用Mybatis编写数据库查询

1.Mybatis简介

• MyBatis 是一款优秀的持久层框架&＃xff0c;它支持自定义 SQL、存储过程以及高级映射。
• MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。
• MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO&＃xff08;Plain Old Java Objects&＃xff0c;普通老式 Java 对象&＃xff09;为数据库中的记录。

2.持久层

数据持久化:将程序的数据在持久状态和瞬间状态转化的过程。

持久层:完成持久化工作的代码块。

3.创建一个Mybatis项目

3.1 mysql数据库建表

3.2 搭建环境

idea导入maven依赖&＃xff08;pom.xml&＃xff09;

<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>dependency>dependencies>


3.3 xml配置

连接mysq数据库(mybatis-config.xml)



<configuration><environments default&＃61;"development"><environment id&＃61;"development"><transactionManager type&＃61;"JDBC"/><dataSource type&＃61;"POOLED"><property name&＃61;"driver" value&＃61;"com.mysql.jdbc.Driver"/><property name&＃61;"url" value&＃61;"jdbc:mysql://localhost:3306/mybatis?useSSL&＃61;false&useUnicode&＃61;true&characterEncoding&＃61;UTF-8"/><property name&＃61;"username" value&＃61;"root"/><property name&＃61;"password" value&＃61;"123456"/>dataSource>environment>environments>


3.4 编写Mybatis工具类

• 拿到Sqlsession对象(utils)

public class MybatisUtils {private static SqlSessionFactory sqlSessionFactory;static {try {//使用Mybatis获取sqlSessionFactory对象String resource &＃61; "mybatis-config.xml";InputStream inputStream&＃61; Resources.getResourceAsStream(resource);sqlSessionFactory &＃61; new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) {e.printStackTrace();}}//通过SqlsessionFactory来获取Sqlsession,Sqlsession完全包含了面向数据库执行SQL命令的所有方法public static SqlSession getSqlSession(){return sqlSessionFactory.openSession();}
}


• 数据实体类(pojo)

public class User {private int id;private String name;private String pwd;//getter and setter...//toString()...
}


3.5 Dao层

• 接口&＃xff08;UserDao&＃xff09;

public interface UserDao {List<User> getUserList();
}


• 接口实现&＃xff08;UserMapper.xml&＃xff09;




<mapper namespace&＃61;"com.ming.dao.UserDao"><select id&＃61;"getUserList" resultType&＃61;"com.ming.pojo.User">select * from mybatis.userselect>
mapper>


3.6 测试

• 测试代码

&＃64;Testpublic void test(){//第一步&＃xff1a;获得SqLsession对象SqlSession sqlSession&＃61;MybatisUtils.getSqlSession();//执行sql:getMapperUserDao userDao&＃61;sqlSession.getMapper(UserDao.class);List<User> userList&＃61;userDao.getUserList();for(User user:userList){System.out.println(user);}sqlSession.close();}


• 运行结果

3.7 常见错误

• 空指针异常

原因&＃xff1a;MybatisUtils类中SqlsessionFactory 已在开头声明过&＃xff0c;

try{}中就不用再声明了。

解决&＃xff1a;将

SqlSessionFactory sqlSessionFactory &＃61; new SqlSessionFactoryBuilder().build(inputStream);


改为&＃xff1a;

sqlSessionFactory &＃61; new SqlSessionFactoryBuilder().build(inputStream);


• 绑定异常

原因&＃xff1a;mapper没有注册

解决&＃xff1a;在mybatis核心配置文件(mybatis-config.xml)中注册

• 初始化异常

原因&＃xff1a;UserMapper.xml没有配置成功&＃xff08;默认配置resource路径下的xml&＃xff09;

解决&＃xff1a;在项目的pom.xml中将UserMapper.xml配置到resource路径下

<build><resources><resource><directory>src/main/javadirectory><includes><include>**/*.xmlinclude><include>**/*.propertiesinclude>includes><filtering>falsefiltering>resource><resource><directory>src/main/resourcesdirectory><includes><include>**/*.xmlinclude><include>**/*.propertiesinclude>includes><filtering>falsefiltering>resource>resources>build>