Mybatis基础概念与高级应用小结 Mybatis基础概念与高级应用小结

Mybatis基础回顾与高级应用

引入依赖
jdbc.properties
sqlMapConfig.xml

案例一-查询用户

案例二-添加用户
案例三-编辑用户
案例四-删除用户
传统开发方式
代理开发方式(使用JDK动态代理产生代理对象,由代理对象执行并且操作)
动态sql语句 if标签

Mybatis复杂映射

一对一
一对多
多对多

Mybatis注解开发

Mybatis注解实现复杂映射开发

Mybatis缓存

基础概念
一级缓存

返回结果为 true ; 测试一级缓存是默认开启的
返回结果为 false

二级缓存

如何使用二级缓存
返回结果为 false

Mybatis基础回顾与高级应用数据库:mysql5.7
jdk:15

引入依赖

org.mybatismybatis3.4.5mysqlmysql-connector-java5.1.6runtimejunitjunit4.12org.projectlomboklombok1.18.22

User实体

@Datapublic class User {private Integer id; private String username; }

jdbc.properties

jdbc.driver=com.mysql.jdbc.Driverjdbc.url=jdbc:mysql://127.0.0.1/zdy_mybatisjdbc.username=rootjdbc.password=root

sqlMapConfig.xml

???

案例一-查询用户 UserMapper.xml

select * from user

@Testpublic void test1() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSession// 默认开启一个事务，但是该事务不会自动提交//在进行增删改操作时，要手动提交事务SqlSession sqlSession = sqlSessionFactory.openSession(); //4.sqlSession调用方法：查询所有selectList查询单个：selectOne 添加：insert修改：update 删除：deleteList users = sqlSession.selectList("user.findAll"); users.forEach(item ->{System.out.println(item); }); sqlSession.close(); ?}

输出结果

User(id=1, username=Tom)
User(id=2, username=Jerry)

案例二-添加用户
UserMapper.xml

insert into user Values (#{id},#{username})

@Testpublic void test2() throws IOException {InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); SqlSession sqlSession = sqlSessionFactory.openSession(); User user = new User(); user.setId(3); user.setUsername("jack"); sqlSession.insert("user.saveUser",user); sqlSession.commit(); sqlSession.close(); }

数据库结果:

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案例三-编辑用户
UserMapper.xml

update user set username = #{username} where id = #{id}

@Testpublic void test3() throws IOException {InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); SqlSession sqlSession = sqlSessionFactory.openSession(); User user = new User(); user.setId(3); user.setUsername("rose"); sqlSession.update("user.updateUser",user); sqlSession.commit(); sqlSession.close(); }

数据库结果:

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案例四-删除用户
UserMapper.xml

delete from user where id =#{id}

@Testpublic void test4() throws IOException {InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); SqlSession sqlSession = sqlSessionFactory.openSession(); ?User user = new User(); user.setId(3); sqlSession.delete("user.deleteUser",3); sqlSession.commit(); ?sqlSession.close(); ?}

数据库结果:

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传统开发方式

public interface IUserDao {?//查询所有用户List findAll() throws IOException; }

UserDaoImpl

public class UserDaoImpl implements IUserDao {?@Overridepublic List findAll() throws IOException {InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); SqlSession sqlSession = sqlSessionFactory.openSession(); List users = sqlSession.selectList("user.findAll"); sqlSession.close(); return users; }}

@Testpublic void test5() throws IOException {UserDaoImpl dao = new UserDaoImpl(); List users = dao.findAll(); System.out.println(users); }

打印结果:

[User(id=1, username=Tom), User(id=2, username=Jerry)]

代理开发方式(使用JDK动态代理产生代理对象,由代理对象执行并且操作)
Mapper接口开发需要遵行以下规范:
mapper.xml文件中的namespace与mapper接口的全限定名相同;
2. mapper接口方法名和mapper.xml中定义的每个statement的id相同
3. mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同
4. mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同
根据上述的规范修改UserMapper.xml

