-config.xml";
        SqlSession openSession = null;
        try {
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            // 获取openSession 不会自动提交数据
            openSession = sqlSessionFactory.openSession(true);
            EmployeeMapper mapper = openSession.getMapper(EmployeeMapper.class);
            Employee employee = mapper.getEmpById(1);
            Employee employee1 = mapper.getEmpById(1);
            System.out.println("测试一级缓存：" + employee);
            System.out.println("测试一级缓存：" + employee1 + "," + (employee == employee1));
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            if (openSession != null) {
                openSession.close();
            }

        }
    }

运行结果：

测试一级缓存：Employee [id=null, lastName=joy33333, email=joy52112225@iclound.com, gender=女]

测试一级缓存：Employee [id=null, lastName=joy33333, email=joy52112225@iclound.com, gender=女],true

运行结果说明：

employee ==employee1 结果为true说明两个地址值相同，说明是同一个employee;
控制台日志显示查询的SQL语句只执行了一次，说明SQL只执行了一次；

 

 一级缓存失效的情况（没有使用到当前一级缓存的情况：效果就是，还需要像数据库发请求）

  1.只要sqlSession不同

  2.sqlSession相同，查询条件不同;原因是当前一级缓存中还没有该数据；

  3.sqlSession相同，两次查询中执行了 增删改 操作也会失效；原因是增删改操作可能会影响数据。

  4.qlSession相同 但是手动清空了一级缓存  

验证实例(实时使用上一篇的目录结构)：

接口文件EmployeeMapper.java：

 同上

映射文件EmployeeMapper.xml

 同上

junit测试类：

@Test
    public void test02() {
        String resource = "mybatis-config.xml";
        SqlSession openSession = null;
        SqlSession openSession2 = null;
        try {
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            // 获取openSession 不会自动提交数据
            openSession = sqlSessionFactory.openSession(true);
            openSession2 = sqlSessionFactory.openSession(true);
            EmployeeMapper mapper = openSession.getMapper(EmployeeMapper.class);
            Employee employee = mapper.getEmpById(1);
            Employee employee1 = mapper.getEmpById(1);
            System.out.println("测试一级缓存：" + employee);
            System.out.println("测试一级缓存：" + employee1 + "," + (employee == employee1));

            EmployeeMapper mapper2 = openSession2.getMapper(EmployeeMapper.class);
            Employee employee2 = mapper2.getEmpById(1);
            System.out.println("sqlSession改变 一级缓存失效：" + employee2 + "," + (employee1 == employee2));
            EmployeeMapper mapper23 = openSession2.getMapper(EmployeeMapper.class);
            Employee employee23 = mapper23.getEmpById(2);
            System.out.println("查询条件改变改变 一级缓存失效：" + employee23 + "," + (employee1 == employee23));

            Employee emp = new Employee();
            emp.setLastName("Tom");
            emp.setGender("男");
            emp.setEmail("678@qq.com");
            mapper23.addEmp(emp);
            Employee employee4 = mapper23.getEmpById(2);
            System.out.println("查询条件相同+sqlSession相同 一级缓存失效：" + employee23 + "," + (employee4 == employee23));

            Employee employee5 = mapper23.getEmpById(2);
            openSession2.clearCache();
            Employee employee6 = mapper23.getEmpById(2);
            System.out.println("手动清楚缓存：" + (employee5 == employee6));
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            if (openSession != null) {
                openSession.close();
            }

        }
    }

运行结果：如图：

 

 

  二级缓存：（全局缓存） 基于namespace级别的一个缓存：

　一个namespace对应一个二级缓存；

  工作机制：

  1.一个会话,查询一条数据，这个数据会被放在一级缓存中；

  2.如果会话关闭，一级缓存中的数据会被保存到二级缓存中；新会话查询信息，就可以参照二级缓存中的内容

  3.sqlSession EmployeeMapper Employee DepartmentMapper Department

  不同namespace查出的数据会放在自己对应的缓存（map）中

  效果，数据会从二级缓存中取，查出的数据都会放在一级缓存中； 只有会话提交或者关闭以后，一级缓存中的数据才会转移到二级缓存中；

   

  使用步骤：

  1.开启全局二级缓存配置：<setting name="cacheEnabled" value="true"/>

  2.去mapper.xml中配置使用二级缓存；

  3.我们的POJO需要实现序列化接口；

验证实例(实时使用上一篇的目录结构)：

 全局文件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="dbconfig.properties"></properties>

