如何获得Mybatis?
<dependency>
<groupId>org.mybatisgroupId>
<artifactId>mybatisartifactId>
<version>3.5.2version>
dependency>
数据持久化
为什么需要持久化?
Dao层、Service层、Controller层
最重要的一点:使用的人多
思路:搭建环境–>导入Mybatis–>编写代码–>测试
搭建数据库
CREATE DATABASE MYBATIS;
USE MYBATIS;
CREATE TABLE user(
id INT(20) NOT NULL PRIMARY KEY,
name VARCHAR(30) DEFAULT NULL,
pwd VARCHAR(30) DEFAULT NULL
)ENGINE = INNODB DEFAULT CHARSET = utf8;
INSERT INTO user(id, name, pwd) VALUES
(1, '果冻奶酪', 123456),
(2, '张三', 138775),
(3, '李四', 175383);
新建项目
1.新建一个普通的maven项目
2.删除src目录
3.导入maven依赖
<dependencies>
<dependency>
<groupId>mysqlgroupId>
<artifactId>mysql-connector-javaartifactId>
<version>5.1.47version>
dependency>
<dependency>
<groupId>org.mybatisgroupId>
<artifactId>mybatisartifactId>
<version>3.5.2version>
dependency>
<dependency>
<groupId>junitgroupId>
<artifactId>junitartifactId>
<version>4.12version>
<scope>testscope>
dependency>
dependencies>
<configuration>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=UTF-8"/>
<property name="username" value="root"/>
<property name="password" value="fj123456"/>
dataSource>
environment>
environments>
configuration>
//sqlSessionFactory --> sqlSession
public class MybatisUtils {
//提升作用域
private static SqlSessionFactory sqlSessionFactory;
static {
try {
//使用Mybatis第一步:获取sqlSessionFactory对象
String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
//既然有了 SqlSessionFactory,顾名思义,我们可以从中获得 SqlSession 的实例。
//sqlSession 完全包含了面向数据库执行SQL 命令所需的所有方法
public static SqlSession getSqlSession(){
return sqlSessionFactory.openSession();
}
}
//实体类
public class User {
private int id;
private String name;
private String pwd;
public User() {
}
public User(int id, String name, String pwd) {
this.id = id;
this.name = name;
this.pwd = pwd;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getPwd() {
return pwd;
}
public void setPwd(String pwd) {
this.pwd = pwd;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", name='" + name + '\'' +
", pwd='" + pwd + '\'' +
'}';
}
}
//Dao接口
public interface UserDao {
List<User> getUserList();
}
<mapper namespace="com.fj.dao.UserDao">
<select id="getUserList" resultType="com.fj.pojo.User">
select * from mybatis.user;
select>
mapper>
注意点:
maven由于约定大于配置,我们之后可以能够遇到我们写的配置文件,无法被导出或者生效的问题,解决方案:
<build>
<resources>
<resource>
<directory>src/main/resourcesdirectory>
<includes>
<include>**/*.propertiesinclude>
<include>**/*.xmlinclude>
includes>
<filtering>truefiltering>
resource>
<resource>
<directory>src/main/javadirectory>
<includes>
<include>**/*.propertiesinclude>
<include>**/*.xmlinclude>
includes>
<filtering>truefiltering>
resource>
resources>
build>
MapperRegistry是什么?
