对于一些对数据实时性要求较高的系统，比如股票行情、在线聊天、微博，实现数据的实时推送是必须的。通常实现实时推送的方式有：
1、轮询：隔一段时间发送数据(如：webqq)
2、socket：以往普通的网页是不支持socket接收消息的。可以通过flash或者applet来作为socket的客户端
3、长连接：指在一个TCP连接上可以连续发送多个数据包，在TCP连接保持期间，如果没有数据包发送，需要双方发检测包以维持此连接，一般需要自己做
在线维持。
----------------------------------------------------------------------------------------------
html5通过window.WebSocket(firefox下是window.MozWebSocket)提供了一种非http的双向连接，这个连接是实时的更是永久的，除非被显示colse。
这就表示只要客户端打开一个Socket并且请求建立了连接(just once)，服务端就能实时接收并发送消息，不需手动检测和维持状态。
WebSocket提供的方法和属性可以在firebug中输入Window.WebSocket.prototype看到。
接下去的代码列出了基本的使用思路：
var location = "ws://localhost:port/serlet/xxx";
//服务端处理的servlet
var webSocket = new WebSocket(location);
//webSocket.readyState
var readyStates = {
"CONNECTING":"正在连接“，
”OPEN“ : "已建立连接",
"CLOSING":"正在关闭连接",
"CLOSED":"已关闭连接"
}
webSocket.send(data);//发送数据到服务端,目前只支持文本类型。JSON.stringify(data);JSON.parse(data);
webSocket.onMessage = function(event){
var data = event.data;//从服务端过来的数据
}
webSocket.onOpen = function(event){
//开始通信
}
webSocket.onClose = function(event){
//结束通信
}
webSocket.close();二、一个基于jetty(java服务器)的例子
目前Apache还不支持WebSocket，各种语言都有各自的方式可以实现它，这里在Java中实现了。
步骤一：下载一个jetty，解压放在任意盘下。jetty7及以上才支持WebSocket，下载地址：download.eclipse.org/jetty/stable-7/dist/
步骤二：下载eclipse（不推荐用MyEclipse，比较麻烦，需要安装其他的插件），必须支持jetty7，版本是越高越好。
步骤三：在eclipse中安装插件，help---Install new software...----url为：eclipse-jetty.sourceforge.net/update/
步骤四：新建一个Dynamic Web Project
目录结构如下
步骤五：拷入如下代码：
TailorWebSocketServlet.java
package com.test;
import java.io.IOException;
import java.util.Date;
import java.util.Set;
import java.util.concurrent.CopyOnWriteArraySet;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.eclipse.jetty.websocket.WebSocket;
import org.eclipse.jetty.websocket.WebSocketServlet;
public class TailorWebSocketServlet extends WebSocketServlet {
private static final long serialVersionUID = -72719281366784056L;
public static String newLine = System.getProperty("line.separator");
private final Set<TailorSocket> _members = new CopyOnWriteArraySet<TailorSocket>();
private ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
public void init() throws ServletException {
super.init();
executor.scheduleAtFixedRate(new Runnable() {
public void run() {
System.out.println("Running Server Message Sending");
for(TailorSocket member : _members){
System.out.println("Trying to send to Member!");
if(member.isOpen()){
System.out.println("Sending!");
try {
member.sendMessage("from server : happy and happiness! "+new Date()+newLine);
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}, 2, 2, TimeUnit.SECONDS);
}
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
getServletContext().getNamedDispatcher("default").forward(request,
response);
}
public WebSocket doWebSocketConnect(HttpServletRequest request,
String protocol) {
return new TailorSocket();
}
class TailorSocket implements WebSocket.OnTextMessage {
private Connection _connection;
public void onClose(int closeCode, String message) {
_members.remove(this);
}
public void sendMessage(String data) throws IOException {
_connection.sendMessage(data);
}
public void onMessage(String data) {
System.out.println("Received: "+data);
}
public boolean isOpen() {
return _connection.isOpen();
}
public void onOpen(Connection connection) {
_members.add(this);
_connection = connection;
try {
connection.sendMessage("onOpen:Server received Web Socket upgrade and added it to Receiver List.");
} catch (IOException e) {
e.printStackTrace();
}
}
}
}test.html
<!DOCTYPE HTML>
<html>
<head>
<meta charset = "utf-8"/>
<title>Chat by Web Sockets</title>
<script type='text/javascript'>
if (!window.WebSocket)
alert("window.WebSocket unsuport!");
function $() {
return document.getElementById(arguments[0]);
}
function $F() {
return document.getElementById(arguments[0]).value;
}
function getKeyCode(ev) {
if (window.event)
return window.event.keyCode;
return ev.keyCode;
}
var server = {
connect : function() {
var location ="ws://localhost:8888/servlet/a";
this._ws =new WebSocket(location);
this._ws.onopen =this._onopen;
this._ws.onmessage =this._onmessage;
this._ws.onclose =this._onclose;
},
_onopen : function() {
server._send('send to server : websockets are open for communications!');
},
_send : function(message) {
if (this._ws)
this._ws.send(message);
},
send : function(text) {
if (text !=null&& text.length >0)
server._send(text);
},
_onmessage : function(m) {
if (m.data) {
var messageBox = $('messageBox');
