在HTML5中定义了WebSocket规范，该规范使得能够实现在页面中通过WebSocket协议和服务器进行双向通信。
在Web应用中一个常见的场景是Server端向Client端推送某些消息，要实现这项功能可以有一下方案：
（1） ajax技术+轮询 ：这种方案仅仅是一个模拟实现，本质还是HTTP请求响应的模式，由于无法预期什么时候推送消息，造成很多无效的请求；
（2） 通过 flash之类 ： 这种方式能够实现双向通信，但有一个前提条件就是依赖第三方插件，而在移动端浏览器大多数都不支持flash.
（3）WebSocket : 原生态支持真正的双向通信，有web TCP 之称，so easy.
关于WebSocket规范和协议参考 http://www.websocket.org/aboutwebsocket.html
这里简单描述一下：WebSocket协议一个设计原则就是能够和现有的Web方式一起工作，建立WebSocket连接是以HTTP连接开始，这个过程为 WebSocket握手 .
下图所示为一个WebSocket建立连接的请求和响应过程：
稍作解释如下：
WebSocket连接建立之后，数据传输通道 和端口使用的都是原来HTTP所使用的TCP/IP通道和端口 ，可以在浏览器和服务器之间进行全双工通信了。
数据帧 支持 text 和 binary 两种方式：在使用text方式传输时，以 0x00 为起始点，以0xFF结束，数据以UTF-8编码放在中间；对于 binary方式则没有结束标识，而是将数据帧长度置于数据前面。
首先需要检测浏览器是否支持WebSocket，代码如下：
var socket=null; window.WebSocket = window.WebSocket || window.MozWebSocket;
if (!window.WebSocket) { alert(‘Error: WebSocket is not supported .‘);
} else{ socket = new WebSocket(‘ws://...‘);}
interface WebSocket : EventTarget {
readonly attribute DOMString url;
// ready state
const unsigned short CONNECTING = 0;
const unsigned short OPEN = 1;
const unsigned short CLOSING = 2;
const unsigned short CLOSED = 3;
readonly attribute unsigned short readyState;
readonly attribute unsigned long bufferedAmount;
// networking
attribute EventHandler onopen;
attribute EventHandler onerror;
attribute EventHandler onclose;
readonly attribute DOMString extensions;
readonly attribute DOMString protocol;
void close([Clamp] optional unsigned short code, optional DOMString reason);
// messaging
attribute EventHandler onmessage;
attribute DOMString binaryType;
void send(DOMString data);
void send(Blob data);
void send(ArrayBuffer data);
void send(ArrayBufferView data);
};
从上面定义中可以很清晰的看出： 这里以Tomcat 7 为例， 需要加载的依赖包为 \lib\catalina.jar、\lib\tomcat-coyote.jar
这里有两个重要的类 ：WebSocketServlet 和 StreamInbound， 前者是一个容器，用来初始化WebSocket环境；后者是用来具体处理WebSocket请求和响应的。
编写一个Servlet类，继承自WebSocket，实现其抽象方法即可，代码如下：
package websocket;
import java.util.concurrent.atomic.AtomicInteger;
import javax.servlet.http.HttpServletRequest;
import org.apache.catalina.websocket.StreamInbound;
import org.apache.catalina.websocket.WebSocketServlet;
public class HelloWebSocketServlet extends WebSocketServlet {
private static final long serialVersionUID = 1L;
private final AtomicInteger connectionIds = new AtomicInteger(0);
@Override
protected StreamInbound createWebSocketInbound(String arg0,
HttpServletRequest request) {
return new HelloMessageInbound(connectionIds.getAndIncrement(), request
.getSession().getId());
}
}
package websocket;
import java.io.IOException;
import java.io.InputStream;
import java.io.Reader;
import java.nio.CharBuffer;
import org.apache.catalina.websocket.StreamInbound;
import org.apache.catalina.websocket.WsOutbound;
public class HelloMessageInbound extends StreamInbound {
private String WS_NAME;
private final String FORMAT = "%s : %s";
private final String PREFIX = "ws_";
private String sessionId = "";
public HelloMessageInbound(int id, String _sessionId) {
this.WS_NAME = PREFIX + id;
this.sessionId = _sessionId;
}
@Override
protected void onTextData(Reader reader) throws IOException {
char[] chArr = new char[1024];
int len = reader.read(chArr);
send(String.copyValueOf(chArr, 0, len));
}
@Override
protected void onClose(int status) {
System.out.println(String.format(FORMAT, WS_NAME, "closing ......"));