核心配置文件中注册mappers
junit测试
@Test
public void test(){
//第一步:获得SqlSession对象
SqlSession sqlSession = MybatisUtils.getSqlSession();
//方式一:getMapper
UserDao userDao = sqlSession.getMapper(UserDao.class);
List<User> userList = userDao.getUserList();
for(User user : userList){
System.out.println(user.toString());
}
//关闭SqlSession
sqlSession.close();
}
可能会遇到的问题:
maven编译出现 Process terminated 错误
Failed to execute goal org.apache.maven.plugins:maven-surefire-plugin:2.12.4:
### Error querying database. Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
namespace中的包名和 Dao/Mapper 接口中的包名一致
选择,查询语句;
1、编写接口
//根据ID查询用户
User getUserByID(int id);
2、编写对应mapper中的sql执行语句
<select id="getUserByID" parameterType="int" resultType="com.fj.pojo.User">
select * from mybatis.user where id = #{id};
select>
3、测试
@Test
public void getUserByID(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = mapper.getUserByID(1);
System.out.println(user);
sqlSession.close();
}
插入语句
1、编写接口
//插入一个用户信息
int addUser(User user);
2、编写对应mapper中的sql执行语句
<insert id="addUser" parameterType="com.fj.pojo.User">
insert into mybatis.user (id, name, pwd) values (#{id}, #{name}, #{pwd});
insert>
3、测试
//增删改需要提交事务
@Test
public void addUser(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = new User(4, "王五", "1324214");
int result = mapper.addUser(user);
if (result > 0){
System.out.println("插入成功!");
}
//提交事务
sqlSession.commit();
sqlSession.close();
}
更新语句
1、编写接口
//修改用户
int updateUser(User user);
2、编写对应mapper中的sql执行语句
<update id="updateUser" parameterType="com.fj.pojo.User">
update mybatis.user set name = #{name}, pwd = #{pwd} where id = #{id};
update>
3、测试
@Test
public void updateUser(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = new User(4, "旺财", "14432434");
mapper.updateUser(user);
sqlSession.commit();
sqlSession.close();
}
删除语句
1、编写接口
//删除用户
int deleteUser(int id);
2、编写对应mapper中的sql执行语句
<delete id="deleteUser" parameterType="int">
delete from mybatis.user where id = #{id};
delete>
3、测试
@Test
public void deleteUser(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
mapper.deleteUser(4);
sqlSession.commit();
sqlSession.close();
}
注意点:
假设我们的实体类,数据库中的表、字段或者参数过多,我们应当考虑使用Map
//万能的Map
int addUser1(Map<String, Object> map);
<insert id="addUser1" parameterType="Map">
insert into mybatis.user (id, name, pwd) values (#{userid}, #{username}, #{password});
insert>
@Test
public void addUser1(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
Map<String, Object> map = new HashMap<>();
map.put("userid", 5);
map.put("username", "赵六");
map.put("password", "144324324");
mapper.addUser1(map);
sqlSession.close();
}
Map传递参数,直接在sql中取出key即可【parameterType=“Map”】
对象传递参数,直接在sql中取对象的属性即可【parameterType=“Object”】
只有一个基本类型参数的情况下,可以直接在sql中取到,不需要parameterType的设置
多个参数用Map,或者注解
模糊查询怎么写?
1、Java代码执行的时候,传递通配符 % %
List<User> list = mapper.getUserLike("%李%");
2、在sql拼接中使用通配符
select * from mybatis.user where name like "%" #{
value} "%";
configuration(配置)
properties(属性)
settings(设置)
typeAliases(类型别名)
typeHandlers(类型处理器)
objectFactory(对象工厂)
plugins(插件)
environments(环境配置)
environment(环境变量)
transactionManager(事务管理器)
dataSource(数据源)
databaseIdProvider(数据库厂商标识)
mappers(映射器)
MyBatis 可以配置成适应多种环境
