在任何网站注册用户的时候，都会检查用户是否已经存在。很久以前的处理方式是将所有数据提交到服务器端进行验证，很显然这种方式的用户体验很不好；后来有 了Ajax，有了异步交互，当用户输完用户名继续填写其他信息的时候，Ajax就将信息发到了服务器去检查该用户名是否已经被注册了，这样如果用户名已经 存在，不用等用户将所有数据都提交就可以给出提示。采用这种方式大大改善了用户体验。

regist.jsp

代码如下:

<%@ page language="java" contentType="text/html; charset=UTF-8"

pageEncoding="UTF-8"%>

用户名:

密 码:

CheckServlet.java

代码如下:

public class CheckServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

public static final String DBDRIVER = "com.microsoft.sqlserver.jdbc.SQLServerDriver";

public static final String DBURL = "jdbc:sqlserver://localhost:1433;DatabaseName=bbs";

public static final String DBUSER = "sa";

public static final String DBPASS = "pass";

public CheckServlet() {

super();

}

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

this.doPost(request, response);

}

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

request.setCharacterEncoding("UTF-8");

response.setContentType("text/html");

Connection conn = null;

PreparedStatement pst = null;

ResultSet rs = null;

PrintWriter out = response.getWriter();

String username = request.getParameter("usernaem");

try{

Class.forName(DBDRIVER);

conn = DriverManager.getConnection(DBURL,DBUSER,DBPASS);

String sql = "select count(username) from user where username=?";

pst = conn.prepareStatement(sql);

pst.setString(1,username);

rs = pst.executeQuery();

if(rs.next()){

if(rs.getInt(1)>0){//用户名已经存在了

out.print("true");

}else{

out.print("false");

}

}

}catch(Exception e){

e.printStackTrace();

}finally{

try{

conn.close();

}catch(Exception e){

e.printStackTrace();

}

}

}

}

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