**

• sql注入问题：sql的特殊关键字与字符串拼接。会造成安全性的问题
• 1输入名随便，输入密码：a’ or ‘a’='a
• sql:select* from user where username=‘123’ and password=‘a’ or ‘a’=‘a’

**

//           1.获取链接
			 conn=JDBCUtils.getConnection();
			//2.定义语句的SQL
			 sql="select* from user where username='"+username+"' and password='"+password+"'";
		     //sql="select* form user where username='"+username+"'and password='"+password+"'";
			//3.获取执行SQL语句的对象
		    stmt=conn.createStatement();
			//4.执行查询
			rs=stmt.executeQuery(sql);

解决：使用Preparestatement

//1.获取链接
			 conn=JDBCUtils.getConnection();
			//2.定义语句的SQL
			 sql="select* from user where username=? and password=?";
			//3.获取执行SQL语句的对象
		    pstmt=conn.prepareStatement(sql);
		    //给？赋值
		    pstmt.setString(1	, username);
		    pstmt.setString(2   , password);
			//4.执行查询
			rs=pstmt.executeQuery();