ajax实现检测用户是否登录

select count(*) from tablename where name="name";

定义一个 boolean变量，则此语句只有两种结果。true  false

返回JSON格式

java文件：

public class CheckUserJSON extends HttpServlet {

    public CheckUserJSON() {

        super();

    }

    public void destroy() {

        super.destroy(); // Just puts "destroy" string in log

        // Put your code here

    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

        response.setContentType("application/json");

        PrintWriter out = response.getWriter();

        Connection conn=null;

        PreparedStatement ps = null;

        ResultSet rs = null;

        int checkresult = 0;

        try{

          Class.forName("com.mysql.jdbc.Driver").newInstance(); 

        }

        catch(Exception e) {    

          e.printStackTrace();    

        } 

        try{

          conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/jquerydb?user=root&password=abc123");    

          ps = conn.prepareStatement("select count(*) from user where username=?");

          ps.setString(1, request.getParameter("username"));

          rs = ps.executeQuery();

          if (rs.next()) {

            checkresult = rs.getInt(1);

          }

          rs.close();

            ps.close();

            conn.close();

        }

        catch(SQLException e) {    

              e.printStackTrace();    

        }

        out.print("{\"result\":");

        out.print(checkresult);

        out.print("}");

        out.flush();

        out.close();

    }

  

    public void doPost(HttpServletRequest request, HttpServletResponse response)

            throws ServletException, IOException {

         doGet(request, response);

    }

    public void init() throws ServletException {

        // Put your code here

    }

}

前端jsp：

$('input').on('blur',function(event){

              if(this.name=="username"&&this.value!=""){

                  $.getJSON('checkUser.php',{username:this.value},function(json){

                      if(json.result)//获取后端返回结果

                          $('#usernmae').next.text('用户已经存在').addClass('error');

                      else

                          $('#usernam').next.text('用户可用').addClass('success');

                      }

                  });

              }

}