执行prepareStatement(sql)时出现空指针异常NullPointerExceptio

在JAVA执行数据库SELECT语句前，必定要记得先connect数据库DriverManager.getConnection(urle,username, password );

否则会报空指针错误java.lang.NullPointerException

statement = conn.prepareStatement(sql);  
rs = statement.executeQuery(sql);