Web客户端与服务器之间基于Ajax（http）的常用通信方式，分为短连接与长轮询。
短连接：客户端和服务器每进行一次HTTP操作，就建立一次连接，任务结束就中断连接。
在长轮询机制中，客户端像传统轮询一样从服务器请求数据。然而，如果服务器没有可以立即返回给客户端的数据，则不会立刻返回一个空结果，
而是保持这个请求等待数据到来（或者恰当的超时：小于ajax的超时时间），之后将数据作为结果返回给客户端。
长轮询机制如下图所示：
web客户端代码如下：
//向后台长轮询消息 ???function longPolling(){ ???????$.ajax({ ???????????async : true,//异步 ???????????url : ‘longPollingAction!getMessages.action‘, ????????????type : ‘post‘, ???????????dataType : ‘json‘, ???????????data :{}, ???????????timeout : 30000,//超时时间设定30秒 ???????????error : function(xhr, textStatus, thrownError) { ???????????????longPolling();//发生异常错误后再次发起请求 ???????????}, ???????????success : function(response) { ???????????????message = response.data.message; ???????????????if(message!="timeout"){ ???????????????????broadcast();//收到消息后发布消息 ???????????????} ???????????????longPolling(); ???????????} ???????}); ???}web服务器端代码如下：
public class LongPollingAction extends BaseAction { ???private static final long serialVersionUID = 1L; ???private LongPollingService longPollingService; ???private static final long TIMEOUT = 20000;// 超时时间设置为20秒 ???public String getMessages() { ???????long requestTime = System.currentTimeMillis(); ???????result.clear(); ???????try { ???????????String msg = null; ???????????while ((System.currentTimeMillis() - requestTime) < TIMEOUT) { ???????????????msg = longPollingService.getMessages(); ???????????????if (msg != null) { ???????????????????break; // 跳出循环，返回数据 ???????????????} else { ???????????????????Thread.sleep(1000);// 休眠1秒 ???????????????} ???????????} ???????????if (msg == null) { ???????????????result.addData("message", "timeout");// 超时 ???????????} else { ???????????????result.addData("message", msg); ???????????} ???????} catch (Exception e) { ???????????e.printStackTrace(); ???????} ???????return SUCCESS; ???} ???????public LongPollingService getLongPollingService() { ???????return longPollingService; ???} ???public void setLongPollingService(LongPollingService longPollingService) { ???????this.longPollingService = longPollingService; ???}}基于HTTP的长轮询简单实现
原文地址：https://www.cnblogs.com/bingyimeiling/p/9639721.html