java - 线程异常

Question

以下错误已通过删除声明得到修复，但出现了另一个以前不存在的错误。

收到两个错误，任何见解都很好，谢谢。

Exception in thread "main" java.lang.NullPointerException at ChatClient.(ChatClient.java:27) at ChatClient.main(ChatClient.java:59)

来自以下 ChatClient:

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

public class ChatClient
{  private Socket socket              = null;
   private BufferedReader  console   = null;
   private BufferedReader  streamIn   = null;
   private DataOutputStream streamOut = null;

   public ChatClient(String serverName, int serverPort, String userName)
   {  System.out.println("Establishing connection. Please wait...");
      try
      {  socket = new Socket(serverName, serverPort);
         System.out.println("Connected: " + socket);
         System.out.println("CTRL+C or type .bye to quit");
         start();
      }
      catch(UnknownHostException uhe)
      {  System.out.println("Host unknown: " + uhe.getMessage());
      }
      catch(IOException ioe)
      {  System.out.println("Unexpected exception: " + ioe.getMessage());
      }
      String line = "";
      while (!line.equals(".bye"))
      {  try
         {  line = console.readLine();
            streamOut.writeBytes(line + '\n'); //Send console data to server socket
            String reply = streamIn.readLine(); //Recieve confirmation msg from server
            System.out.println( reply ); //Print the msg
            streamOut.flush();
         }
         catch(IOException ioe)
         {  System.out.println("Sending error: " + ioe.getMessage());
         }
      }
   }
   public void start() throws IOException
   {  console = new BufferedReader(new InputStreamReader(System.in)); //Changed console to BufferedReader
      streamIn  = new BufferedReader(new InputStreamReader(socket.getInputStream()));
      streamOut = new DataOutputStream(socket.getOutputStream());
   }
   public void stop()
   {  try
      {  if (console   != null)  console.close();
         if (streamOut != null)  streamOut.close();
         if (streamIn != null)  streamIn.close(); //Is it good practice to close
         if (socket    != null)  socket.close();
      }
      catch(IOException ioe)
      {  System.out.println("Error closing ...");
      }
   }
   public static void main(String args[])
   {  ChatClient client = null;
      if (args.length != 3)
         System.out.println("Usage: java ChatClient host port username");
      else
         client = new ChatClient(args[0], Integer.parseInt(args[1]), args[2]);
   }
}

这个错误:

Exception in thread "Thread-1" java.lang.NullPointerException
        at ChatServerThread.handleClient(ChatServerThread.java:41)
        at ChatServerThread.run(ChatServerThread.java:17)

来自聊天服务器线程:

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

//public class ChatServerThread implements Runnable
public class ChatServerThread extends Thread
{  private Socket          socket   = null;
   private ChatServer      server   = null;
   private int             ID       = -1;
   private BufferedReader streamIn =  null;
   private DataOutputStream streamOut = null;

   public ChatServerThread(ChatServer _server, Socket _socket)
   {  server = _server;  socket = _socket;  ID = socket.getPort();
   }
   public void run() {
   try {
       handleClient();
   } catch( EOFException eof ) {
        System.out.println("Client closed the connection.");
   } catch( IOException ioe ) {
        ioe.printStackTrace();
   }
}

   public void handleClient() throws IOException {
      boolean done = false;
      try {
      System.out.println("Server Thread " + ID + " running.");
      while (!done) {
        String nextCommand = streamIn.readLine();
        if( nextCommand.equals(".bye") ) {
           System.out.println("Client disconnected with bye.");
           done = true;
        } else {
           System.out.println( nextCommand );
           String nextReply = "You sent me: " + nextCommand.toUpperCase() + '\n';
           streamOut.writeBytes ( nextReply );
        }
     }
   } finally {
     streamIn.close();
     streamOut.close();
     socket.close();
   }
   }
   public void open() throws IOException
   {
      streamIn = new BufferedReader(new InputStreamReader(socket.getInputStream()));
      streamOut = new DataOutputStream(socket.getOutputStream());
   }
   public void close() throws IOException
   {  if (socket != null)    socket.close();
      if (streamIn != null)  streamIn.close();
      if (streamOut != null) streamOut.close();
   }
}

Answer 1

是的，就是这一行:

line = console.readLine();

console 仍然是 null。即使您正在调用 start()，它也不会像您想的那样工作:

public void start() throws IOException
{  BufferedReader console = new BufferedReader(new InputStreamReader(System.in));

这声明了一个名为 console 的新本地 变量。它不会更改名为 console 的 instance 变量的值。为此，您应该删除声明部分:

public void start() throws IOException
{  
    console = new BufferedReader(new InputStreamReader(System.in));
    ...

即使进行了这种更改，您也可能会遇到问题 - 因为如果那确实抛出异常，那么您在构造函数中使用它进行的操作如下:

catch(IOException ioe)
{  System.out.println("Unexpected exception: " + ioe.getMessage());
}

然后你就好像什么都没发生一样继续。不要那样做。您并没有真正“处理”异常 - 所以您几乎可以肯定要么不首先捕获它，要么在您的 catch block 中重新抛出它。

顺便说一句，您的音梁风格非常密集、非常规且不一致。我强烈建议不要在左大括号之后(在同一行)包含代码。事实上，您的代码对于习惯于(非常)更常见的约定的任何人来说都很难阅读:

if (foo) {
     // Do something
}

或

if (foo)
{
     // Do something
}