?select * from user

public interface IUserDao {?//查询所有用户List findAll() throws IOException; }

@Testpublic void test6() throws IOException {InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); ?IUserDao mapper = sqlSession.getMapper(IUserDao.class); List all = mapper.findAll(); all.forEach(item ->{System.out.println(all); }); }

输出结果:

User(id=1, username=Tom)
User(id=2, username=Jerry)

动态sql语句 if标签

public interface IUserDao {?//多条件组合查询：演示ifpublic List findByCondition(User user); }

select * from user?and id = #{id}and username = #{username}

@Testpublic void test7() throws IOException {InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); ?IUserDao mapper = sqlSession.getMapper(IUserDao.class); ?User user1 = new User(); user1.setId(1); user1.setUsername("Tom"); ?List all = mapper.findByCondition(user1); for (User user : all) {System.out.println(user); }}

输出结果:

User(id=1, username=Tom)

动态sql语句 foreach标签

public interface IUserDao {?//多值查询：演示foreachpublic List findByIds(int[] ids); }

#{id}

@Testpublic void test8() throws IOException {InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml"); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream); SqlSession sqlSession = sqlSessionFactory.openSession(); ?IUserDao mapper = sqlSession.getMapper(IUserDao.class); ?int[] arr = {1,2}; ?List all = mapper.findByIds(arr); for (User user : all) {System.out.println(user); }}

输出结果:

User(id=1, username=Tom)
User(id=2, username=Jerry)

Mybatis复杂映射
一对一
User实体

@Datapublic class User {?private Integer id; ?private String username; ?//该用户所具有的订单信息private List orders; ?//该用户所具有的角色信息private List roles; }

Order实体

@Datapublic class Order {?private Integer id; ?private String orderTime; ?private BigDecimal total; ?//表明该订单属于哪个用户private User user; }

Role实体

@Datapublic class Role {?private Integer id; ?private String roleName; }

public interface IOrderMapper {?/*** 查询订单的同时还查询该订单所属的用户* @return*/public List findOrderAndUser(); }

??select * from orders o,user uwhere o.uid = u.id

@Testpublic void test1() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessionSqlSession sqlSession = sqlSessionFactory.openSession(); IOrderMapper mapper = sqlSession.getMapper(IOrderMapper.class); List orderAndUser = mapper.findOrderAndUser(); orderAndUser.forEach(order -> {System.out.println(order); }); }

运行结果:

Order(id=1, orderTime=2022-05-01, total=1000.00, user=User(id=1, username=Tom, orders=null, roles=null))
Order(id=2, orderTime=2022-05-10, total=2000.00, user=User(id=2, username=Jerry, orders=null, roles=null))
Order(id=3, orderTime=2022-05-20, total=3000.00, user=User(id=2, username=Jerry, orders=null, roles=null))

一对多

public interface IUserMapper {?/*** 查询所有用户信息,同时查询出每个用户关联的订单信息* @return*/public List findAll(); }

??select u.*,o.id oid,o.order_time,o.total,o.uid from user u left join orders o on u.id = o.uid

@Testpublic void test2() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessionSqlSession sqlSession = sqlSessionFactory.openSession(); IUserMapper mapper = sqlSession.getMapper(IUserMapper.class); List users = mapper.findAll(); users.forEach(user -> {System.out.println(user.getUsername()); System.out.println(user.getOrders()); }); }

运行结果:

Tom
[Order(id=1, orderTime=2022-05-01, total=1000.00, user=null)]
Jerry
[Order(id=2, orderTime=2022-05-10, total=2000.00, user=null), Order(id=3, orderTime=2022-05-20, total=3000.00, user=null)]

多对多

public interface IUserMapper {?/*** 查询所有用户信息,同事查询出每个用户关联的角色信息* @return*/public List findUserAndRole(); }

??SELECT * FROM USER uLEFT JOIN sys_user_role sur ON u.id = sur.user_idLEFT JOIN sys_role sr ON sur.role_id = sr.id

