从 XML 中构建 SqlSessionFactory
每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为核心的。
SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。
而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或一个预先配置的 Configuration 实例来构建出 SqlSessionFactory 实例。
String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
从 XML 文件中构建 SqlSessionFactory 的实例非常简单,建议使用类路径下的资源文件进行配置。 但也可以使用任意的输入流(InputStream)实例,比如用文件路径字符串或 file:// URL 构造的输入流。MyBatis 包含一个名叫 Resources 的工具类,它包含一些实用方法,使得从类路径或其它位置加载资源文件更加容易。
<?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>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="org/mybatis/example/BlogMapper.xml"/>
</mappers>
</configuration>
XML 配置文件中包含了对 MyBatis 系统的核心设置,包括获取数据库连接实例的数据源(DataSource)以及决定事务作用域和控制方式的事务管理器(TransactionManager)。后面会再探讨 XML 配置文件的详细内容,这里先给出一个简单的示例:
粗俗的解释:就是从SqlSessionFactory 这个工厂中 获取到 一个SqlSession 然后利用SqlSession 来执行已经映射的SQL 语句。
try (SqlSession session = sqlSessionFactory.openSession()) {
BlogMapper mapper = session.getMapper(BlogMapper.class);
Blog blog = mapper.selectBlog(101);
}
但是我们在具体中 会将 获取这个SqlSession 这个方法 提取成一个 工具类 直接 获取到 而不是像文档中这样
public class MybatisUtil {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource="mybatis-config.xml";
InputStream inputStream=Resources.getResourceAsStream(resource);
sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSession(){
return sqlSessionFactory.openSession();
}
}
这一步就是看 我们自己获取到的SqlSession 我们拿它来干嘛啦
我们执行的sql 语句在哪里 ? 一个语句既可以通过 XML 定义,也可以通过注解定义
<?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="org.mybatis.example.BlogMapper">
<select id="selectBlog" resultType="Blog">
select * from Blog where id = #{id}
</select>
</mapper>
注解配置
public interface BlogMapper {
@Select("SELECT * FROM blog WHERE id = #{id}")
Blog selectBlog(int id);
}
**注意:**使用注解来映射简单语句会使代码显得更加简洁,但对于稍微复杂一点的语句,Java 注解不仅力不从心
通过什么获取到sql 语句 ?
通过核心配置文件中的 mappers 配置 这是其中一种
<mappers>
<mapper resource="mapper/UserMapper.xml"/>
</mappers>
第一步是配置环境
第二步是进行配置
mybatis 核心配置文件
<?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>
<!-- STDOUT_LOGGING 打印到控制台 -->
<setting name="logImpl" value="STDOUT_LOGGING"/>
<!-- 开启驼峰 -->
<setting name="mapUnderscoreToCamelCase" value="true"/>
</settings>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"></transactionManager>
<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>
</environments>
<mappers>
<!-- test1() <package name="com.study.dao"/>-->
<mapper resource="mapper/UserMapper.xml"/>
</mappers>
</configuration>
MySQL 的连接数据
jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/test
jdbc.username=root
jdbc.password=123456
这是后面用到的sql 映射的 xml 文件
<?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="com.study.dao.UserMapper">
<select id="selectAll" resultType="com.study.pojo.User">
select * from user
</select>
</mapper>
导入包 加资源 过滤
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.12</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.49</version>
</dependency>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.5.5</version>
</dependency>
</dependencies>
<build>
<resources>
<resource>
<directory>src/main/java</directory>
<includes>
<include>**/*.properties</include>
<include>**/*.xml</include>
</includes>
<filtering>true</filtering>
</resource>
</resources>
</build>
建立User 类
public class User {
private Integer id;
private String username;
private String password;
public User() {
}
public User(Integer id, String username, String password) {
this.id = id;
this.username = username;
this.password = password;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", password='" + password + '\'' +
'}';
}
}
UserMapper
public interface UserMapper {
@Select("SELECT * FROM user WHERE id = #{id}")
User selectId( @Param("id") Integer id);
List<User> selectAll();
}
mybatis 提取出来的工具类
public class MybatisUtil {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource="mybatis-config.xml";
InputStream inputStream=Resources.getResourceAsStream(resource);
sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSession(){
return sqlSessionFactory.openSession();
}
}
测试
/*
* 给定一个id
* 然后进行查询
* 使用注解查询
* */
@org.junit.Test
public void test1(){
SqlSession session = MybatisUtil.getSession();
UserMapper mapper = session.getMapper(UserMapper.class);
User user = mapper.selectId(1);
System.out.println(user);
session.close();
}
/*
* 查询全部 user 数据
* 使用xml 配置 select 语句
* */
@org.junit.Test
public void test2(){
SqlSession session = MybatisUtil.getSession();
UserMapper mapper = session.getMapper(UserMapper.class);
List<User> users = mapper.selectAll();
System.out.println(users);
session.close();
}