Tomcat7中WebSocket初探

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.


1、建立WebSocket连接过程


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:

  1.  浏览器向服务器发送一个 Upgrade 请求头,告诉服务器 “我想从 HTTP 协议 切换到 WebSocket 协议”;
  2. 服务器端接受请求,如果服务器支持 WebSocket ,则返回 Upgrade 响应头;
  3. 浏览器接受响应头,从这个时间点开始,由原来的HTTP协议转为 WebSocket协议,Websocket连接建立起来了.

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。


2、浏览器端 WebSocket编程接口

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://...‘);}
        

Websocket接口定义如下:

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:
  • 通过send()发送数据,
  • 通过close()关闭连接,
  • 通过注册事件函数onopen,onerror,onmessage,onclose来处理服响应。

3、服务器端WebSocket编程


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 {
	}
}

在Web.xml中进行Servlet配置:

<?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>

在index.jsp中编写浏览器端代码如下:

<!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>

4、结果

 

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,古老的榕树,5-wow.com

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