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在J2ME中实现基于UDP协议通讯程序

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  在GCF中提供了DatagramConnection和Datagram两个接口,借助他们我们可以在J2ME中基于UDP协议开发联网应用程序。在MIDP2.0中,添加了UDPDatagramConnection这个接口,他扩展了DatagramConnection并添加了两个方法getLocalAddress()和getLocalPort()。我们知道UDP服务是不可靠的,如果你希望开发更可靠的联网应用的话可以采用SocketConnection,因为TCP服务是面向连接且可靠的。我们还必须清楚地一点是以上所说的各种连接方式都不是MIDP规范中规定必须实现的。因此在使用之前请参考特定设备的开发文档。MIDP中只有Http连接是必须支持的。

  同样,我们要获得DatagramConnection的话,必须通过Connector的open方法,其中的URL应该满足如下的形式。
datagram://localhost:5555 这样的话表示建立了一个客户端模式的连接。在指定ip:localhost和指定端口:5555
datagram://:5555 这样建立的是一个服务器端模式的连接,在本地的5555端口。
  建立连接后,我们可以通过DatagramConnection的newDatagram()方法构造一个Datagram,然后调用DatagramConnection的send()方法。这样数据报将会发送到指定的接受方。例如你可以构建这个一个负责发送数据的Sender类。

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;
    }
  }

}


  注意联网的时候我们应该在另外一个线程中而不是在主线程中。

服务器端的目的就是启动后监听指定的端口,当客户端连接过来后接受数据并记录下客户端的地址,以便服务器端向客户端发送数据。

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()
  {
  }

}


客户端代码则是建立连接后向服务器端发送数据,并等待接受服务器返回的数据。

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()
  {
  }

}


  本文的代码取自WTK demo中的例子,您可以参考demo中的源代码!下面给出MIDlet的代码

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();
      }
    }
  }

}

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