（二十一）Mybatis一级查询缓存

注：代码已托管在GitHub上，地址是：https://github.com/Damaer/Mybatis-Learning ，项目是mybatis-17-sqlsessionCache，需要自取，需要配置maven环境以及mysql环境(sql语句在resource下的test.sql中)，觉得有用可以点个小星星，小菜鸟在此Thanks~
查询缓存的使用，主要是为了提高查询访问速度，不用每次都从数据库查询数据，可以提高访问速度，而且可以减少数据库查询次数，减小数据库压力。

一级查询缓存

1.mybatis一级缓存是基于org.apache.ibatis.cache.impl.PerpetualCache类的HashMap本地缓存，作用域是SqlSession，也就是在同一个sqlsession中两次执行相同的sql查询语句，第一次执行完毕之后，就会将查询结果写入缓存中，第二次会从缓存里面直接获取数据，不需要去数据库查询。
2.当一个SqlSession结束，这个SqlSession的以及缓存就不存在了，mybatis默认开启一级缓存，而且不可以关闭
3.一级缓存存的时候是根据sql语句的id，不是根据sql的具体内容。

证明一级缓存的存在

在这里不放大部分代码，只放上核心代码，sql的接口如下：

public interface IStudentDao {
    public Student selectStudentById(int id);
}

mapper.xml与之对应的sql语句：

    
    <select id="selectStudentById" resultType="Student">
        select * from student where id=#{xxx}

    select>

单元测试：

    @Test
    public void testselectStudentById(){

        // 第一次查询
        Student student=dao.selectStudentById(17);
        System.out.println(student);
        // 第二次查询
        Student student2=dao.selectStudentById(17);
        System.out.println(student2);
    }

结果,我们可以看到我们只执行了一次查询，第二次查询的时候直接走的一级缓存，没有对数据库进行查询，如果是对数据库进行查询，那么会有sql语句打印出来：

[service] 2018-07-21 09:48:22,534 - dao.IStudentDao.selectStudentById -1349 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 09:48:22,635 - dao.IStudentDao.selectStudentById -1450 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 09:48:22,677 - dao.IStudentDao.selectStudentById -1492 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]
Student [id=17, name=hello, age=14, score=94.6]

从缓存中读取数据的依据是sql的id还是sql本身？

我们此时有一个疑问，缓存到底是根据什么进行缓存的，那么我们需要做实验，写两个拥有不同的id但是sql完全一致的sql就知道结果了。下面是在同一个namespace中进行：
sql接口：

public interface IStudentDao {
    public Student selectStudentById(int id);
    // 测试不同的id的sql
    public Student selectStudentById2(int id);
}

mapper文件：

<mapper namespace="dao.IStudentDao">

    
    <select id="selectStudentById" resultType="Student">
        select * from student where id=#{xxx}

    select>

    
    <select id="selectStudentById2" resultType="Student">
        select * from student where id=#{xxx}

    select>
mapper>

测试代码：

    @Test
    public void testDiffereentId2(){

        // 第一次查询
        Student student=dao.selectStudentById(17);
        System.out.println(student);
        // 第二次查询，测试不同的id，同一个namespace
        Student student2=dao.selectStudentById2(17);
        System.out.println(student2);
    }

结果如下,我们可以看到里面执行了两次sql，那么就证明了一级缓存不是依据sql本身来的，而是依据sql的id缓存的：

[service] 2018-07-21 10:26:32,844 - dao.IStudentDao.selectStudentById -957  [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 10:26:32,954 - dao.IStudentDao.selectStudentById -1067 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 10:26:32,989 - dao.IStudentDao.selectStudentById -1102 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]
[service] 2018-07-21 10:26:32,990 - dao.IStudentDao.selectStudentById2 -1103 [main] DEBUG dao.IStudentDao.selectStudentById2  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 10:26:32,991 - dao.IStudentDao.selectStudentById2 -1104 [main] DEBUG dao.IStudentDao.selectStudentById2  - ==> Parameters: 17(Integer)
[service] 2018-07-21 10:26:32,996 - dao.IStudentDao.selectStudentById2 -1109 [main] DEBUG dao.IStudentDao.selectStudentById2  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]