尽管可以配置多个环境,但每个 SqlSessionFactory 实例只能选择一种环境。
学会使用配置多套运行环境
Mybatis默认的事务管理器是JDBC,连接池:POOLED
我们可以通过properties属性来实现引用配置文件
这些属性都是可外部配置且可动态替换的,既可以在典型的 Java 属性文件中配置,也可以在 properties 元素的子元素中来传递。【db.properties】
编写一个配置文件
db.properties
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/mybatis?useSSL=false&useUnicode=false&serverTimezone=GMT
username=root
password=fj123456
在核心配置文件中引入
<properties resource="db.properties">
<property name="username" value="root"/>
<property name="password" value="122434"/>
properties>
<typeAliases>
<typeAlias type="com.fj.pojo.User" alias="User"/>
typeAliases>
也可以指定一个包名,MyBatis 会在包名下面搜索需要的 Java Bean,比如:
扫描实体类的包,它的默认别名就为这个类的 类名,首字母小写(大写也可以)
<typeAliases>
<package name="com.fj.pojo"/>
typeAliases>
在实体类比较少的情况,使用第一种方式
如果实体类十分多,建议使用第二种
第一种可以DIY别名,第二种则不行,如果非要改,需要在实体类上增加注解
@Alias("hello")
public class User {
}
这是 MyBatis 中极为重要的调整设置,它们会改变 MyBatis 的运行时行为。
MapperRegistry:注册绑定我们的Mapper文件;
方式一:【推荐使用】
<mappers>
<mapper resource="com/fj/dao/UserMapper.xml"/>
mappers>
<mappers>
<mapper class="com.fj.dao.UserMapper"/>
mappers>
注意点:
<mappers>
<package name="com.fj.dao"/>
mappers>
注意点:
生命周期和作用域是至关重要的,因为错误的使用会导致非常严重的并发问题。
SqlSessionFactoryBuilder
SqlSessionFactory
SqlSession
数据库中的字段
新建一个项目,拷贝之前的,测试实体类字段不一致的情况
public class User {
private int id;
private String name;
private String pwd;
}
select * from mybatis.user where id = #{id};
类型处理器
select id,name,pwd from mybatis.user where id = #{id};
解决方案:
select id,name,pwd as password from mybatis.user where id = #{id};
结果集映射
<resultMap id="UserMap" type="com.fj.pojo.User">
<result column="id" property="id"/>
<result column="name" property="name"/>
<result column="pwd" property="password"/>
resultMap>
<select id="getUserByID" parameterType="int" resultMap="UserMap">
select * from mybatis.user where id = #{id};
select>
如果一个数据库操作,出现了异常,我们需要排错,那么日志是最好的助手
在Mybatis中具体使用哪一日志实现,在设置中设定
STDOUT_LOGGING 标准日志输出
<settings>
<setting name="logImpl" value="STDOUT_LOGGING"/>
settings>
什么是LOG4J?
1、先导入LOG4J的包
<dependency>
<groupId>log4jgroupId>
<artifactId>log4jartifactId>
<version>1.2.17version>
dependency>
2、log4j.properties
#将等级为DEBUG的日志信息输出到console和file这两个目的地,console和file的定义在下面的代码
log4j.rootLogger=DEBUG,console,file
#控制台输出的相关设置
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.Target=System.out
log4j.appender.console.Threshold=DEBUG
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=[%c]-%m%n
#文件输出的相关设置
log4j.appender.file=org.apache.log4j.RollingFileAppender
log4j.appender.file.File=./logs/fj.log
log4j.appender.file.MaxFileSize=10mb
log4j.appender.file.Threshold=DEBUG
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=[%p][%d{yy-MM-dd}][%c]%m%n
#日志输出级别
log4j.logger.org.mybatis=DEBUG
log4j.logger.java.sql=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.ResultSet=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG
3、配置log4j为日志的实现
<settings>
<setting name="logImpl" value="LOG4J"/>
settings>
4、Log4j的使用,直接测试运行查询
简单使用
1、在要使用 Log4j 的类中,导入包:import org.apache.log4j.Logger;
2、日志对象,参数为当前类的class
static Logger logger = Logger.getLogger(UserDaoTest.class);
3、日志级别
logger.info("info:进入了testLog4j方法");
logger.debug("debug:进入了testLog4j方法");
logger.error("error:进入了testLog4j方法");
思考:为什么要分页?
