java 网络编程 java.NullPointerException

[jiangtianyin]

今天刚接触Java网络编程，跟着书上做了一个简单的客户端与服务器通信例子，结果老是报异常，不知道那里出错了，由于是第一次接触，不知道是不是需要配置什么东西，我直接就开始写代码了，特来请教各位高手

EchoServer.java 为服务器端程序，代码如下：

import java.io.*;
import java.net.*;

public class EchoServer
{
	private int port=8000;
	private ServerSocket svSocket;
	
	public EchoServer() throws IOException
	{
		ServerSocket svSocket=new ServerSocket(port);
		System.out.println("服务器启动！");
	}
	
	public String echo(String msg){
		return "echo: "+msg;
	}
	
	private PrintWriter getWriter(Socket sk) throws IOException{
		OutputStream out=sk.getOutputStream();
		return new PrintWriter(out,true);
	}
	
	private BufferedReader getReader(Socket sk) throws IOException{
		InputStream in=sk.getInputStream();
		return new BufferedReader(new InputStreamReader(in));
	}
	
	public void service()
	{
		while(true)
		{
			Socket sk=null;	
			try
			{
				sk=svSocket.accept(); //通过使用输出语句测试好像是这里抛出了异常
				System.out.println("new connection accepted: "+sk.getInetAddress()+":"+sk.getPort());
				BufferedReader br=getReader(sk);
				PrintWriter pw=getWriter(sk);
				
				String msg=null;
				while((msg=br.readLine())!=null)
				{
					System.out.println(msg);
					pw.println(echo(msg));
					if(msg.equals("bye"))
					{
						break;
					}
				}
			}
			catch(IOException ex)
			{
					ex.printStackTrace();
			}
			finally
			{
					try
					{
						if(sk!=null)
							sk.close();
					}
					catch(IOException ex)
					{
						ex.printStackTrace();
					}
			}
		}
	}
			
	public static void main(String [] args) throws IOException
	{
			System.out.println("hello");
			new EchoServer().service();
	}
}

运行时抛出异常：
Exception in thread "main" java.lang.NullPointerException
at EchoServer.service(EchoServer.java:38)
at EchoServer.main(EchoServer.java:77)

EchoClient.java为客户端程序,代码如下：

import java.io.*;
import java.net.*;
import java.util.*;

public class EchoClient{
	private String host="localhost";
	private int port=8000;
	private Socket sk;
	
	public EchoClient() throws IOException{
		sk=new Socket(host,port);
	}
		
	public static void main(String[] args) throws IOException{
		new EchoClient().talk();
	}
	private PrintWriter getWriter(Socket sk) throws IOException{
		OutputStream out=sk.getOutputStream();
		return new PrintWriter(out,true);
	}
	
	private BufferedReader getReader(Socket sk) throws IOException{
		InputStream in=sk.getInputStream();
		return new BufferedReader(new InputStreamReader(in));
	}
	
	public void talk() throws IOException{
		try{
			BufferedReader br=getReader(sk);
			PrintWriter pw=getWriter(sk);
			BufferedReader localReader=new BufferedReader(new InputStreamReader(System.in));
			
			String msg=null;
			while((msg=localReader.readLine())!=null){
				pw.println(msg);
				System.out.println(br.readLine());
				if(msg.equals("bye"))
					break;
			}
		}catch(IOException e){
			e.printStackTrace();
		}finally{
			try{
				sk.close();
			}catch(IOException e){
				e.printStackTrace();
			}
		}
	}
}

抛出异常：Exception in thread "main" java.net.ConnectException: Connection refused: connect
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(Unknown Source)
at java.net.PlainSocketImpl.connectToAddress(Unknown Source)
at java.net.PlainSocketImpl.connect(Unknown Source)
at java.net.SocksSocketImpl.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at java.net.Socket.<init>(Unknown Source)
at EchoClient.<init>(EchoClient.java:11)
at EchoClient.main(EchoClient.java:15)

[火龙果被占用了]

EchoServer 38 行报错了呗，不知道 38 行是哪一行

[fulinkster]

private ServerSocket svSocket;

public EchoServer() throws IOException
{
ServerSocket svSocket=new ServerSocket(port);
System.out.println("服务器启动！");
}

然后你public void service()里的 sk=svSocket.accept(); 根本没实例化！

[jiangtianyin]

不是这个问题吧，我的main()方法中写的是 new EchoServer().service();会先到用构造方法
public EchoServer() throws IOException
{
ServerSocket svSocket=new ServerSocket(port);
System.out.println("服务器启动！");
}
在这里就已经实例化了