var spanText = document.createElement('span');
spanText.className ='text';
spanText.innerHTML = m.data;
var lineBreak = document.createElement('br');
messageBox.appendChild(spanText);
messageBox.appendChild(lineBreak);
messageBox.scrollTop = messageBox.scrollHeight
- messageBox.clientHeight;
}
},
_onclose : function(m) {
this._ws =null;
}
};
</script>
<style type='text/css'>
div {
border: 0px solid black;
}
div#messageBox {
clear: both;
width: 40em;
height: 20ex;
overflow: auto;
background-color: #f0f0f0;
padding: 4px;
border: 1px solid black;
}
div#input {
clear: both;
width: 40em;
padding: 4px;
background-color: #e0e0e0;
border: 1px solid black;
border-top: 0px
}
div.hidden {
display: none;
}
span.alert {
font-style: italic;
}
</style>
</head>
<body>
<div id='messageBox'></div>
<div id='input'>
<div>
<input id='connect' type='submit' name='Connect'
value='Connect' />
</div>
</div>
<script type='text/javascript'>
$('connect').onclick =function(event) {
server.connect();
returnfalse;
};
</script>
<p>
JAVA Jetty for WebSocket
</p>
</body>
</html>web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"> <display-name>tailor</display-name> <servlet> <servlet-name>WebSocket</servlet-name> <servlet-class>com.test.TailorWebSocketServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>WebSocket</servlet-name> <url-pattern>/servlet/*</url-pattern> </servlet-mapping> <welcome-file-list> <welcome-file>test.html</welcome-file> </welcome-file-list> </web-app>
websocket.xml
<?xml version="1.0" encoding="ISO-8859-1"?> <!DOCTYPE Configure PUBLIC "-//Jetty//Configure//EN" "http://www.eclipse.org/jetty/configure.dtd"> <!-- ================================================================== Configure and deploy the test web application in $(jetty.home)/webapps/test Note. If this file did not exist or used a context path other that /test then the default configuration of jetty.xml would discover the test webapplication with a WebAppDeployer. By specifying a context in this directory, additional configuration may be specified and hot deployments detected. ===================================================================== --> <Configure class="org.eclipse.jetty.webapp.WebAppContext"> <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> <!-- Required minimal context configuration : --> <!-- + contextPath --> <!-- + war OR resourceBase --> <!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --> <Set name="contextPath">/</Set> <Set name="defaultsDescriptor"><SystemProperty name="jetty.home" default="."/>/etc/webdefault.xml</Set> <Set name="overrideDescriptor"><SystemProperty name="jetty.home" default="."/>/contexts/test.d/override-web.xml</Set> <!-- virtual hosts <Set name="virtualHosts"> <Array type="String"> <Item>www.myVirtualDomain.com</Item> <Item>localhost</Item> <Item>127.0.0.1</Item> </Array> </Set> --> <!-- disable cookies <Get name="sessionHandler"> <Get name="sessionManager"> <Set name="usingCookies" type="boolean">false</Set> </Get> </Get> --> <Get name="securityHandler"> <Set name="loginService"> <New class="org.eclipse.jetty.security.HashLoginService"> <Set name="name">Test Realm</Set> <Set name="config"><SystemProperty name="jetty.home" default="."/>/etc/realm.properties</Set> <!-- To enable reload of realm when properties change, uncomment the following lines --> <!-- changing refreshInterval (in seconds) as desired --> <!-- <Set name="refreshInterval">5</Set> <Call name="start"></Call> --> </New> </Set> <Set name="checkWelcomeFiles">true</Set> </Get> <!-- Non standard error page mapping --> <!-- <Get name="errorHandler"> <Call name="addErrorPage"> <Arg type="int">500</Arg> <Arg type="int">599</Arg> <Arg type="String">/dump/errorCodeRangeMapping</Arg> </Call> </Get> --> <!-- Add context specific logger <Set name="handler"> <New id="RequestLog" class="org.eclipse.jetty.server.handler.RequestLogHandler"> <Set name="requestLog"> <New id="RequestLogImpl" class="org.eclipse.jetty.server.NCSARequestLog"> <Set name="filename"><Property name="jetty.logs" default="./logs"/>/test-yyyy_mm_dd.request.log</Set> <Set name="filenameDateFormat">yyyy_MM_dd</Set> <Set name="append">true</Set> <Set name="LogTimeZone">GMT</Set> </New> </Set> </New> </Set> --> </Configure>
步骤六：
跑后的效果：
浏览器访问：localhost:8080
eclipse控制台上：
恭喜你，到这儿就算success了！
三、性能
对于这种类型的连接，各种服务器需要消耗的性能也不同，java下可以通过JDK的bin目录下的Jconsole来查看，单个连接内存消耗在2.5M左右，但是对于并发量还没有做测试。这里不贴图了
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