websocket实现android消息推送

前段时间做一个项目，需要android客户端作为管理工具与web服务器后台实时交互，想了很多方法，包括androidpn、openfire+smack、xmpp协议，要么太繁琐，要么无法满足实时性。想来还是用socket，经人提醒想到了websocket。
websocket协议是近些年随html5发展而诞生的，主要用于解决web服务器与客户端无法双向交互的问题。如今已经被W3C收入标准协议。服务器支持：tomcat、jetty的最新版本都已支持websocket；如果不想更换现有服务器，也可用spring4.0作为替代。据说新版本的jre将收入websocket类，没具体接触。客户端支持：目前的主流浏览器都已经实现了websocket，但由于前期协议版本变化太快，很多厂商没有跟上。android默认浏览器就不支持websocket，IE也直到IE10才支持。
网上已有通过html实现websocket client的例子，这里我们用java实现客户端连接。废话不说，上Demo：

1.服务器端 服务器用了tomcat 7.0，直接使用tomcat的websocket实现 1）连接管理类

import java.io.IOException;import java.nio.CharBuffer;import java.util.ArrayList;import java.util.List;import org.apache.catalina.websocket.MessageInbound;import org.apache.catalina.websocket.WsOutbound;public class MessageCenter{private static MessageCenter        instance           = new MessageCenter();private List        socketList;private MessageCenter(){this.socketList = new ArrayList();}public static MessageCenter getInstance(){return instance;}public void addMessageInbound(MessageInbound inbound){socketList.add(inbound);}public void removeMessageInbound(MessageInbound inbound){socketList.remove(inbound);}public void broadcast(CharBuffer msg) throws IOException{for (MessageInbound messageInbound : socketList){CharBuffer buffer = CharBuffer.wrap(msg);WsOutbound outbound = messageInbound.getWsOutbound();outbound.writeTextMessage(CharBuffer.wrap("broadcasting:" + msg));// outbound.writeTextMessage(buffer);outbound.flush();}}}

2）消息入口类

import java.io.IOException;import java.nio.ByteBuffer;import java.nio.CharBuffer;import java.util.Date;import org.apache.catalina.websocket.MessageInbound;import org.apache.catalina.websocket.WsOutbound;public class MyMessageInbound extends MessageInbound {@Overrideprotected void onBinaryMessage(ByteBuffer arg0) throws IOException {// TODO Auto-generated method stub}@Overrideprotected void onTextMessage(CharBuffer msg) throws IOException {System.out.println("Received:"+msg);MessageCenter.getInstance().broadcast(msg);}@Overrideprotected void onClose(int status) {System.out.println("close:"+new Date());MessageCenter.getInstance().removeMessageInbound(this);super.onClose(status);}@Overrideprotected void onOpen(WsOutbound outbound) {System.out.println("open :"+new Date());super.onOpen(outbound);MessageCenter.getInstance().addMessageInbound(this);}}

3）Websocket servlet

import javax.servlet.http.HttpServletRequest;import org.apache.catalina.websocket.StreamInbound;import org.apache.catalina.websocket.WebSocketServlet;public class MeWebSocketServlet extends WebSocketServlet{private static final long serialVersionUID = -7178893327801338294L;@Overrideprotected StreamInbound createWebSocketInbound(String subProtocol, HttpServletRequest request){System.out.println("##########client login#########");return new MyMessageInbound();}}

4）添加servlet到web.xml

        < servlet>               < servlet-name> android               < servlet-class> MyWebSocketServlet                 < servlet-mapping>               < servlet-name> android               < url-pattern> *.do

2.客户端 客户端使用java实现websocket client，网上有人实现了Java-websocket： https://github.com/TooTallNate/Java-WebSocket 可以取得源码，用maven编译。最新jar包下载地址： http://download.csdn.net/detail/chifengxin/6524283
引用jar包后，实现简单消息连接：

import java.net.URI;import java.net.URISyntaxException;import org.java_websocket.client.WebSocketClient;import org.java_websocket.drafts.Draft;import org.java_websocket.drafts.Draft_10;import org.java_websocket.drafts.Draft_17;import org.java_websocket.framing.Framedata;import org.java_websocket.handshake.ServerHandshake;/** This example demonstrates how to create a websocket connection to a server. Only the most important callbacks are overloaded. */public class ExampleClient extends WebSocketClient {        public ExampleClient( URI serverUri , Draft draft ) {                super( serverUri, draft );        }        public ExampleClient( URI serverURI ) {                super( serverURI );        }        @Override        public void onOpen( ServerHandshake handshakedata ) {                System.out.println( "opened connection" );                // if you plan to refuse connection based on ip or httpfields overload: onWebsocketHandshakeReceivedAsClient        }        @Override        public void onMessage( String message ) {                System.out.println( "received: " + message );        }        @Override        public void onFragment( Framedata fragment ) {                System.out.println( "received fragment: " + new String( fragment.getPayloadData().array() ) );        }        @Override        public void onClose( int code, String reason, boolean remote ) {                // The codecodes are documented in class org.java_websocket.framing.CloseFrame                System.out.println( "Connection closed by " + ( remote ? "remote peer" : "us" ) );        }        @Override        public void onError( Exception ex ) {                ex.printStackTrace();                // if the error is fatal then onClose will be called additionally        }        public static void main( String[] args ) throws URISyntaxException {                ExampleClient c = new ExampleClient( new URI( "ws://localhost:8080/myweb/android.do" ), new Draft_17() );                 c.connectBlocking();                c.send("handshake");        }}

注意，连接中使用的new Draft_17()就是使用的协议version 17( RFC 6455)，Tomcat 7.0使用的协议版本为RFC 6455。

总结websocket的利弊 优点： 与socket相比，可以节省额外的端口占用，直接使用一个公网域名访问。另外协议对报文的流量消耗做了优化。
缺点： 毕竟websocket底层也是socket连接，因而当大并发用户连接时目测会消耗较多资源。
参考： Tomcat 7.0 api http://tomcat.apache.org/tomcat-7.0-doc/api/

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