今天学了ajax技术，特地在此写下来作为复习。
一、什么是ajax?
客户端（特指PC浏览器）与服务器，可以在【不必刷新整个浏览器】的情况下，与服务器进行异步通讯的技术 即，AJAX是一个【局部刷新】的【异步】通讯技术,
说白了就是局部刷新。
二、ajax的原理如下图
附上ajax与服务器之间的几种状态，但 4是所有浏览器都支持的的
三、ajax包含的技术如下图
四、ajax开发步骤
步一：创建ajax对象，例如：ajax = createAjax();
步二：开启异步对象：例如：ajax.open(method,url)
步三：如果是POST请求则要设置请求头，例如：ajax.setRequestHeader("content-type","application/x-www-form-urlencoded");这句话的作用是转码，如果遇到中文，将会自动转成UTF-8编码；
步四：发送请求：例如：如果是GET请求，ajax.send(null),因为GET请求的数据在请求头中就已经被发送了，即地址栏后的用?username=""
如果是POST请求，content一般写成这种形式，content = "username="+username; ajax.send(content);
步五：AJAX不断的监听服务端响应的状态变化，例如：ajax.onreadystatechange，后面写一个无名处理函数
步六：在无名处理函数中，获取AJAX的数据后，按照DOM规则，用JS语言来操作Web页面
五、ajax的应用例子，以用户登录为例，
1）使用GET方式，要注意IE浏览器，使用下面这条语句，防止IE自动缓存相同内容不刷新，
var url = "${pageContext.request.contextPath}/AjaxUserServlet?username="+username+"&time="+new Date().getTime();
jsp代码：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>通过使用GET,POST方式来使用ajax验证用户名是否重复</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
用户名：
<input type="text" id="username" maxlength="4"/> <span id="rsID"></span>
<hr/>
<!-- ajax对象 -->
<script type="text/javascript">
function createAjax()
{
var ajax =null;
try{
ajax = new ActiveXObject("microsoft.xmlhttp");
}catch(e1)
{
try{
ajax = new XMLHttpRequest();
}catch(e2)
{
alert("你的浏览器不支持AJAX异步对象");
}
}
return ajax;
}
</script>
<script type="text/javascript">
document.getElementById("username").onblur = function()
{
var username = this.value;
if(username.length == 0)
{
document.getElementById("rsID").innerHTML="用户名不能为空";
}
else
{
//对汉字进行编码
username = encodeURI(username);
//1)首先创建Ajax对象
var ajax = createAjax();
//2)打开连接
var method = "GET";
var url = "${pageContext.request.contextPath}/AjaxUserServlet?username="+username+"&time="+new Date().getTime();
ajax.open(method,url);
//3)真正发送数据
ajax.send(null);
//4)Ajax对象不断监听服务器响应的状态
ajax.onreadystatechange = function()
{
if(ajax.readyState == 4)
{
//5)响应码是200的话，从ajax对象中取出数据
if(ajax.status == 200)
{
var rsText = ajax.responseText;
document.getElementById("rsID").innerHTML=rsText;
}
}
}
}
}
</script>
</body>
</html>
java后台代码：在ajax与java后台之间使用流来传递数据的
package cn.itcast.js.user;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class AjaxUserServlet extends HttpServlet {
/**
* The doGet method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to get.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String username = request.getParameter("username");
byte [] buf = username.getBytes("ISO8859-1");
username = new String(buf,"UTF-8");
String tip = "<font color='green'>可以注册</font>";
if("杰克".equals(username))
{
tip = "<font color='red'>该用户已注册</font>";
}
System.out.println(username);
response.setContentType("text/html;charset=UTF-8");
PrintWriter pw = response.getWriter();
pw.write(tip);
pw.flush();
pw.close();
}
}
GET实现效果：
2）、POST方式实现ajax登录验证
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>通过使用GET,POST方式来使用ajax验证用户名是否重复</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
用户名：
<input type="text" id="username" maxlength="4"/>
<span id="rsID">
<!--
-->
</span>
<hr/>
<!-- 引入外部js文件 -->
<script type="text/javascript" src="js/ajax.js"> </script>
<script type="text/javascript">
document.getElementById("username").onblur = function()
{
var username = this.value;
//1)
var ajax = createAjax();
//2)
method = "POST";
url = "${pageContext.request.contextPath}/AjaxUserServlet?time="+new Date().getTime();
ajax.open(method,url);
//post方式提交要设置请求头自动将中文转换为UTF-8
ajax.setRequestHeader("content-type","application/x-www-form-urlencoded");
//3)
var content = "username="+username;
ajax.send(content);
//on ready state change
ajax.onreadystatechange = function()
{
if(ajax.readyState == 4)
{
if(ajax.status == 200)
{
//5)
var tip = ajax.responseText;