    <settings>
        <setting name="cacheEnabled" value="true" />
    </settings>

    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc.driver}" />
                <property name="url" value="${jdbc.url}" />
                <property name="username" value="${jdbc.username}" />
                <property name="password" value="${jdbc.password}" />
            </dataSource>
        </environment>
        <environment id="development_mysql2">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${jdbc2.driver}" />
                <property name="url" value="${jdbc2.url}" />
                <property name="username" value="${jdbc2.username}" />
                <property name="password" value="${jdbc2.password}" />
            </dataSource>
        </environment>
    </environments>

    <databaseIdProvider type="DB_VENDOR">
        <!-- 为不同的数据库厂商取别名 -->
        <property name="MySQL" value="mysql" />
        <property name="Oracle" value="oracle" />
        <property name="SQL Server" value="sqlserver" />
    </databaseIdProvider>
    <mappers>
        <package name="dao" />
    </mappers>
</configuration>

映射文件EmployeeMapper.xml：使用二级缓存<cache></cache>

cache 标签的属性说明：

• eviction:缓存回收策略；
• flushInterval：缓存刷新间隔，多长时间清空一次，默认不清空
• readOnly：缓存只读；true： mybatis认为所有从缓存中获取数据的操作都是只读操作，不会修改数据；
• mybatis为了加快速度，直接把数据在缓存中的引用交给用户；不安全；
• false：非只读；mybatis局的数据可能会被修改。mybatis会利用序列化和反序列化的技术克隆一份新数据给用户；安全，速度慢
• size：存放多少元素；
• type：指定自定义缓存的全类名；实现cache接口即可；

<?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="dao.EmployeeMapper">
<cache></cache>
    <insert id="addEmp" parameterType="entity.Employee" useGeneratedKeys="true" keyProperty="id">
        insert into tbl_employee(last_name,gender,email) values(
        #{lastName},#{gender},#{email}
        )
    </insert>
    
    <select id="getEmpById" parameterType="Integer" resultType="entity.Employee">
        select LAST_NAME AS
        lastName,gender as gender,email as email from
        tbl_employee where id =#{asdsdfsdf}
    </select>

</mapper>

Employee.java实体类实现序列化接口：

package entity;

import java.io.Serializable;

public class Employee implements Serializable {

    /**
     * 
     */
    private static final long serialVersionUID = -6962919367201266002L;
    private Integer id;
    private String lastName;
    private String email;
    private String gender;
    private Department department;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Department getDepartment() {
        return department;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }

    @Override
    public String toString() {
        return "Employee [id=" + id + ", lastName=" + lastName + ", email=" + email + ", gender=" + gender + "]";
    }

}

接口文件EmployeeMapper.java：

void addEmp(Employee employee);
    Employee getEmpById(int id);

junit测试类：

 

@Test
    public void test01() {
        String resource = "mybatis-config.xml";
        SqlSession openSession = null;
        SqlSession openSession2 = null;
        try {
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            // 获取openSession 不会自动提交数据
            openSession = sqlSessionFactory.openSession(true);
            openSession2 = sqlSessionFactory.openSession(true);
            EmployeeMapper mapper = openSession.getMapper(EmployeeMapper.class);
            EmployeeMapper mapper2 = openSession2.getMapper(EmployeeMapper.class);
            Employee employee = mapper.getEmpById(1);

            System.out.println("测试二级缓存：" + employee);
            openSession.close();
            Employee employee1 = mapper2.getEmpById(1);
            System.out.println("测试二级缓存：" + employee1);
            openSession2.close();
        } catch (Exception e) {
            // TODO: handle exception
        } finally {
            if (openSession != null) {
                openSession.close();
                openSession2.close();
            }

        }
    }

运行结果：

运行结果说明：

Cache Hit Ratio [dao.EmployeeMapper]: 0.5 且没有 发送相同的SQL，说明 ：第二次的记录是从缓存Cache Hit Ratio中取出的；

 

  和缓存有关的设置和属性：

  1.cacheEnabled = true; false会关闭二级缓存；不会关闭一级缓存

  2.每个select 都有 useCache="true" 若为false 关闭的是二级缓存一级缓存没有关闭

  3. 每个增删标签的：flushCache="true" 增删改操作执行完成后就清除缓存 一级缓存失效；二级缓存也会失效

  每个select 都有 flushCache="false" 若为true 一级缓存失效；二级缓存也会失效

即：

<select id="getEmpById" parameterType="Integer" resultType="entity.Employee" flushCache="true">
        select LAST_NAME AS
        lastName,gender as gender,email as email from
        tbl_employee where id =#{asdsdfsdf}
    </select>

运行结果如下图：发送了两次sql;一级缓存失效；二级缓存也会失效;

  4. openSession2.clearCache(); 是清空的是一级缓存，不会影响二级缓存

  5.localCacheScope:本地缓存作用域(一级缓存session 当前会话的所有数据保存在会话中) statement可以禁用一级缓存