在J2ME中实现基于UDP协议通讯程序
2024年03月07日
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();
}
}
}
}