@Testpublic void test3() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessionSqlSession sqlSession = sqlSessionFactory.openSession(); IUserMapper mapper = sqlSession.getMapper(IUserMapper.class); List users = mapper.findUserAndRole(); users.forEach(user -> {System.out.println(user.getUsername()); System.out.println(user.getRoles()); }); }

运行结果:

Tom
[Role(id=null, roleName=董事长), Role(id=null, roleName=经理)]
Jerry
[Role(id=null, roleName=董事长), Role(id=null, roleName=经理)]

Mybatis注解开发 Mybasits常用注解:

@Insert: 实现新增@Update: 实现更新@Delete: 实现删除@Select: 实现查询@Result: 实现结果集封装; 他代替的是标签,该注解中可以使用单个@Result注解,也可以使用@Result集合,使用格式:@Results({@Result(),@Result()}) 或 @Results(@Result())@Results: 可以与@Result一起使用,封装多个结果集@One: 实现一对一结果集封装@Many: 实现一对多结果集封装

测试案例

public interface IUserMapper {?//添加用户@Insert("insert into user values(#{id},#{username})")public void addUser(User user); ?//更新用户@Update("update user set username = #{} where id = #{id}")public void updateUser(User user); ?//查询用户@Select("select * from user")public List getAllUser(); ?//删除用户@Delete("delete from user where id=#{id}")public void delUser(Integer id); }

private IUserMapper mapper; ?@Beforepublic void before() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessiontrue:事务自动提交SqlSession sqlSession = sqlSessionFactory.openSession(true); mapper = sqlSession.getMapper(IUserMapper.class); }?@Testpublic void addUser(){User user = new User(); user.setId(3); user.setUsername("jack"); mapper.addUser(user); }?@Testpublic void updateUser(){User user = new User(); user.setId(3); user.setUsername("rose"); mapper.updateUser(user); }?@Testpublic void getAllUser(){List userList = mapper.getAllUser(); userList.forEach(item -> {System.out.println(item); }); }?@Testpublic void delUser(){mapper.delUser(3); }

运行结果:

文章图片

Mybatis注解实现复杂映射开发
一对一

public interface IOrderMapper {?/*** 查询订单的同时还查询该订单所属的用户* @return*/@Results({@Result(property = "id",column = "id"),@Result(property = "orderTime",column = "order_time"),@Result(property = "total",column = "total"),@Result(property = "user",column = "uid",javaType = User.class,one = @One(select = "com.yun.mapper.IUserMapper.getUserById")),})@Select("select * from orders")public List findOrderAndUser(); }?public interface IUserMapper {@Select("select * from user where id = #{id}")public User getUserById(Integer id); }

@Testpublic void oneToOne(){List orderAndUser = orderMapper.findOrderAndUser(); orderAndUser.forEach(item -> {System.out.println(item); }); }

运行结果:

Order(id=1, orderTime=2022-05-01, total=1000.00, user=User(id=1, username=Tom, orders=null, roles=null))
Order(id=2, orderTime=2022-05-10, total=2000.00, user=User(id=2, username=Jerry, orders=null, roles=null))
Order(id=3, orderTime=2022-05-20, total=3000.00, user=User(id=2, username=Jerry, orders=null, roles=null))

一对多

public interface IUserMapper {?/*** 查询所有用户信息,同时查询出每个用户关联的订单信息* @return*/@Select("select * from user")@Results({@Result(property = "id",column = "id"),@Result(property = "username",column = "username"),@Result(property = "orders",column = "id",javaType = List.class,many = @Many(select = "com.yun.mapper.IOrderMapper.getOrderByUid"))})public List findAll(); }?public interface IOrderMapper {@Select("select * from orders where uid = #{uid}")public List getOrderByUid(Integer uid); }

@Testpublic void oneToMore(){List users = mapper.findAll(); users.forEach(item -> {System.out.println(item); }); ?}

运行结果:

User(id=1, username=Tom, orders=[Order(id=1, orderTime=null, total=1000.00, user=null)], roles=null)
User(id=2, username=Jerry, orders=[Order(id=2, orderTime=null, total=2000.00, user=null), Order(id=3, orderTime=null, total=3000.00, user=null)], roles=null)