不同的namespace下的相同id呢？

不同的namespace。就算是同一个id，那么一级缓存也是不生效的，因为sql缓存的时候是根据namespace不同来区分的。
两个sql接口如下：

public interface IStudentDao {
    public Student selectStudentById(int id);
}

public interface IStudentDao2 {
    //不同的namespace下的相同id的测试
    public Student selectStudentById(int id);
}

两个mapper文件如下:

        
        
<mapper namespace="dao.IStudentDao">

<select id="selectStudentById" resultType="Student">
    select * from student where id=#{xxx}
select>
mapper>

<mapper namespace="dao.IStudentDao2">
    
    <select id="selectStudentById" resultType="Student">
        select * from student where id=#{xxx}
    select>

mapper>

单元测试如下：

    @Test
    public void testDiffereentNamespaceSameId(){

        // 第一次查询
        Student student=dao.selectStudentById(17);
        System.out.println(student);
        // 第二次查询，测试相同的id，不同的namespace
        IStudentDao2 dao2= sqlSession.getMapper(IStudentDao2.class);
        Student student2=dao2.selectStudentById(17);
        System.out.println(student2);
    }

结果如下,我们可以看到执行了两次sql。也就证明了一级缓存根据的是同一个namespace下的同一个id，就算是同一个id，但是namespace也不会生效，因为一级缓存是根据namespace来区分存放的。：

[service] 2018-07-21 10:37:36,916 - dao.IStudentDao.selectStudentById -1545 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 10:37:37,154 - dao.IStudentDao.selectStudentById -1783 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 10:37:37,194 - dao.IStudentDao.selectStudentById -1823 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]
[service] 2018-07-21 10:37:37,202 - dao.IStudentDao2.selectStudentById -1831 [main] DEBUG dao.IStudentDao2.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 10:37:37,204 - dao.IStudentDao2.selectStudentById -1833 [main] DEBUG dao.IStudentDao2.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 10:37:37,210 - dao.IStudentDao2.selectStudentById -1839 [main] DEBUG dao.IStudentDao2.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]

增删改对一级缓存的影响

增删改操作无论是否提交sqlsession.commit()，都会清空以及查询缓存，让查询缓存再次从DB中查询。
sql的接口如下：

public interface IStudentDao {
    public Student selectStudentById(int id);
    // 测试不同的id的sql
    public Student selectStudentById2(int id);


    // 增加学生
    public void insertStudent(Student student);
    // 根据id删除学生
    public void deleteStudentById(int id);
    // 更新学生的信息
    public void updateStudent(Student student);
}

mapper.xml文件：

<mapper namespace="dao.IStudentDao">

    
    <select id="selectStudentById" resultType="Student">
        select * from student where id=#{xxx}

    select>

    
    <select id="selectStudentById2" resultType="Student">
        select * from student where id=#{xxx}
select>

    <insert id="insertStudent" parameterType="Student">
        insert into student(name,age,score) values(#{name},#{age},#{score})
    insert>
    
    <delete id="deleteStudentById">
        delete from student where id=#{id}
        
    delete>

    <update id="updateStudent">
        update student set name=#{name},age=#{age},score=#{score} where id=#{id}
    update>
mapper>

单元测试：

    public void test03(){

        // 第一次查询
        Student student=dao.selectStudentById(17);
        System.out.println(student);
        //插入学生
        Student student1 = new Student("12112",12,21.6);
        //dao.insertStudent(student1);
        dao.updateStudent(student1);
        student=dao.selectStudentById(17);
        System.out.println(student);
    }

