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在J2ME中基于UDP協(xié)議編程  
［ 作者：mingjava    轉(zhuǎn)貼自：本站原創(chuàng)    點擊數(shù)：236    更新時間：2004-11-26    文章錄入：mingjava ］ 
 
    在GCF中提供了DatagramConnection和Datagram兩個接口，借助他們我們可以在J2ME中基于UDP協(xié)議開發(fā)聯(lián)網(wǎng)應(yīng)用程序，在MIDP2.0中，添加了UDPDatagramConnection這個接口。他擴展了DatagramConnection并添加了兩個方法getLocalAddress()和getLocalPort()。我們知道UDP服務(wù)是不可靠的，如果你希望開發(fā)更可靠的聯(lián)網(wǎng)應(yīng)用的話可以采用SocketConnection，因為TCP服務(wù)是面向連接且可靠的。我們還必須清楚地一點是以上所說的各種連接方式都不是MIDP規(guī)范中規(guī)定必須實現(xiàn)的。因此在使用之前請參考特定設(shè)備的開發(fā)文檔。MIDP中只有Http連接是必須支持的。

    同樣，我們要獲得DatagramConnection的話，必須通過Connector的open方法，其中的URL應(yīng)該滿足如下的形式。

datagram://localhost:5555 這樣的話表示建立了一個客戶端模式的連接。在指定ip:localhost和指定端口:5555 
datagram://:5555  這樣建立的是一個服務(wù)器端模式的連接，在本地的5555端口。
建立連接后，我們可以通過DatagramConnection的newDatagram()方法構(gòu)造一個Datagram，然后調(diào)用DatagramConnection的send()方法。這樣數(shù)據(jù)報將會發(fā)送到指定的接受方。例如你可以構(gòu)建這個一個負責(zé)發(fā)送數(shù)據(jù)的Sender類。

package com.siemens.datagramtest;

import javax.microedition.io.Datagram;
import javax.microedition.io.DatagramConnection;

public class Sender extends Thread
{

    private DatagramConnection dc;

    private String address;

    private String message;

    public Sender(DatagramConnection dc)
    {
        this.dc = dc;
        start();
    }

    public synchronized void send(String addr, String msg)
    {
        address = addr;
        message = msg;
        notify();
    }

    public synchronized void run()
    {

        while (true)
        {

            // If no client to deal, wait until one connects
            if (message == null)
            {
                try
                {
                    wait();
                } catch (InterruptedException e)
                {
                }
            }

            try
            {
                byte[] bytes = message.getBytes();
                Datagram dg = null;
                // Are we a sender thread for the client ? If so then there's
                // no address parameter
                if (address == null)
                {
                    dg = dc.newDatagram(bytes, bytes.length);
                } else
                {
                    dg = dc.newDatagram(bytes, bytes.length, address);
                    System.out.println(address);
                }
                dc.send(dg);
            } catch (Exception ioe)
            {
                ioe.printStackTrace();
            }

            // Completed client handling, return handler to pool and
            // mark for wait
            message = null;
        }
    }

}
注意聯(lián)網(wǎng)的時候我們應(yīng)該在另外一個線程中而不是在主線程中。

     服務(wù)器端的目的就是啟動后監(jiān)聽指定的端口，當(dāng)客戶端連接過來后接受數(shù)據(jù)并記錄下客戶端的地址，以便服務(wù)器端向客戶端發(fā)送數(shù)據(jù)。
package com.siemens.datagramtest;

import java.io.IOException;

import javax.microedition.io.Connector;
import javax.microedition.io.Datagram;
import javax.microedition.io.DatagramConnection;
import javax.microedition.io.UDPDatagramConnection;
import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.StringItem;
import javax.microedition.lcdui.TextField;

public class Server implements Runnable, CommandListener
{

    private DatagramMIDlet parent;

    private Display display;

    private Form f;

    private StringItem si;

    private TextField tf;

    private Command sendCommand = new Command("Send", Command.ITEM, 1);

    Sender sender;

    private String address;

    public Server(DatagramMIDlet m)
    {
        parent = m;
        display = Display.getDisplay(parent);
        f = new Form("Datagram Server");
        si = new StringItem("Status:", " ");
        tf = new TextField("Send:", "", 30, TextField.ANY);
        f.append(si);
        f.append(tf);
        f.addCommand(sendCommand);
        f.setCommandListener(this);
        display.setCurrent(f);
    }

    public void start()
    {

        Thread t = new Thread(this);
        t.start();
    }

    public void run()
    {
        try
        {

            si.setText("Waiting for connection");
            DatagramConnection dc =(DatagramConnection)Connector.open("datagram://:5555");
           

            sender = new Sender(dc);

            while (true)
            {
                Datagram dg = dc.newDatagram(100);
                dc.receive(dg);
                address = dg.getAddress();
                si.setText("Message received - "
                        + new String(dg.getData(), 0, dg.getLength()));
              