多对多

public interface IUserMapper {/*** 查询所有用户信息,同事查询出每个用户关联的角色信息* @return*/@Select("select * from user")@Results({@Result(property = "id",column = "id"),@Result(property = "username",column = "username"),@Result(property = "roles",column = "id",javaType = List.class,many = @Many(select ="com.yun.mapper.IRoleMapper.getAll"))})public List findUserAndRole(); }?public interface IRoleMapper {?@Select("select * from sys_role sr,sys_user_role sur where sr.id = sur.role_id and sur.user_id = #{uid}")public List getAll(Integer uid); }

@Testpublic void moreToMore(){List users = mapper.findUserAndRole(); users.forEach(item -> {System.out.println(item); }); }

运行结果:

User(id=1, username=Tom, orders=null, roles=[Role(id=1, roleName=董事长), Role(id=2, roleName=经理)])
User(id=2, username=Jerry, orders=null, roles=[Role(id=1, roleName=董事长), Role(id=2, roleName=经理)])

Mybatis缓存
基础概念

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缓存就是内存中的数据,常常来自对数据库查询结果的保存,使用缓存,我们可以避免频繁的与数据库进行交互,进而提高响应速度.
一级缓存是SqlSession,在操作数据库时需要构造sqlSession对象,在对象中有一个数据结构(HashMap)用于存储缓存数据.不同的sqlSession之间互不影响.
二级缓存是mapper级别的缓存,多个sqlSession去操作同一个mapper的sql语句,多个sqlSession可以共用二级缓存,二级缓存是跨sqlSession的.

一级缓存
demo

public class CacheTest {?private IUserMapper mapper; private SqlSession sqlSession; ?@Beforepublic void before() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessiontrue:事务自动提交sqlSession = sqlSessionFactory.openSession(true); mapper = sqlSession.getMapper(IUserMapper.class); }?@Testpublic void test1() {?//第一次查询id为1的用户User user1 = mapper.getUserById(1); ?//第二次查询id为1的用户User user2 = mapper.getUserById(1); ?System.out.println(user1 == user2); }}

返回结果为 true ; 测试一级缓存是默认开启的结论:第一次发起查询用户id为1的用户信息,先去找缓存中是否有id为1的用户,如果没有,则从数据库查询用户信息,得到用户信息并且将用户信息存储到一级缓存中,第二次发起查询用户id为1的用户信息,先去找缓存中是否有id为1的用户,如果缓存中有,直接从缓存中获取用户信息; 现在我们变换一下上面的demo

public class CacheTest {?private IUserMapper mapper; private SqlSession sqlSession; ?@Beforepublic void before() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessiontrue:事务自动提交sqlSession = sqlSessionFactory.openSession(); mapper = sqlSession.getMapper(IUserMapper.class); }?@Testpublic void test1() {?//第一次查询id为1的用户User user1 = mapper.getUserById(1); ?User user = new User(); user.setId(1); user.setUsername("Lucy"); mapper.updateUser(user); ?sqlSession.commit(); ?//第二次查询id为1的用户User user2 = mapper.getUserById(1); ?System.out.println(user1 == user2); }}

返回结果为 false

结论:做增删改操作,并进行了事物的提交,就会刷新以及缓存; 或者还可以通过 sqlSession.clearCache()清楚缓存; 这样做的目的就是为了让缓存中存储的是最新的信息,避免脏读;

二级缓存
二级缓存的原理和一级缓存原理一样,第一次查询会将数据放入缓存中,然后第二次查询则会直接从缓存中获取,但是一级缓存是基于sqlSession的,而二级缓存是基于mapper文件的namespace,也就是说,多个sqlSession可以共享一个mapper中的二级缓存,并且如果两个mapper的namespace相同,即使是两个mapper,那么这两个mapper执行sql查询到的数据也将存在相同的二级缓存区域中.

