数据库

为什么我两次sql查询，查出来的对象引用是同一个？

Posted by zhangtao on Wednesday, July 22, 2020

回答这个问题，我们需要掌握MyBatis的缓存概念，如果我们两次查询，一次是数据库查询，而第二次直接取得缓存中的数据，那么这两次查询查询的数据就是同一个对象

Mybatis的一级缓存是Session级别的缓存。一级缓存的作用域默认是一个SqlSession。Mybatis默认开启一级缓存。

在同一个SqlSession中，执行相同的查询SQL，第一次会去数据库进行查询，并把对象放入缓存中，第二次以后是直接去缓存中取。

当执行SQL查询中间发生了事务提交（增删改都会触发自动提交）的操作，都会把当前SqlSession的缓存清空。

只要两条SQL的下列五个值相同，即可以认为是相同的SQL。

Statement Id + Offset + Limmit + Sql + Params

CacheKey cacheKey = new CacheKey();
//MappedStatement的id
// id 就是Sql语句的所在位置 包名 + 类名 + SQL名称
cacheKey.update(ms.getId());
// offset 就是 0
cacheKey.update(rowBounds.getOffset());
// limit 就是 Integer.MAXVALUE
cacheKey.update(rowBounds.getLimit());
// 具体的SQL语句
cacheKey.update(boundSql.getSql());
//后面是update了sql中带的参数
cacheKey.update(value);
...

下面我以例子说明：

一级缓存开启

Demo1:

@Test
public void testLocalCache() throws Exception {
   
    SqlSession sqlSession = factory.openSession(true);
    StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    //第一次经过数据库查询
    System.out.println(studentMapper.getStudentById(1));
    // 同一个sqlSession,走缓存
    System.out.println(studentMapper.getStudentById(1));
    System.out.println(studentMapper.getStudentById(1));
    sqlSession.close();
}

执行结果

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
StudentEntity{
   id=1, name='小岑', age=16, className='null'}

我们发现，当我们开启一级缓存，同一个sql查询只有第一次经过了数据库，后面两次都走了缓存

Demo2:

@Test
public void testLocalCacheClear() throws Exception {
   
    SqlSession sqlSession = factory.openSession(true); // true为自动提交事务
    StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    System.out.println(studentMapper.getStudentById(1));
    // 自动提交事务，清空了缓存
    System.out.println("增加了" + studentMapper.addStudent(buildStudent()) + "个学生");
    // 缓存为空，重新查询数据库
    System.out.println(studentMapper.getStudentById(1));
    sqlSession.close();
}

执行结果

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - ==>  Preparing: INSERT INTO student(name,age) VALUES(?, ?) 
DEBUG [main] - ==> Parameters: 明明(String), 20(Integer)
DEBUG [main] - <==    Updates: 1
增加了1个学生
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
StudentEntity{
   id=1, name='小岑', age=16, className='null'}

提交事务会把缓存清空

Demo3

@Test
public void testLocalCacheScope() throws Exception {
   
    SqlSession sqlSession1 = factory.openSession(true); // true为自动提交事务
    SqlSession sqlSession2 = factory.openSession(true);
    // 两个不同的sqlSession
    StudentMapper studentMapper = sqlSession1.getMapper(StudentMapper.class);
    StudentMapper studentMapper2 = sqlSession2.getMapper(StudentMapper.class);
    // 第一次从数据库中读
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentById(1));
    // 一个session,从一级缓存中读
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentById(1));
    // 清空sqlSession2的一级缓存
    System.out.println("studentMapper2更新了" + studentMapper2.updateStudentName("小岑",1) + "个学生的数据");
    // sqlSession1的一级缓存还在
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentById(1));
    // sqlSession2一级缓存没了，所以重新从数据库读
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentById(1));
}

执行结果：

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
studentMapper读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
studentMapper读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - ==>  Preparing: UPDATE student SET name = ? WHERE id = ? 
DEBUG [main] - ==> Parameters: 小岑(String), 1(Integer)
DEBUG [main] - <==    Updates: 1
studentMapper2更新了1个学生的数据
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
studentMapper读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
studentMapper2读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}

一级缓存只会影响一个session，不同的session互不影响

mybatis中的二级缓存是mapper级别的缓存。

二级缓存开启后，同一个namespace下的所有操作语句，都影响着同一个Cache，即二级缓存被多个SqlSession共享。

当开启缓存后，数据的查询执行的流程就是 二级缓存 -> 一级缓存 -> 数据库。

需要注意的是，只有当事务提交或者Session关闭之后，才会把查到的数据放到二级缓存中

img

我们可以在mapper.xml中添加来启用二级缓存

二级缓存的执行原理和前面提到的一级缓存是差不多的，二级缓存与一级缓存区别在于二级缓存的范围更大，多个sqlSession可以共享一个mapper中的二级缓存区域。

下面我以例子说明：

一级、二级缓存开启

Demo1

@Test
public void testCacheWithoutCommitOrClose() throws Exception {
   
    SqlSession sqlSession1 = factory.openSession(true); // true为自动提交事务
    SqlSession sqlSession2 = factory.openSession(true);
    // 两个不同的sqlSession
    StudentMapper studentMapper = sqlSession1.getMapper(StudentMapper.class);
    StudentMapper studentMapper2 = sqlSession2.getMapper(StudentMapper.class);
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentById(1));
    // Mapper的nameSpace相同，不管是不是同一个session，都可以用二级缓存
    // 但是因为sqlSession1未提交，所以没有存入二级缓存。
    // 只能从数据库查出
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentById(1));
}