//6)创建img标签
var imgElement = document.createElement("img");
//设置img标签的src/width/height
imgElement.src = tip;
imgElement.style.width = "12px";
imgElement.style.height = "12px";
//定位span标签
var spanElement = document.getElementById("rsID");
//清空span标签的内容，防止重复出现图片
spanElement.innerHTML = "";
//将img标签加入到span标签
spanElement.appendChild(imgElement);
}
}
}
}
</script>
</body>
</html>
外部js代码：
//外部js文件
function createAjax()
{
var ajax =null;
try{
ajax = new ActiveXObject("microsoft.xmlhttp");
}catch(e1)
{
try{
ajax = new XMLHttpRequest();
}catch(e2)
{
alert("你的浏览器不支持AJAX异步对象");
}
}
return ajax;
}
java后台代码：
package cn.itcast.js.user;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.AsyncContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class AjaxUserServlet extends HttpServlet {
public void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
req.setCharacterEncoding("UTF-8");
String username = req.getParameter("username");
String tip = "images/MsgSent.gif";
if("杰克".equals(username))
{
tip = "images/MsgError.gif";
}
System.out.println(username);
resp.setContentType("text/html;charset=UTF-8");
PrintWriter pw = resp.getWriter();
pw.write(tip);
pw.flush();
pw.close();
}
}
}
}
PSOT实现效果：
六、除了这种方式，当然后续还会利用xml实现验证二级下拉框联动
注意：使用xml传输数据和使用html传输数据相应的response设置也不一样,
xml:response.setContentType("text/xml;charset=UTF-8");
xml:response.setContentType("text/html;charset=UTF-8");
jsp代码如下：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>通过使用GET,POST方式来使用ajax验证用户名是否重复</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
<!--
<link rel="stylesheet" type="text/css" href="styles.css">
-->
</head>
<body>
用户名：
<select id="provinceID" style="111px">
<option>请选择省份</option>
<option>湖北</option>
<option>广东</option>
</select>
<select id="cityID" style="111px">
<option>请选择城市</option>
</select>
<hr/>
<!-- 引入外部js文件 -->
<script type="text/javascript" src="js/ajax.js"> </script>
<script type="text/javascript">
document.getElementById("provinceID").onchange = function()
{
//清除下拉列框之前的内容，第一项除外
var cityElement = document.getElementById("cityID");
cityElement.options.length = 1;
//获取选中省份的名字
var index = this.selectedIndex;
var optionElement = this[index];
var province = optionElement.innerHTML;
//去掉提示"请选择省份"
if("请选择省份" != province)
{
//NO1)
var ajax = createAjax();
//NO2)
var method = "POST";
var url = "${pageContext.request.contextPath}/ProvinceCityServlet?time="+new Date().getTime();
ajax.open(method,url);
//设置请求头
ajax.setRequestHeader("content-type", "application/x-www-form-urlencoded");
//NO3)
var content = "province="+province;
ajax.send(content);
//NO4)
ajax.onreadystatechange = function()
{
if(ajax.readyState == 4)
{
if(ajax.status == 200)
{
//NO5)从ajax异步对象中获取服务器XML数据
var xmlDocument = ajax.responseXML;
//N06)按照DOM规则，解析xml数据
var cityElementArray = xmlDocument.getElementsByTagName("city");
var size = cityElementArray.length;
for(var i=0;i<size;i++)
{
//innerHTML只能用在html标签中
var city = cityElementArray[i].innerHTML;
//<option></option>
var optionElement = document.createElement("option");
//<option>深圳</option>
optionElement.innerHTML = city;
//<select><option>广东</option></select>
//appendeChild()不会去除之前的内容
cityElement.appendChild(optionElement);
}
}
}
}
}
}
</script>
</body>
</html>
外部js代码：
//外部js文件
function createAjax()
{
var ajax =null;
try{
ajax = new ActiveXObject("microsoft.xmlhttp");
}catch(e1)
{
try{
ajax = new XMLHttpRequest();
}catch(e2)
{
alert("你的浏览器不支持AJAX异步对象");
}
}
return ajax;
}
java代码实现
package cn.itcast.js.province;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class ProvinceCityServlet extends HttpServlet {
/**
* The doPost method of the servlet. <br>
*
* This method is called when a form has its tag value method equals to post.