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如何使用二级缓存首先在全局配置文件sqlMapconfig.xml文件加入如下代码

注意,该配置需要放在properties标签下,具体顺序,可百度了解
其次,在xxxMapper.xml文件中开启缓存(如果当前操作时基于注解开发的话,使用注解@CacheNamespace)

demo2

public class CacheTest {?private IUserMapper mapper; private SqlSession sqlSession; private SqlSessionFactory sqlSessionFactory; ?@Beforepublic void before() throws IOException {//1.Resources工具类，配置文件的加载，把配置文件加载成字节输入流InputStream inputStream = Resources.getResourceAsStream("sqlMApConfig.xml"); //2.解析了配置文件，并创建了sqlSessionFactory工厂sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); //3.生产sqlSessionsqlSession = sqlSessionFactory.openSession(); mapper = sqlSession.getMapper(IUserMapper.class); }?@Testpublic void test2() {?SqlSession sqlSession1 = sqlSessionFactory.openSession(); SqlSession sqlSession2 = sqlSessionFactory.openSession(); SqlSession sqlSession3 = sqlSessionFactory.openSession(); ?IUserMapper mapper1 = sqlSession1.getMapper(IUserMapper.class); IUserMapper mapper2 = sqlSession2.getMapper(IUserMapper.class); IUserMapper mapper3 = sqlSession3.getMapper(IUserMapper.class); ?User user1 = mapper1.getUserById(1); //清空一级缓存sqlSession1.close(); User user2 = mapper2.getUserById(1); ?System.out.println(user1 == user2); }}

运行结果为 false

结论:通过debug断点显示,实际上,第二次查询则会直接从缓存中获取用户信息了,不过二级缓存缓存的不是对象,而是缓存的对象中的数据,所以查询结果为false;

注意,二级缓存底层还是HashMap结构,所以 po类需要实现序列化接口 ; 因为二级缓存数据存储介质多种多样,不一定只存在内存中,有可能存在硬盘中,如果我们要在取这个缓存的话,就需要反序列化了,所以mybatis中的pojo都去实现Serializable接口;
变换一下demo2

@Testpublic void test2() {SqlSession sqlSession1 = sqlSessionFactory.openSession(); SqlSession sqlSession2 = sqlSessionFactory.openSession(); SqlSession sqlSession3 = sqlSessionFactory.openSession(); IUserMapper mapper1 = sqlSession1.getMapper(IUserMapper.class); IUserMapper mapper2 = sqlSession2.getMapper(IUserMapper.class); IUserMapper mapper3 = sqlSession3.getMapper(IUserMapper.class); User user1 = mapper1.getUserById(1); //清空一级缓存sqlSession1.close(); User user = new User(); user.setId(1); user.setUsername("Tom"); mapper3.updateUser(user); sqlSession3.commit(); User user2 = mapper2.getUserById(1); System.out.println(user1 == user2); }

返回结果为 false 结论:做增删改操作,并进行了事物的提交,就会刷新以及缓存; 这样做的目的就是为了让缓存中存储的是最新的信息,避免脏读;
此外mybatis中还可以配置useCache和flushCache等配置项;
useCache
是用来设置是否禁用二级缓存的,在statement中设置useCache=false可以禁用当前select语句的二级缓存,即每次查询都会会发出sql去查询,默认情况是true,即该sql使用二级缓存,例如

select u.*,o.id oid,o.order_time,o.total,o.uid from user u left join orders o on u.id = o.uid

使用sql注解方式可以使用@Options(useCache = false)的方式
flushCache
在mapper的同一个namespace中,如果有其他的insert,update,delete操作数据后需要刷新缓存,如果不执行刷新缓存会出现脏读,设置statememt配置中的
flushCache = "true"属性,默认情况下为true,即刷新缓存,如果改成false则不会刷新,使用缓存时如果手动修改数据库表中的查询数据会出现脏读例如

select u.*,o.id oid,o.order_time,o.total,o.uid from user u left join orders o on u.id = o.uid

一般下执行完commit操作都需要刷新缓存,flushCache=true表示刷新缓存,这样可以避免数据库脏读,所以我们不用设置,默认即可
至此,mybatis基础概念及应用回顾完成!
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