语法:SELECT * FROM user limit startIndex,pageSize;
SELECT * FROM user limit n; #[0,n]
使用Mybatis实现分页,核心SQL
1、接口
//分页
List<User> getUserByLimit(Map<String,Integer> map);
2、Mapper.xml
<select id="getUserByLimit" parameterType="map" resultMap="UserMap">
select * from mybatis.user limit #{startIndex}, #{pageSize};
select>
3、测试
@Test
public void getUserByLimit(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
Map<String,Integer> map = new HashMap<>();
map.put("startIndex", 0);
map.put("pageSize", 3);
List<User> userByLimit = mapper.getUserByLimit(map);
for (User user : userByLimit){
System.out.println(user);
}
sqlSession.close();
}
不再使用SQL实现分页
1、接口
//分页2
List<User> getUserByRowBounds();
2、Mapper.xml
<select id="getUserByLimit" parameterType="map" resultMap="UserMap">
select * from mybatis.user limit #{startIndex}, #{pageSize};
select>
3、测试
@Test
public void getUserByRowBounds(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
//RowBoundS实现
RowBounds rowBounds = new RowBounds(1, 2);
//通过Java代码层面实现分页
List<User> userList = sqlSession.selectList("com.fj.dao.UserMapper.getUserByRowBounds", null, rowBounds);
for (User user : userList){
System.out.println(user);
}
sqlSession.close();
}
分页插件地址
关于接口的理解
三个面向区别
1、注解在接口上实现
@Select("select * from user")
List<User> getUsers();
2、在核心配置文件中绑定接口
<!-- 绑定接口 -->
<mappers>
<mapper class="com.fj.dao.UserMapper"/>
</mappers>
3、测试
@Test
public void test(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
//底层主要应用反射
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
List<User> users = mapper.getUsers();
for (User user : users){
System.out.println(user);
}
sqlSession.close();
}
本质:反射机制实现
底层:动态代理
我们可以在工具类创建的时候实现自动提交事务
public static SqlSession getSqlSession(){
return sqlSessionFactory.openSession(true);
}
编写接口,添加注解
@Select("select * from user")
List<User> getUsers();
// 方法中如果有多个参数,那么每一个参数前都必须加上@Param("")
@Select("select * from user where id = #{id}")
User getUserByID(@Param("id") int id);
// User getUserByID(@Param("id") int id, @Param("name") String name);
@Insert("insert into user (id, name, pwd) values (#{id}, #{name}, #{password})")
int addUser(User user);
@Update("update user set name = #{name}, pwd = #{password} where id = #{id}")
int updateUser(User user);
@Delete("delete from user where id = #{id}")
int deleteUser(@Param("id") int id);
测试类
【注意,我们必须要将接口绑定到核心配置文件中】
mybatis-config.xml文件中
<mappers>
<mapper class="com.fj.dao.UserMapper"/>
mappers>
关于@Param()注解
Project Lombok is a java library that automatically plugs into your editor and build tools, spicing up your java.
Never write another getter or equals method again, with one annotation your class has a fully featured builder, Automate your logging variables, and much more.
使用步骤:
1、在IDEA中安装Lombok插件
2、在项目中导入Lombok的jar包
<dependency>
<groupId>org.projectlombokgroupId>
<artifactId>lombokartifactId>
<version>1.18.10version>
dependency>
3、在实体类上加注解
@Data
@AllArgsConstructor
@NoArgsConstructor
@Getter and @Setter
@FieldNameConstants
@ToString
@EqualsAndHashCode
@AllArgsConstructor, @RequiredArgsConstructor and @NoArgsConstructor
@Log, @Log4j, @Log4j2, @Slf4j, @XSlf4j, @CommonsLog, @JBossLog, @Flogger, @CustomLog
@Data
@Builder
@SuperBuilder
@Singular
@Delegate
@Value
@Accessors
@Wither
@With
@SneakyThrows
说明
@Data:无参构造、get、set、tostring、hashcode、equals
@AllArgsConstructor
@NoArgsConstructor
@EqualsAndHashCode
@ToString
SQL:
CREATE TABLE teacher(
id INT(10) NOT NULL,
name VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (id)
)ENGINE=INNODB;
INSERT INTO teacher (id, name) VALUES (1, '果冻奶酪');
CREATE TABLE student(
id INT(10) NOT NULL,
name VARCHAR(30) DEFAULT NULL,
tid INT(10) DEFAULT NULL,
PRIMARY KEY(id),
