读取操作系统环境变量，模拟创建连接池，连接MySQL数据库

1、读取操作系统设置的环境变量信息

// 读取环境变量来连接数据库
		String hais_db_host = System.getenv("hais_db_host");
		String hais_db_port = System.getenv("hais_db_port");
		String hais_db_name = System.getenv("hais_db_name");
		String hais_db_user_name = System.getenv("hais_db_user_name");
		String hais_db_password = System.getenv("hais_db_password");
		String url = "jdbc:mysql://" + hais_db_host + ":" + hais_db_port + "/" + hais_db_name
				+ "?characterEncoding=UTF-8";

2、模拟创建连接池，通过拼接方式连接数据库

//创建连接池，包括url，passwd和user等
			PooledDataSource pooledDataSource = new PooledDataSource("com.mysql.jdbc.Driver", url, hais_db_user_name,
					hais_db_password);
			//创建工厂事务
			TransactionFactory transactionFactory = new JdbcTransactionFactory();
			Environment environment = new Environment("cybatis", transactionFactory, pooledDataSource);
			Configuration configuration = new Configuration(environment);
			configuration.addMapper(SQLMapper.class);
			//创建会话工厂
			sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);
		


//创建会话
	public static SqlSession getSession() throws IOException {
		SqlSession sqlSession = sqlSessionFactory.openSession();
		return sqlSession;
	}
//开始连接
		SqlSession sqlSession = getSession();
		SQLMapper sqlMapper = (SQLMapper) sqlSession.getMapper(SQLMapper.class);
		List<String> results= sqlMapper.getResults();