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 {

	@Override
	protected void onBinaryMessage(ByteBuffer arg0) throws IOException {
		// TODO Auto-generated method stub
		
	}

	@Override
	protected void onTextMessage(CharBuffer msg) throws IOException {
		System.out.println("Received:"+msg);
		MessageCenter.getInstance().broadcast(msg);
		
	}

	@Override
	protected void onClose(int status) {
		System.out.println("close:"+new Date());
		MessageCenter.getInstance().removeMessageInbound(this);
		super.onClose(status);
	}

	@Override
	protected 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;

	@Override
	protected 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/