*
* @param request the request send by the client to the server
* @param response the response send by the server to the client
* @throws ServletException if an error occurred
* @throws IOException if an error occurred
*/
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
String province = request.getParameter("province");
System.out.println(province);
String city = "";
if("广东".equals(province))
{
city="<city>广州</city>"+"<city>深圳</city>"+"<city>惠州</city>";
}else if("湖北".equals(province))
{
city = "<city>武汉</city>"+"<city>黄冈</city>"+"<city>宜昌</city>";
}
response.setContentType("text/xml;charset=utf-8");
PrintWriter pw = response.getWriter();
pw.write("<?xml version='1.0' encoding='UTF-8'?>");
pw.write("<root>");
pw.write(city);
pw.write("</root>");
pw.flush();
pw.close();
}
//需求实现从数据库中读取数据并添加到xml文件中
}
最后实现效果：
七、最后一点根据省份从数据库中取出对应的城市数据，话不多说直接上代码
package cn.itcast.js.province; import java.io.IOException; import java.io.PrintWriter; import java.sql.Connection; import java.sql.DriverManager; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import java.util.ArrayList; import java.util.LinkedList; import java.util.List; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; public class ProvinceCityServlet extends HttpServlet { private static PreparedStatement pst = null; private static Connection con = null; private static ResultSet rs = null; String sql = ""; //首先要指定ArrayList初始化 private List<String> list = new ArrayList<String>(); /** * The doPost method of the servlet. <br> * * This method is called when a form has its tag value method equals to post. * * @param request the request send by the client to the server * @param response the response send by the server to the client * @throws ServletException if an error occurred * @throws IOException if an error occurred */ public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setCharacterEncoding("UTF-8"); //十分重要，清除之前存在的数据 list.clear(); String province = request.getParameter("province"); //System.out.println(province); //String city = ""; //从数据库中查询获得 try { Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver"); con = DriverManager.getConnection("jdbc:sqlserver://localhost:1433;DatabaseName=Employee","sa","*******"); //从两个表中去查找 sql = "select c.cName from t_city c,t_province p where p.pName =? and c.pName = p.pName"; pst = con.prepareStatement(sql); //下标从1开始,设置占位符必须在con.prepareStatement(sql);语句之后 pst.setString(1, province); rs = pst.executeQuery(); while(rs.next()) { //System.out.println(rs.getString(1)); list.add(rs.getString(1)); } } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); }finally{ if(con != null) try { con.close(); } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); } if(pst != null) try{ pst.close(); } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); } if(rs != null) try { rs.close(); } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); } } /**if("广东".equals(province)) { //city="<city>广州</city>"+"<city>深圳</city>"+"<city>惠州</city>"; }else if("湖北".equals(province)) { city = "<city>武汉</city>"+"<city>黄冈</city>"+"<city>宜昌</city>"; }*/ response.setContentType("text/xml;charset=utf-8"); PrintWriter pw = response.getWriter(); pw.write("<?xml version='1.0' encoding='UTF-8'?>"); pw.write("<root>"); for(int i=0;i<list.size();i++) { pw.write("<city>"+list.get(i)+"</city>"); } // pw.write(city); pw.write("</root>"); pw.flush(); pw.close(); } //需求实现从数据库中读取数据并添加到xml文件中 }
jsp文件同上,这里出了两个问题,一是rs.getString("pName");会报错,原因是写错了rs.getString("cName");或者rs.getString(1);下标从一开始,第二个就是list集合元素要清空,如果不清空就会出现下面这种情况
,