KEY fktid (tid),
CONSTRAINT fktid FOREIGN KEY (tid) REFERENCES teacher (id)
)ENGINE=INNODB;
INSERT INTO student (id, name, tid) VALUES (1, '小明', 1);
INSERT INTO student (id, name, tid) VALUES (2, '小红', 1);
INSERT INTO student (id, name, tid) VALUES (3, '小张', 1);
INSERT INTO student (id, name, tid) VALUES (4, '小李', 1);
INSERT INTO student (id, name, tid) VALUES (5, '小王', 1);
1、导入lombok
<dependencies>
<!-- https://mvnrepository.com/artifact/org.projectlombok/lombok -->
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
<version>1.18.10</version>
</dependency>
</dependencies>
2、新建实体类Teacher、Student
@Data
public class Teacher {
private int id;
private String name;
}
@Data
public class Student {
private int id;
private String name;
//学生需要关联一个老师
private Teacher teacher;
}
3、建立Mapper接口
public interface TeacherMapper {
@Select("select * from teacher where id = #{id}")
Teacher getTeacher(@Param("id") int id);
}
public interface StudentMapper {
}
4、建立Mapper.xml文件
<mapper namespace="com.fj.dao.StudentMapper">
mapper>
<mapper namespace="com.fj.dao.TeacherMapper">
mapper>
5、核心配置文件中绑定注册Mapper接口或者文件
<mappers>
<mapper class="com.fj.dao.TeacherMapper"/>
<mapper class="com.fj.dao.StudentMapper"/>
mappers>
6、测试查询是否成功
@Test
public void test(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
Teacher teacher = mapper.getTeacher(1);
System.out.println(teacher);
sqlSession.close();
}
<select id="getStudent" resultMap="StudentTeacher">
select * from student;
select>
<resultMap id="StudentTeacher" type="Student">
<result property="id" column="id"/>
<result property="name" column="name"/>
<association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
resultMap>
<select id="getTeacher" resultType="Teacher">
select * from teacher where id = #{id};
select>
<select id="getStudent2" resultMap="StudentTeacher2">
select s.id sid, s.name sname, t.name tname
from student s, teacher t
where s.tid = t.id
select>
<resultMap id="StudentTeacher2" type="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<association property="teacher" javaType="Teacher">
<result property="name" column="tname"/>
association>
resultMap>
MySQL多对一查询方式:
比如一个老师有多个学生
对于老师而言,就是一对多的关系
1、环境搭建
实体类
@Data
public class Student {
private int id;
private String name;
private int tid;
}
@Data
public class Teacher {
private int id;
private String name;
//一个老师拥有多个学生
private List<Student> students;
}
<select id="getTeacher" resultMap="TeacherStudent">
select s.id sid, s.name sname, t.name tname,t.id tid
from teacher t, student s
where t.id = s.tid and t.id = #{tid};
select>
<resultMap id="TeacherStudent" type="Teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<collection property="students" ofType="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
collection>
resultMap>
<select id="getTeacher2" resultMap="TeacherStudent2">
select * from mybatis.teacher where id = #{tid};
select>
<resultMap id="TeacherStudent2" type="Teacher">
<result property="id" column="id"/>
<result property="name" column="name"/>
<collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherID" column="id"/>
resultMap>
<select id="getStudentByTeacherID" resultType="Student">
select * from mybatis.student where tid = #{tid};
select>
1、关联 - association【多对一】
2、集合 - collection【一对多】
3、javaType & ofType
注意点:
慢SQL:
面试必问:
什么是动态SQL:动态SQL就是根据不同的条件生成不同的SQL语句
利用动态 SQL,可以彻底摆脱这种痛苦。
如果你之前用过 JSTL 或任何基于类 XML 语言的文本处理器,你对动态 SQL 元素可能会感觉似曾相识。在 MyBatis 之前的版本中,需要花时间了解大量的元素。借助功能强大的基于 OGNL 的表达式,MyBatis 3 替换了之前的大部分元素,大大精简了元素种类,现在要学习的元素种类比原来的一半还要少。
if
choose (when, otherwise)
trim (where, set)
foreach
CREATE TABLE blog(
id varchar(50) NOT NULL COMMENT '博客id',
title varchar(100) NOT NULL COMMENT '博客标题',
