用数据包socket实现双工通信报参数不合法错误

[javacankillyou]

虚心请教大家。我用数据包socket实现一个双工通信的过程。而在其中客户端和服务器的代码基本一样，知识send和receive换了一个位置。但是运行时，发现一个没错，一个报出了参数不合法异常。请大家帮帮忙看哪里出错了。

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.SocketException;


public class myDatagramSocket extends DatagramSocket {
    final int max_len=1000;
	public myDatagramSocket(int myport) throws SocketException {
		super(myport);
	}
	public void sendMessage(InetAddress receiverAddr,int receiverPort,String message){
		byte[] buffer=new byte[max_len];
		buffer=message.getBytes();
		DatagramPacket mypacket=new DatagramPacket(buffer,max_len,receiverAddr,receiverPort );
		try {
			this.send(mypacket);
		} catch (IOException e) {
			// TODO 自动生成的 catch 块
			e.printStackTrace();
		}
	}
	public String receiveMessage(){
		byte[] buffer=new byte[max_len];
		DatagramPacket mypacket=new DatagramPacket(buffer, max_len);
		try {
			this.receive(mypacket);
		} catch (IOException e) {
			// TODO 自动生成的 catch 块
			e.printStackTrace();
		}
		String message=new String(buffer);
		return message;
	}

}

继承datagramSocket的类。

import java.net.InetAddress;
import java.net.SocketException;
import java.net.UnknownHostException;


public class Example2SenderReceiver {

	public static void main(String[] arg){
		if(arg.length!=4){
			System.out.println("此程序需要输入四个命令参数");
			return;
		}else{
			try {
				InetAddress addr=InetAddress.getByName(arg[0]);
				int myport=Integer.parseInt(arg[1]);
				int receiverPort=Integer.parseInt(arg[2]);
				String message=arg[3];
				myDatagramSocket mysocket=new myDatagramSocket(myport);
				mysocket.sendMessage(addr,receiverPort,message);
				System.out.println(mysocket.receiveMessage());
				mysocket.close();
			} catch (UnknownHostException e) {
				// TODO 自动生成的 catch 块
				e.printStackTrace();
			} catch (SocketException e) {
				// TODO 自动生成的 catch 块
				e.printStackTrace();
			}
			
		}
		
	}
}

客户端

import java.net.InetAddress;
import java.net.SocketException;
import java.net.UnknownHostException;


public class Example2ReceiverSender {

	public static void main(String[] arg){
		if(arg.length!=4){
			System.out.println("此程序需要输入四个命令参数");
			return;
		}else{
			try {
				InetAddress addr=InetAddress.getByName(arg[0]);
				int myport=Integer.parseInt(arg[1]);
				int receiverPort=Integer.parseInt(arg[2]);
				String message=arg[3];
				myDatagramSocket mysocket=new myDatagramSocket(myport);
				System.out.println(mysocket.receiveMessage());
				mysocket.sendMessage(addr, receiverPort, message);
				mysocket.close();
			} catch (UnknownHostException e) {
				// TODO 自动生成的 catch 块
				e.printStackTrace();
			} catch (SocketException e) {
				// TODO 自动生成的 catch 块
				e.printStackTrace();
			}
			
		}
		
	}
}

服务端。

Exception in thread "main" java.lang.IllegalArgumentException: illegal length or offset
	at java.net.DatagramPacket.setData(Unknown Source)
	at java.net.DatagramPacket.<init>(Unknown Source)
	at java.net.DatagramPacket.<init>(Unknown Source)
	at myDatagramSocket.sendMessage(myDatagramSocket.java:16)
	at Example2SenderReceiver.main(Example2SenderReceiver.java:19)

报错信息。
我客户端我设置的参数是：127.0.0.1 8000 9000 客户端发给服务器
服务端设置的信息参数是：127.0.0.1 9000 8000 服务端发给客户端

谢谢大家。

[javacankillyou]

知道了 DatagramPacket mypacket=new DatagramPacket(buffer,max_len,receiverAddr,receiverPort );这里出错了。不要用max_len。应该用buffer.length