当我们执行第一次查询之后，执行一次插入操作的时候，我们发现一级缓存已经被更新了：

[service] 2018-07-21 13:07:27,136 - dao.IStudentDao.selectStudentById -1059 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 13:07:27,247 - dao.IStudentDao.selectStudentById -1170 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 13:07:27,288 - dao.IStudentDao.selectStudentById -1211 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]
[service] 2018-07-21 13:07:27,289 - dao.IStudentDao.insertStudent -1212 [main] DEBUG dao.IStudentDao.insertStudent  - ==>  Preparing: insert into student(name,age,score) values(?,?,?) 
[service] 2018-07-21 13:07:27,291 - dao.IStudentDao.insertStudent -1214 [main] DEBUG dao.IStudentDao.insertStudent  - ==> Parameters: 12112(String), 12(Integer), 21.6(Double)
[service] 2018-07-21 13:07:27,295 - dao.IStudentDao.insertStudent -1218 [main] DEBUG dao.IStudentDao.insertStudent  - <==    Updates: 1
[service] 2018-07-21 13:07:27,295 - dao.IStudentDao.selectStudentById -1218 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 13:07:27,295 - dao.IStudentDao.selectStudentById -1218 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 13:07:27,302 - dao.IStudentDao.selectStudentById -1225 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]

当我们执行第一次查询之后，再执行一次更新操作，就算更新的不是查询的数据，但是属于同一张表,一级缓存同样被更新：

[service] 2018-07-21 23:43:28,073 - dao.IStudentDao.selectStudentById -1081 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 23:43:28,202 - dao.IStudentDao.selectStudentById -1210 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 23:43:28,236 - dao.IStudentDao.selectStudentById -1244 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]
[service] 2018-07-21 23:43:28,239 - dao.IStudentDao.updateStudent -1247 [main] DEBUG dao.IStudentDao.updateStudent  - ==>  Preparing: update student set name=?,age=?,score=? where id=? 
[service] 2018-07-21 23:43:28,241 - dao.IStudentDao.updateStudent -1249 [main] DEBUG dao.IStudentDao.updateStudent  - ==> Parameters: 12112(String), 12(Integer), 21.6(Double), 18(Integer)
[service] 2018-07-21 23:43:28,246 - dao.IStudentDao.updateStudent -1254 [main] DEBUG dao.IStudentDao.updateStudent  - <==    Updates: 1
[service] 2018-07-21 23:43:28,246 - dao.IStudentDao.selectStudentById -1254 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 23:43:28,246 - dao.IStudentDao.selectStudentById -1254 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 23:43:28,251 - dao.IStudentDao.selectStudentById -1259 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]

当我们执行一次查询操作之后，执行一次删除操作，那么一级缓存同样会被更新：

[service] 2018-07-21 23:44:49,296 - dao.IStudentDao.selectStudentById -1172 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 23:44:49,457 - dao.IStudentDao.selectStudentById -1333 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 23:44:49,504 - dao.IStudentDao.selectStudentById -1380 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]
[service] 2018-07-21 23:44:49,505 - dao.IStudentDao.deleteStudentById -1381 [main] DEBUG dao.IStudentDao.deleteStudentById  - ==>  Preparing: delete from student where id=? 
[service] 2018-07-21 23:44:49,505 - dao.IStudentDao.deleteStudentById -1381 [main] DEBUG dao.IStudentDao.deleteStudentById  - ==> Parameters: 18(Integer)
[service] 2018-07-21 23:44:49,508 - dao.IStudentDao.deleteStudentById -1384 [main] DEBUG dao.IStudentDao.deleteStudentById  - <==    Updates: 1
[service] 2018-07-21 23:44:49,509 - dao.IStudentDao.selectStudentById -1385 [main] DEBUG dao.IStudentDao.selectStudentById  - ==>  Preparing: select * from student where id=? 
[service] 2018-07-21 23:44:49,509 - dao.IStudentDao.selectStudentById -1385 [main] DEBUG dao.IStudentDao.selectStudentById  - ==> Parameters: 17(Integer)
[service] 2018-07-21 23:44:49,517 - dao.IStudentDao.selectStudentById -1393 [main] DEBUG dao.IStudentDao.selectStudentById  - <==      Total: 1
Student [id=17, name=hello, age=14, score=94.6]