super.onClose(status);
}
@Override
protected void onOpen(WsOutbound outbound) {
super.onOpen(outbound);
try {
send("hello, my name is " + WS_NAME);
send("session id = " + this.sessionId);
} catch (IOException e) {
e.printStackTrace();
}
}
private void send(String message) throws IOException {
message = String.format(FORMAT, WS_NAME, message);
System.out.println(message);
getWsOutbound().writeTextMessage(CharBuffer.wrap(message));
}
@Override
protected void onBinaryData(InputStream arg0) throws IOException {
}
}
<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0" 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_3_0.xsd">
<display-name>websocket demo</display-name>
<servlet>
<servlet-name>wsHello</servlet-name>
<servlet-class>websocket.HelloWebSocketServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>wsHello</servlet-name>
<url-pattern>/websocket/say</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>
<!DOCTYPE HTML>
<html>
<head>
<title>WebSocket demo</title>
<style>
body { padding: 40px;}
#outputPanel{margin:10px;padding:10px; background:#eee; border:1px solid #000; min-height: 300px;}
</style>
</head>
<body>
<input type="text" id="messagebox" size="60" />
<input type="button" id="buttonSend" value="Send Message" />
<input type="button" id="buttonConnect" value="Connect to server" />
<input type="button" id="buttonClose" value="Disconnect" />
<br>
<%
out.println("Session ID = " + session.getId());
%>
<div id="outputPanel"></div>
</body>
<script type="text/javascript">
var infoPanel = document.getElementById(‘outputPanel‘);
var textBox = document.getElementById(‘messagebox‘);
var sendButton = document.getElementById(‘buttonSend‘);
var connButton = document.getElementById(‘buttonConnect‘);
var discButton = document.getElementById(‘buttonClose‘);
var console = {
log : function(text) {
infoPanel.innerHTML += text + "<br>";
}
};
var demo = {
socket : null,
host : ‘‘,
connect : function() {
window.WebSocket = window.WebSocket || window.MozWebSocket;
if (!window.WebSocket) {
console.log(‘Error: WebSocket is not supported .‘);
return;
}
this.socket = new WebSocket(this.host);
this.socket.onopen = function() {
console.log("websocket is opened .");
};
this.socket.onclose = function() {
console.log("websocket is closed .");
demo.socket = null;
};
this.socket.onmessage = function(message) {
console.log(message.data);
};
},
send : function(message) {
if (this.socket) {
this.socket.send(message);
return true;
}
console.log(‘please connect to the server first !!!‘);
return false;
}
};
demo.host = window.location.host + ‘/Hello/websocket/say‘;
if (window.location.protocol == ‘http:‘) {
demo.host = ‘ws://‘ + demo.host;
} else {
demo.host = ‘wss://‘ + demo.host;
}
sendButton.onclick = function() {
var message = textBox.value;
if (!message)
return;
if (!demo.send(message))
return;
textBox.value = ‘‘;
};
connButton.onclick = function() {
if (!demo.socket) {
demo.connect();
} else {
console.log(‘websocket already exists .‘);
}
};
discButton.onclick = function() {
if (demo.socket) {
demo.socket.close();
} else {
console.log(‘websocket is not found .‘);
}
};
</script>
</html>
这里看到 WebSocket建立的连接所访问的Session和HTTP访问的Session是一致的。
Tomcat7中WebSocket初探,布布扣,bubuko.com
原文：http://blog.csdn.net/whucyl/article/details/20153207
