Servlets 简明教程

Servlets - Auto Page Refresh

考虑一个显示现场比赛比分、股票市场状况或汇率的网页。对于所有这些类型的页面，您都需要使用浏览器的刷新或重新加载按钮定期刷新您的网页。

Java Servlet 通过为您提供一种机制来简化此项工作，您可以在这种机制中以这样的方式制作网页，使其在给定的时间间隔后自动刷新。

刷新网页的最简单方法是使用响应对象的 setIntHeader() 方法。以下是该方法的签名 -

public void setIntHeader(String header, int headerValue)

此方法将标头 "Refresh" 连同表示时间间隔（以秒为单位）的整数值发送回浏览器。

Auto Page Refresh Example

此示例演示 servlet 如何使用 setIntHeader() 方法来设置 Refresh 标头以执行自动页面刷新。

// Import required java libraries
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

// Extend HttpServlet class
public class Refresh extends HttpServlet {

   // Method to handle GET method request.
   public void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

      // Set refresh, autoload time as 5 seconds
      response.setIntHeader("Refresh", 5);

      // Set response content type
      response.setContentType("text/html");

      // Get current time
      Calendar calendar = new GregorianCalendar();
      String am_pm;
      int hour = calendar.get(Calendar.HOUR);
      int minute = calendar.get(Calendar.MINUTE);
      int second = calendar.get(Calendar.SECOND);

      if(calendar.get(Calendar.AM_PM) == 0)
        am_pm = "AM";
      else
        am_pm = "PM";

      String CT = hour+":"+ minute +":"+ second +" "+ am_pm;

      PrintWriter out = response.getWriter();
      String title = "Auto Page Refresh using Servlet";
      String docType =
         "<!doctype html public \"-//w3c//dtd html 4.0 " + "transitional//en\">\n";

      out.println(docType +
         "<html>\n" +
         "<head><title>" + title + "</title></head>\n"+
         "<body bgcolor = \"#f0f0f0\">\n" +
         "<h1 align = \"center\">" + title + "</h1>\n" +
         "<p>Current Time is: " + CT + "</p>\n"
      );
   }

   // Method to handle POST method request.
   public void doPost(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

      doGet(request, response);
   }
}

现在让我们编译上述 servlet，并在 web.xml 中创建以下项

....
 <servlet>
     <servlet-name>Refresh</servlet-name>
     <servlet-class>Refresh</servlet-class>
 </servlet>

 <servlet-mapping>
     <servlet-name>Refresh</servlet-name>
     <url-pattern>/Refresh</url-pattern>
 </servlet-mapping>
....

现在使用 URL [role="bare"] [role="bare"]http://localhost:8080/Refresh 调用此 servlet，它将在每 5 秒后显示当前系统时间，如下所示。只需运行 servlet 并等待查看结果 -

Auto Page Refresh using Servlet
Current Time is: 9:44:50 PM