【前端开发系列】—— 别说你不会Ajax

之前一直都是用封装好的Ajax,所以一直很好奇它是如何使用和实现的。这里正好就进行一下学习，下面是Ajax的一个时间图。

设置触发条件

　　这里模拟一个使用场景，就是在用户登陆时，异步的对用户名以及密码进行验证。所以使用onBlur()触发，onBlur函数在输入框焦点遗失时，就会触发。

     <form name="loginForm">

         <table>

             <tr>

                 <td>用戶名:<input type="text" name="username" onBlur="checkUsername()"></td>

             </tr>

             <tr>

                 <td><div id="usernameDiv"></div></td>

             </tr>

             <tr>

                 <td>密 码:<input type="password" name="password" onBlur="checkPassword()"></td>

             </tr>

             <tr>

                 <td><div id="passwordDiv"></div></td>

             </tr>

         </table>

     </form>

创建Ajax的对象

　　由于Ajax并不是一个统一的标准，因此各个浏览器有自己的创建方式，因此在创建时，需要进行浏览器的验证，使用如下的验证方法即可。

 if(window.XMLHttpRequest){

             req = new XMLHttpRequest();

         }else if(window.ActiveXObject){

             req = new ActiveXObject("Microsoft.XMLHTTP");

         }

open() onreadystatechante() send()

3.1 open(),这个函数有三个参数，第一个是http的发送方式，常见的有GET和POST，我们这里使用GET，来传送参数；第二个参数是一个url，这个url第一个作用是与后台的servlet进行匹配，第二个作用是传送前台的数据到后台；第三个参数是个bool值，如果为true标识用异步的方式发送请求。

req.open("GET",url+"?username="+document.loginForm.username.value,true);

3.2 onreadystatechange 指定回调函数，当数据返回时，使用该函数进行处理。我们的处理函数是自己定义的usernameCallback

req.onreadystatechange = usernameCallback;

3.3 send函数用来发送创建好的请求。由于前面的参数已经添加到url后面，因此这里直接使用null作为参数就行了。

req.send(null);

　　自己定义的usernameCallback函数，用于把返回的字符串使用DOM方式写入指定的div中。

　　在函数返回时，可以使用之前创建好的req对象，调用它的readystate 以及 status属性，4表示XMLHttpRequest对象响应正常结束。200表示HTTP请求获得正确的响应。

　　只有这两个条件都满足，才表明异步请求请求成功。

function usernameCallback(){

        if(req.readyState == 4){

            if(req.status == 200){

                var str = req.responseText;

                if(str.split("!")[0] == "Success"){

                    document.getElementById("usernameDiv").className = "black";

                }else{

                    document.getElementById("usernameDiv").className = "red";

                }

                document.getElementById("usernameDiv").innerHTML = str;

            }else{

                alert("username failed!");

            }

        }else{

        }

    }

通过url与后台的servlet关联

<?xml version="1.0" encoding="GBK"?>

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">

  <servlet>

      <servlet-name>checkUsername</servlet-name>

    <servlet-class>usernameServlet</servlet-class>

  </servlet>

  <servlet-mapping>

      <servlet-name>checkUsername</servlet-name>

      <url-pattern>/checkUsername</url-pattern>

  </servlet-mapping>

  <servlet>

      <servlet-name>checkPassword</servlet-name>

    <servlet-class>passwordServlet</servlet-class>

  </servlet>

  <servlet-mapping>

      <servlet-name>checkPassword</servlet-name>

    <url-pattern>/checkPassword</url-pattern>

  </servlet-mapping>

</web-app>

简单反馈

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

        response.setContentType("text/xml;charset=UTF-8");

        response.setHeader("Cache-Control", "no-cache");

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

//        System.out.println("username"+username);

        String str = "";

        if("xingoo".equals(username)){

            str += "Success!" + username;

        }else{

            str += "failed!" + username;

        }

        response.getWriter().write(str);

    }

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

        doGet(request,response);

    }

另一个验证的servlet采用同样的方式，这里就不再赘述了。下面会贴出所有的代码。

源码暴露

 <!DOCTYPE html>

 <html>

 <head>

 <meta charset="UTF-8">

 <title>Insert title here</title>

 <style type="text/css">

      .red {

          color:red;

          font-size: 20;