执行结果

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
studentMapper读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
studentMapper2读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}

session未提交的话，是不会放入二级缓存的

Demo2

@Test
public void testCacheWithCommitOrClose() throws Exception {
   
    SqlSession sqlSession1 = factory.openSession(true); // true为自动提交事务
    SqlSession sqlSession2 = factory.openSession(true);
    StudentMapper studentMapper = sqlSession1.getMapper(StudentMapper.class);
    StudentMapper studentMapper2 = sqlSession2.getMapper(StudentMapper.class);
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentById(1));
    // sqlSession1关闭或事务提交，则放入二级缓存
    sqlSession1.close();
    // 从二级缓存中取出
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentById(1));
}

执行结果

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 小岑, 16
DEBUG [main] - <==      Total: 1
studentMapper读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.5
studentMapper2读取数据: StudentEntity{
   id=1, name='小岑', age=16, className='null'}

同一个nameSpace，session1提交后，可从二级缓存查出

Demo3

@Test
public void testCacheWithUpdate() throws Exception {
   
    SqlSession sqlSession1 = factory.openSession(true);
    SqlSession sqlSession2 = factory.openSession(true);
    SqlSession sqlSession3 = factory.openSession(true);
    StudentMapper studentMapper = sqlSession1.getMapper(StudentMapper.class);
    StudentMapper studentMapper2 = sqlSession2.getMapper(StudentMapper.class);
    StudentMapper studentMapper3 = sqlSession3.getMapper(StudentMapper.class);
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentById(1));
    sqlSession1.close();
    // sqlSession1关闭后，可从二级缓存中查出
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentById(1));
    studentMapper3.updateStudentName("方方",1);
    // 提交会清空二级缓存
    sqlSession3.commit();
    // 从数据库查
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentById(1));
}

执行结果

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 方方, 16
DEBUG [main] - <==      Total: 1
studentMapper读取数据: StudentEntity{
   id=1, name='方方', age=16, className='null'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.5
studentMapper2读取数据: StudentEntity{
   id=1, name='方方', age=16, className='null'}
DEBUG [main] - ==>  Preparing: UPDATE student SET name = ? WHERE id = ? 
DEBUG [main] - ==> Parameters: 方方(String), 1(Integer)
DEBUG [main] - <==    Updates: 1
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.3333333333333333
DEBUG [main] - ==>  Preparing: SELECT id,name,age FROM student WHERE id = ? 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age
TRACE [main] - <==        Row: 1, 方方, 16
DEBUG [main] - <==      Total: 1
studentMapper2读取数据: StudentEntity{
   id=1, name='方方', age=16, className='null'}

Demo4

@Test
public void testCacheWithDiffererntNamespace() throws Exception {
   
    SqlSession sqlSession1 = factory.openSession(true); // 自动提交事务
    SqlSession sqlSession2 = factory.openSession(true); // 自动提交事务
    SqlSession sqlSession3 = factory.openSession(true); // 自动提交事务
    StudentMapper studentMapper = sqlSession1.getMapper(StudentMapper.class);
    StudentMapper studentMapper2 = sqlSession2.getMapper(StudentMapper.class);
    ClassMapper classMapper = sqlSession3.getMapper(ClassMapper.class);
    System.out.println("studentMapper读取数据: " + studentMapper.getStudentByIdWithClassInfo(1));
    sqlSession1.close();
    // 可从二级缓存中查出
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentByIdWithClassInfo(1));
    // 提交只清空了classMapper的二级缓存，没有清空StudentMapper的二级缓存，所以读到了脏值
    classMapper.updateClassName("特色一班",1);
    sqlSession3.commit();
    // 仍然从缓存中查出
    System.out.println("studentMapper2读取数据: " + studentMapper2.getStudentByIdWithClassInfo(1));
}

执行结果

DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.0
DEBUG [main] - ==>  Preparing: SELECT s.id,s.name,s.age,class.name as className FROM classroom c JOIN student s ON c.student_id = s.id JOIN class ON c.class_id = class.id WHERE s.id = ?; 
DEBUG [main] - ==> Parameters: 1(Integer)
TRACE [main] - <==    Columns: id, name, age, className
TRACE [main] - <==        Row: 1, 方方, 16, 特色一班
DEBUG [main] - <==      Total: 1
studentMapper读取数据: StudentEntity{
   id=1, name='方方', age=16, className='特色一班'}
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.5
studentMapper2读取数据: StudentEntity{
   id=1, name='方方', age=16, className='特色一班'}
DEBUG [main] - ==>  Preparing: UPDATE class SET name = ? WHERE id = ? 
DEBUG [main] - ==> Parameters: 特色一班(String), 1(Integer)
DEBUG [main] - <==    Updates: 1
DEBUG [main] - Cache Hit Ratio [mapper.StudentMapper]: 0.6666666666666666
studentMapper2读取数据: StudentEntity{
   id=1, name='方方', age=16, className='特色一班'}

资源地址 https://github.com/zntzhang/mybatis-cache-demo.git

参考资料：

MyBatis 核心配置综述之 细说一级缓存

MyBatis中的二级缓存

聊聊MyBatis缓存机制

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