author varchar(30) NOT NULL COMMENT '博客作者',
create_time datetime NOT NULL COMMENT '创建时间',
views int(30) NOT NULL COMMENT '浏览量'
)ENGINE=InnoDB;
创建一个基础工程
1、导包
2、编写配置文件
3、编写实体类
4、编写对应实体类的Mapper接口
5、编写对应Mapper.xml文件
6、测试
<select id="queryBlogIF" parameterType="map" resultType="Blog">
select * from mybatis.blog where 1 = 1
<if test="title != null">
and title = #{title}
if>
<if test="author != null">
and author = #{author}
if>
select>
<select id="queryBlogChoose" parameterType="map" resultType="blog">
select * from mybatis.blog
<where>
<choose>
<when test="title != null">
title = #{title}
when>
<when test="author != null">
and author = #{author}
when>
<otherwise>
and views = #{views}
otherwise>
choose>
where>
select>
<update id="updateBlog" parameterType="map">
update mybatis.blog
<set>
<if test="title != null">
title = #{title},
if>
<if test="author != null">
author = #{author}
if>
set>
where id = #{id}
update>
所谓的动态SQL,本质还是SQL语句,只是我们可以在SQL层面,去执行一个逻辑代码
if
where , choose , set , when
有时候,我们可能会将一些公共的部分抽取出来,方便复用
1、用sql标签抽取公共的部分
<sql id="if_title_author">
<if test="title != null">
title = #{title}
if>
<if test="author != null">
and author = #{author}
if>
sql>
2、在需要的地方用include标签引用
<select id="queryBlogIF" parameterType="map" resultType="blog">
select * from mybatis.blog
<where>
<include refid="if_title_author">include>
where>
select>
注意事项:
select * from user where 1=1 and
<foreach item="id" collection="ids"
open="(" separator="or" close=")">
#{id}
</foreach>
(id = 1 or id = 2 or id = 3)
<select id="queryBlogForeach" parameterType="map" resultType="blog">
select * from mybatis.blog
<where>
<foreach item="id" collection="ids" open="(" separator="or" close=")">
id = #{id}
foreach>
where>
select>
动态SQL就是拼接SQL语句,我们只要保证SQL语句的正确性,按照SQL的格式,去排列组合就可以了
建议:
查询: 连接数据库,消耗资源
一次查询的结果,可以暂存在内存(缓存)中
再次查询相同的数据时直接走缓存,不需要走数据库
1、什么是缓存[ Cache ]?
2、为什么使用缓存?
3、什么样的数据能使用缓存?
测试步骤:
1、开启日志
<settings>
<setting name="logImpl" value="STDOUT_LOGGING"/>
settings>
2、测试在一个Session中查询两次相同记录
@Test
public void test(){
SqlSession sqlSession = MybatisUtils.getSqlSession();
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
User user = mapper.queryUserByID(1);
System.out.println(user);
System.out.println("=========================");
User user2 = mapper.queryUserByID(1);
System.out.println(user2);
System.out.println(user == user2);
sqlSession.close();
}
缓存失效的情况:
1、查询不同的东西
2、增删改操作,可能会改变原来的数据,所以必定会刷新缓存
小结:一级缓存默认是开启的,只在一次SqlSession中有效,也就是拿到连接到关闭连接这个区间段
步骤:
1、开启全局缓存
<setting name="cacheEnabled" value="true"/>
2、在要使用二级缓存的Mapper中开启
<cache/>
也可以自定义参数
<cache
eviction="FIFO"
flushInterval="60000"
size="512"
readOnly="true"/>
3、测试
1、问题:我们需要将实体类序列化,否则就会报错
Caused by:java.io.NotSerializableException: com.fj.pojo.User
public class User implements Serializable {
}
小结:
Ehcache是一种广泛使用的开源java分布式缓存,主要面向通用缓存
要在程序中使用,先导包
<dependency>
<groupId>org.mybatis.cachesgroupId>
<artifactId>mybatis-ehcacheartifactId>
<version>1.1.0version>
dependency>
在mapper中指定使用我们的ehcache缓存实现
<cache type="org.mybatis.caches.ehcache.EhcacheCache"/>
ecache.xml
<ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://ehcache.org/ehcache.xsd"
updateCheck="false">
<diskStore path="./tmpdir/Tmp_EhCache"/>
<defaultCache
eternal="false"
maxElementsInMemory="10000"
overflowToDisk="false"
diskPersistent="false"
timeToIdleSeconds="1800"
timeToLiveSeconds="259200"
memoryStoreEvictionPolicy="LRU"/>
<cache
name="cloud_user"
eternal="false"
maxElementsInMemory="5000"
overflowToDisk="false"
diskPersistent="false"
timeToIdleSeconds="1800"
timeToLiveSeconds="1800"
memoryStoreEvictionPolicy="LRU"/>
ehcache>
Redis数据库来做缓存,K-V
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