      }

      .black {

          color: black;

          font-size: 20;

      }

 </style>

 <script language="javascript">

     function checkUsername(){

         var url = "checkUsername";

         if(window.XMLHttpRequest){

             req = new XMLHttpRequest();

         }else if(window.ActiveXObject){

             req = new ActiveXObject("Microsoft.XMLHTTP");

         }

         if(req){

             req.open("GET",url+"?username="+document.loginForm.username.value,true);

             req.onreadystatechange = usernameCallback;

             req.send(null);

         }

     }

     function checkPassword(){

         var url = "checkPassword";

         if(window.XMLHttpRequest){

             req = new XMLHttpRequest();

         }else if(window.ActiveXObject){

             req = new ActiveXObject("Microsoft.XMLHTTP");

         }

         if(req){

             req.open("GET",url+"?password="+document.loginForm.password.value,true);

             req.onreadystatechange = passwordCallback;

             req.send(null);

         }

     }

     function usernameCallback(){

         if(req.readyState == 4){

             if(req.status == 200){

                 var str = req.responseText;

                 if(str.split("!")[0] == "Success"){

                     document.getElementById("usernameDiv").className = "black";

                 }else{

                     document.getElementById("usernameDiv").className = "red";

                 }

                 document.getElementById("usernameDiv").innerHTML = str;

             }else{

                 alert("username failed!");

             }

         }else{

         }

     }

     function passwordCallback(){

         if(req.readyState == 4){

             if(req.status == 200){

                 var str = req.responseText;

                 if(str.split("!")[0] == "Success"){

                     document.getElementById("passwordDiv").className = "black";

                 }else{

                     document.getElementById("passwordDiv").className = "red";

                 }

                 document.getElementById("passwordDiv").innerHTML = str;

             }else{

                 alert("password failed!");

             }

         }else{

         }

     }

 </script>

 </head>

 <body>

     <form name="loginForm">

         <table>

             <tr>

                 <td>用戶名:<input type="text" name="username" onBlur="checkUsername()"></td>

             </tr>

             <tr>

                 <td><div id="usernameDiv"></div></td>

             </tr>

             <tr>

                 <td>密 码:<input type="password" name="password" onBlur="checkPassword()"></td>

             </tr>

             <tr>

                 <td><div id="passwordDiv"></div></td>

             </tr>

         </table>

     </form>

 </body>

 </html>

web.xml

<?xml version="1.0" encoding="GBK"?>

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">

  <servlet>

      <servlet-name>checkUsername</servlet-name>

    <servlet-class>usernameServlet</servlet-class>

  </servlet>

  <servlet-mapping>

      <servlet-name>checkUsername</servlet-name>

      <url-pattern>/checkUsername</url-pattern>

  </servlet-mapping>

  <servlet>

      <servlet-name>checkPassword</servlet-name>

    <servlet-class>passwordServlet</servlet-class>

  </servlet>

  <servlet-mapping>

      <servlet-name>checkPassword</servlet-name>

    <url-pattern>/checkPassword</url-pattern>

  </servlet-mapping>

</web-app>

usernameServlet.java

import java.io.IOException;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class usernameServlet extends HttpServlet {

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

        response.setContentType("text/xml;charset=UTF-8");

        response.setHeader("Cache-Control", "no-cache");

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

//        System.out.println("username"+username);

        String str = "";

        if("xingoo".equals(username)){

            str += "Success!" + username;

        }else{

            str += "failed!" + username;

        }

        response.getWriter().write(str);

    }

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

        doGet(request,response);

    }

}

passwordServlet.java

import java.io.IOException;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

public class passwordServlet extends HttpServlet {

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

        response.setContentType("text/xml;charset=UTF-8");

        response.setHeader("Cache-Control", "no-cache");

        String password = request.getParameter("password");

//        System.out.println("password"+password);

        String str = "";

        if("123".equals(password)){

            str += "Success!" + password;

        }else{

            str += "failed!" + password;

        }

        response.getWriter().write(str);

    }

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

        doGet(request,response);

    }

}

运行效果

这里直接判断返回状态，进行样式的改变

成功时的效果！

失败时的效果！