            }

        } catch (IOException ioe)
        {
            Alert a = new Alert("Server", "Port 5000 is already taken.", null,
                    AlertType.ERROR);
            a.setTimeout(Alert.FOREVER);
            a.setCommandListener(this);
            display.setCurrent(a);
        } catch (Exception e)
        {
            e.printStackTrace();
        }
    }

    public void commandAction(Command c, Displayable s)
    {
        if (c == sendCommand && !parent.isPaused())
        {
            if (address == null)
            {
                si.setText("No destination address");
            } else
            {
                sender.send(address, tf.getString());
            }
        }
        if (c == Alert.DISMISS_COMMAND)
        {
            parent.destroyApp(true);
            parent.notifyDestroyed();
        }
    }

    public void stop()
    {
    }

}

客戶端代碼則是建立連接后向服務(wù)器端發(fā)送數(shù)據(jù)，并等待接受服務(wù)器返回的數(shù)據(jù)。
package com.siemens.datagramtest;

import java.io.IOException;

import javax.microedition.io.ConnectionNotFoundException;
import javax.microedition.io.Connector;
import javax.microedition.io.Datagram;
import javax.microedition.io.DatagramConnection;
import javax.microedition.lcdui.Alert;
import javax.microedition.lcdui.AlertType;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.StringItem;
import javax.microedition.lcdui.TextField;

public class Client implements Runnable, CommandListener
{

    private DatagramMIDlet parent;

    private Display display;

    private Form f;

    private StringItem si;

    private TextField tf;

    private Command sendCommand = new Command("Send", Command.ITEM, 1);

    Sender sender;

    public Client(DatagramMIDlet m)
    {
        parent = m;
        display = Display.getDisplay(parent);
        f = new Form("Datagram Client");
        si = new StringItem("Status:", " ");
        tf = new TextField("Send:", "", 30, TextField.ANY);
        f.append(si);
        f.append(tf);
        f.addCommand(sendCommand);
        f.setCommandListener(this);
        display.setCurrent(f);

    }

    public void start()
    {
        Thread t = new Thread(this);
        t.start();
    }

    public void run()
    {
        try
        {

            DatagramConnection dc = (DatagramConnection) Connector
                    .open("datagram://localhost:5555");
            

            si.setText("Connected to server");

            sender = new Sender(dc);

            while (true)
            {
                Datagram dg = dc.newDatagram(100);
                dc.receive(dg);
                // Have we actually received something or is this just a timeout
                // ?
                if (dg.getLength() > 0)
                {
                    si.setText("Message received - "
                            + new String(dg.getData(), 0, dg.getLength()));
                }
            }

        } catch (ConnectionNotFoundException cnfe)
        {
            Alert a = new Alert("Client", "Please run Server MIDlet first",
                    null, AlertType.ERROR);
            a.setTimeout(Alert.FOREVER);
            display.setCurrent(a);
        } catch (IOException ioe)
        {
            ioe.printStackTrace();
        }
    }

    public void commandAction(Command c, Displayable s)
    {
        if (c == sendCommand && !parent.isPaused())
        {
            sender.send(null, tf.getString());
        }
    }

    public void stop()
    {
    }

}
本文的代碼取自WTK demo中的例子，您可以參考demo中的源代碼！下面給出MIDlet的代碼
package com.siemens.datagramtest;

import javax.microedition.lcdui.Choice;
import javax.microedition.lcdui.ChoiceGroup;
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.midlet.MIDlet;

public class DatagramMIDlet extends MIDlet implements CommandListener
{

    private static final String SERVER = "Server";

    private static final String CLIENT = "Client";

    private static final String[] names = { SERVER, CLIENT };

    private static Display display;

    private Form f;

    ChoiceGroup cg;

    private boolean isPaused;

    private Command exitCommand = new Command("Exit", Command.EXIT, 1);

    private Command startCommand = new Command("Start", Command.ITEM, 1);

    public DatagramMIDlet()
    {
        display = Display.getDisplay(this);
        f = new Form("Datagram Demo");
        cg = new ChoiceGroup("Please select peer", Choice.EXCLUSIVE, names,
                null);
        f.append(cg);

        f.addCommand(exitCommand);
        f.addCommand(startCommand);
        f.setCommandListener(this);

        display.setCurrent(f);
    }

    public static Display getDisplay()
    {
        return display;
    }

    public boolean isPaused()
    {
        return isPaused;
    }

    public void startApp()
    {
        isPaused = false;
    }

    public void pauseApp()
    {
        isPaused = true;
    }

    public void destroyApp(boolean unconditional)
    {
    }

    public void commandAction(Command c, Displayable s)
    {
        if (c == exitCommand)
        {
            destroyApp(true);
            notifyDestroyed();
        } else if (c == startCommand)
        {
            String name = cg.getString(cg.getSelectedIndex());
            if (name.equals(SERVER))
            {
                Server server = new Server(this);
                server.start();
            } else
            {
                Client client = new Client(this);
                client.start();
            }
        }
    }

}