19、Ajax剖析

Ajax&＃xff08;Asynchronous Javascript and XML&＃xff09;&＃xff0c;异步的Javascript与XML&＃xff0c;强调异步。

所谓异步&＃xff0c;就是前一个操作还没完成&＃xff0c;就可以进行下一个操作&＃xff0c;提高用户体验&＃xff0c;页面可以局部刷新&＃xff0c;减少网络传输流量

1、Ajax中的一个重要对象是XMLHttpRequest。

2、使用Ajax准备向服务器端发送请求&＃xff1a;xmlHttpRequest.open("GET","AjaxServlet",true);

一个Ajax使用实例&＃xff1a;

<%&＃64; page language&＃61;"java" import&＃61;"java.util.*" pageEncoding&＃61;"UTF-8"%>
<%
String path &＃61; request.getContextPath();
String basePath &＃61; request.getScheme()&＃43;"://"&＃43;request.getServerName()&＃43;":"&＃43;request.getServerPort()&＃43;path&＃43;"/";
%>



">













调用的Servlet&＃xff1a;

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 AjaxServlet extends HttpServlet
{
&＃64;Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
PrintWriter out &＃61; resp.getWriter();
System.out.println("doGet invoke");
out.println("hello world");
out.flush();
}
}


对于上面的程序&＃xff0c;如果在firefox中进行测试&＃xff0c;点击一次按钮&＃xff0c;就会在MyEclipse控制台打印一次doGet invoke&＃xff0c;但是如果使用IE浏览器&＃xff0c;则只是在第一次点击按钮时打印doGet invoke&＃xff0c;此后再按就没有打印信息了。

上述原因是因为IE缓存的问题&＃xff0c;将Servlet进行修改如下&＃xff1a;

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 AjaxServlet extends HttpServlet
{
&＃64;Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
PrintWriter out &＃61; resp.getWriter();
try
{
Thread.sleep(5000);
}
catch(Exception e)
{
e.printStackTrace();
}
System.out.println("doGet invoke");
resp.setHeader("pragma", "no-cache");
resp.setHeader("cache-control", "no-cache");
out.println("hello world");
out.flush();
}
}


IE每次点击按钮都打印。

3、带参数的ajax实例 分别使用GET和POST进行传递&＃xff1a;

页面ajax.jsp

<%&＃64; page language&＃61;"java" import&＃61;"java.util.*" pageEncoding&＃61;"UTF-8"%>
<%
String path &＃61; request.getContextPath();
String basePath &＃61; request.getScheme()&＃43;"://"&＃43;request.getServerName()&＃43;":"&＃43;request.getServerPort()&＃43;path&＃43;"/";
%>



">


















接收请求的Servlet&＃xff1a;

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 AjaxServlet extends HttpServlet
{
&＃64;Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
process(req, resp);
}
private void process(HttpServletRequest req, HttpServletResponse resp)
throws IOException
{
String v1 &＃61; req.getParameter("v1");
String v2 &＃61; req.getParameter("v2");
System.out.println("v1&＃61;" &＃43; v1 &＃43; " v2&＃61;" &＃43; v2);
String v3 &＃61; String.valueOf(Integer.valueOf(v1) &＃43; Integer.valueOf(v2));
PrintWriter out &＃61; resp.getWriter();
/*
try
{
Thread.sleep(5000);
}
catch(Exception e)
{
e.printStackTrace();
}
*/
System.out.println("doGet invoke");
resp.setHeader("pragma", "no-cache");
resp.setHeader("cache-control", "no-cache");
out.println(v3);
out.flush();
}
&＃64;Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
System.out.println("doPost invoke");
this.process(req, resp);
}
}