Mybatis入门（一）
在实际项目中，Mybatis通常是与Spring Boot相结合的，Spring Boot帮我们做了大量自动化配置。 这次我们仅仅使用maven来构建Mybatis项目，学习Mybatis的用法。
maven依赖
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.5.6</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.23</version>
</dependency>
配置XML
Mybatis的核心是SqlSessionFactory，而SqlSessionFactory是通过SqlSessionFactoryBuilder获得。 SqlSessionFactoryBuilder则需要从XML文件或Configuration实例中获得。我们选择使用XML。在resources目录下 创建mybatis-config.xml。driver里填入驱动，这边使用的是MySQL数据库，所以是com.mysql.cj.jdbc.Driver。url、username、password是数据库信息。
<?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="com/hunter/mapper/ArticleMapper.xml"/>
</mappers>
</configuration>
构建SqlSessionFactoryBuilder以及SqlSessionFactory
在项目下面建一个Main类
public class Main {
public static void main(String[] args) throws IOException {
String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
//通过Builder获得Factory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//获得SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
ArticleMapper mapper = sqlSession.getMapper(ArticleMapper.class);
List<Article> allArticles = mapper.getAllArticles();
System.out.println(allArticles);
sqlSession.close();
}
}
Article类
public class Article {
private Integer id;
private String name;
private Integer watch;
private Boolean enabled;
@Override
public String toString() {
return "Article{" +
"id=" + id +
", name='" + name + '\'' +
", watch=" + watch +
", enabled=" + enabled +
'}';
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getWatch() {
return watch;
}
public void setWatch(Integer watch) {
this.watch = watch;
}
public Boolean getEnabled() {
return enabled;
}
public void setEnabled(Boolean enabled) {
this.enabled = enabled;
}
}
ArticleMapper
public interface ArticleMapper {
List<Article> getAllArticles();
}
相应的xml
<?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.hunter.mapper.ArticleMapper">
<select id="getAllArticles" resultType="com.hunter.model.Article">
select * from article
</select>
</mapper>
启动项目，发现报错了。
这是一个新手非常容易犯的错误，问题不在于Mybatis而是Maven。可以看项目编译后的结果，配置的xml文件不见了， 因为默认Maven会过滤java包里的除java之外的文件。
解决方法有2个
- 修改Maven配置
<build> <resources> <resource> <directory>src/main/java</directory> <includes> <include>**/*.xml</include> </includes> </resource> <resource> <directory>src/main/resources</directory> </resource> </resources> </build>OK，修改完之后发现可以查出结果了
-
把xml放到resources包下面，同时记得修改Mybatis配置文件的mapper位置
OK，发现也能查询出结果。
项目改进
接下来我们需要对项目进行改进
-
首先是配置，如果每次新增一个mapper都要在这里加，过于麻烦。
<mappers> <mapper resource="com/hunter/mapper/ArticleMapper.xml"/> </mappers>我们采用统一的包扫描，这样只要在这个包下面新增mapper.xml，就会自动识别
<mappers> <package name="com.hunter.mapper"/> </mappers> -
Factory是由Builder构造的，采用一个静态单例模式创造
public class SqlSessionFactoryUtil { private static SqlSessionFactory FACTORY = null; public static SqlSessionFactory getInstance() { if (FACTORY == null) { try { return new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml")); } catch (IOException e) { e.printStackTrace(); } } return FACTORY; } }这样我们的Main方法也变成了
public class Main { public static void main(String[] args) throws IOException { SqlSessionFactory factory = SqlSessionFactoryUtil.getInstance(); SqlSession sqlSession = factory.openSession(); ArticleMapper mapper = sqlSession.getMapper(ArticleMapper.class); List<Article> allArticles = mapper.getAllArticles(); System.out.println(allArticles); sqlSession.close(); } } -
把Sql语句打印到控制台，方便调试。加入log4j相关依赖
<dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-api</artifactId> <version>2.14.1</version> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.17</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-log4j12</artifactId> <version>RELEASE</version> </dependency>配置log4j
### 设置### log4j.rootLogger = debug,stdout ### 输出信息到控制抬 ### log4j.appender.stdout = org.apache.log4j.ConsoleAppender log4j.appender.stdout.Target = System.out log4j.appender.stdout.layout = org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%nOK，现在可以在控制台看到相关SQL语句了。
-
取别名
在mybatis配置文件里，可以对类取别名，如
<typeAliases> <typeAlias alias="Article" type="com.hunter.model.Article"/> </typeAliases>这样查询的时候可以不用全部输入了
<select id="getAllArticles" resultType="Article"> select * from article </select>