MyBatis系列-动态连接数据库

前言

MyBatis是目前java项目连接数据库的最流行的orm框架了。常用的使用方法是配置datasource，编写dao和mapper，最后通过依赖注入调用dao的方法来操作数据。本篇使用MyBatis来动态生成MyBatis的对象，动态配置数据库连接操作数据库。

动态数据库连接

使用MyBaits常规方式是在xml或者properties来配置MyBatis，但是这样会把配置参数写死。如果想要动态访问数据库，我们能自己手动实例化MyBatis对象。

DataSource对象

还记得我们在经常在xml配置文件中，配置一个Datasource对象，设置数据连接相关参数。这里我们使用代码动态实例化一个PooledDataSource

PooledDataSource dataSource = new PooledDataSource();
dataSource.setDriver("com.mysql.cj.jdbc.Driver");
dataSource.setUrl("jdbc:mysql://127.0.0.1:3306/test");
dataSource.setUsername("root");
dataSource.setPassword("123456");

mapper路径

ResourcePatternResolver resolver = new PathMatchingResourcePatternResolver();
Resource[] resources = resolver.getResources("classpath:/mapper/*Mapper.xml");

MybatisSqlSessionFactoryBean对象

MybatisSqlSessionFactoryBean sqlSessionFactoryBean = new MybatisSqlSessionFactoryBean();
sqlSessionFactoryBean.setMapperLocations(resources);
sqlSessionFactoryBean.setDataSource(dataSource);

SqlSessionFactory对象

SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBean.getObject();

SqlSession对象

记住，这里的sqlsession生成后，请保存到一个HashMap就像一个数据库连接池一样，可以反复使用。否则，每次数据库操作都会打开一个新的数据库连接，造成数据库连接超限，导致数据库瘫痪。

SqlSession sqlsession = sqlSessionFactory.openSession();
// 调用数据库操作方法
sqlsession.close()

我们平常使用MyBatis时，会在xml或者properties文件中配置的参数，然后MyBatis会使用这些参数实例化以上对象。

动态sql语句

动态配置了MyBatis的数据库连接，并生成MyBatis对象。接下来，就是常规的利用MyBatis的Mapper模板来实现数据的增删改查了。

DynamicMapper类

public interface DynamicMapper {
    List<Map<String, Object>> selectSql(@Param("tableName") String tableName, @Param("fields") String fields, @Param("whereCondition") String whereCondition, @Param("orderCondition") String orderCondition);

    Integer insertSql(@Param("tableName") String tableName, @Param("result") String result);

    Integer updateSql(@Param("tableName") String tableName, @Param("result") String result, @Param("whereCondition") String whereCondition);

    Integer deleteSql(@Param("tableName") String tableName, @Param("whereCondition") String whereCondition);
}

DynamicMapper.xml

<mapper namespace="com.camellibby.pangu.openapi.mapper.DynamicMapper">
	<select id="selectSql" resultType="java.util.LinkedHashMap">
		select ${fields} from ${tableName}
		<if test="whereCondition != null and whereCondition != ''">
			where ${whereCondition}
		if>
		<if test="orderCondition != null and orderCondition != ''">
			order by ${orderCondition}
		if>
	select>

	<insert id="insertSql">
		insert into ${tableName} ${result}
	insert>

	<update id="updateSql">
		UPDATE ${tableName} SET ${result} WHERE ${whereCondition}
	update>

	<delete id="deleteSql">
		delete from ${tableName} WHERE ${whereCondition}
	delete>
mapper>

DynamicMapper动态运行sql语句

DynamicMapper dynamicMapper = sqlsession.getMapper(DynamicMapper.class);
List<Map<String, Object>> users = dynamicMapper.selectSql(
		"users",
		"username, password, enabled",
		"enabled=0",
		"username"
);
users.forEach(user ->
        user.keySet().forEach(key ->
			System.out.println(key + ":" + user.get(key))
		)
);

测试

运行测试用例，进行sql查询，输出如下

03:32:46.245 [main] DEBUG org.mybatis.spring.transaction.SpringManagedTransaction - JDBC Connection [com.mysql.cj.jdbc.ConnectionImpl@68044f4] will not be managed by Spring
03:32:46.260 [main] DEBUG DynamicMapper.selectSql - ==>  Preparing: select username, password, enabled from users where enabled=0 order by username 
03:32:46.338 [main] DEBUG DynamicMapper.selectSql - ==> Parameters: 
03:32:46.385 [main] DEBUG DynamicMapper.selectSql - <==      Total: 6
username:Chandler
password:{bcrypt}$2a$10$/8MMuGgPbNXoiMgvS3uabuQiRMW3f96wLP/ifZ3VcSHo28OuzB8Hu
enabled:false
username:Joey
password:{bcrypt}$2a$10$/8MMuGgPbNXoiMgvS3uabuQiRMW3f96wLP/ifZ3VcSHo28OuzB8Hu
enabled:false
username:Monica
password:{bcrypt}$2a$10$/8MMuGgPbNXoiMgvS3uabuQiRMW3f96wLP/ifZ3VcSHo28OuzB8Hu
enabled:false
username:Phoebe
password:{bcrypt}$2a$10$/8MMuGgPbNXoiMgvS3uabuQiRMW3f96wLP/ifZ3VcSHo28OuzB8Hu
enabled:false
username:Rachel
password:{bcrypt}$2a$10$/8MMuGgPbNXoiMgvS3uabuQiRMW3f96wLP/ifZ3VcSHo28OuzB8Hu
enabled:false
username:Ross
password:{bcrypt}$2a$10$/8MMuGgPbNXoiMgvS3uabuQiRMW3f96wLP/ifZ3VcSHo28OuzB8Hu
enabled:false

和数据库查询结果一致，结果如下