写入服务器时的NullPointerExpetion [英] NullPointerExpetion when writing to server

问题描述

你好。



这两天我试图弄清楚我在下面的代码中做错了什么：

Hello there.

For two days i''m trying to figure out what am i doing wrong in the code bellow:

(receiving NullPointerException on actionPerformed method of sendButton, on line output.println(messageToServer);

public class ChatApplication implements ActionListener{

	//here i've declared my variables JTextAreas, clientSocket, buttons, JFrame, String userName
	
	public ChatApplication(){}
	
	public ChatApplication(String userName,String serverName) throws UnknownHostException, IOException{
		//super(userName);
//		try{
		this.userName=userName;
		clientSocket=new Socket(serverName,11111);
		input=new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
		output=new PrintWriter(new OutputStreamWriter(clientSocket.getOutputStream()),true);
		output.println(userName);
		new ServerMessageThread().start();
		
//		}catch(IOException e){
//			System.err.println(e.getMessage());
//		}
	}
	
	//creating my interface etc.
	sendButton.addActionListener(this);		
		connectButton.addActionListener(new ActionListener(){			
			public void actionPerformed(ActionEvent e){
				
				String userName=JOptionPane.showInputDialog(window,"Name: "," ",JOptionPane.PLAIN_MESSAGE);
				try{
					String serverName="localhost";
					new ChatApplication(userName, serverName);
					window.setTitle(userName);
										
				}catch(IOException ex){
					JOptionPane.showMessageDialog(window,"Clientul nu se poate conecta."+ex.getMessage().toString());
				}				
			}			
		});
		
		startServerButton.addActionListener(new ActionListener(){
			
			public void actionPerformed(ActionEvent e){
				Thread thread=new Thread(){
					public void run(){
						
						ChatServer server=new ChatServer();
						try {
							server.process();
						} catch (Exception e) {
							System.err.println(e.getMessage());
							e.printStackTrace();
						}
					}
				};
				
				thread.start();		
			}			
		});		
	}
	
	public static void main(String[] args) {
			
			ChatApplication program=new ChatApplication();
			program.createInterface();
	}
	

	public void actionPerformed(ActionEvent e) {
		
		if(e.getSource()==sendButton){
			sendFlag=true;
			
			try{
			String messageToServer=new String("");
			messageToServer=messageField.getText();
			//PipedOutputStream pipedOut=new PipedOutputStream();
				
			output.println(messageToServer);//<- this is where i get NullPointerException
			
			}catch(NullPointerException ex){
				System.err.println(ex.getMessage());
				ex.printStackTrace();
			}
		}
	}
	
	
	
	/*
	 * clasa se ocupa cu crearea unui thread pentru a putea citi de la server
	 */
	
	class ServerMessageThread extends Thread{
		
		
		public void run(){
			
			String serverResponse=new String();
			
				try {
					while(true){
					serverResponse=input.readLine();
					System.out.println(serverResponse);
					output.println(serverResponse); //and here sending a message to the server works
					ChatApplication.messagesTextArea.append(serverResponse+"\n");
					
					}
				} catch (IOException e) {
					ChatApplication.messagesTextArea.setForeground(Color.RED);
					ChatApplication.messagesTextArea.setText("Eroare citire de la server: "+e.getMessage());
				}
		}
	}
	
	/*
	 * clasa care se ocupa cu crearea firelor de executie a clientilor
	 */

}

当我点击sendButton时，我得到NullPointerException。而且我有点卡在那里。 outputStream在ServerMessageThread中工作，但在单击我的按钮时则不行。我知道它可以修复，但目前我无法理解。



ChatServer可以正常工作，因为我已经测试过它而不使用客户端界面，在控制台中测试它。对于每个客户我都在创建一个帖子。



我只需要一个想法。谢谢。最好的问候。



PS：我知道在问题中加入很多代码并不是一个好习惯，但我想告诉你我是如何实现客户端的app。

When i click the sendButton i get NullPointerException. And I''m kinda stuck there. The outputStream works in the ServerMessageThread, but not when clicking my button. I know that it can be fixed, but at the moment i can''t figure it out.

The ChatServer works, because i''ve tested it without using a client interface, tested it in the console. For every client i''m creating a thread.

I just need an idea.Thank you. Best regards.

P.S: i know that it isn''t a good practice to put much code in the question, but i wanted to show you how i implemented the client app.

推荐答案

public static void main(String[] args) {
			
	ChatApplication program=new ChatApplication(); //calling noarg constructor
	program.createInterface();
}

我明白了，您在参数化的ctor（构造函数）中进行所有流初始化，但是您正在调用默认的ctor 。所有东西都保持为null，NullPointerException。



现在这里

I see, You do all stream initialisation in the parameterized ctor (constructor) but you are calling the default ctor. Every thing is kept null so, NullPointerException.

Now here

sendButton.addActionListener(this);		
		connectButton.addActionListener(new ActionListener(){			
			public void actionPerformed(ActionEvent e){
				
				String userName=JOptionPane.showInputDialog(window,"Name: "," ",JOptionPane.PLAIN_MESSAGE);
				try{
					String serverName="localhost";
					new ChatApplication(userName, serverName); //call to appropriate constructor
					window.setTitle(userName);
										
				}catch(IOException ex){
					JOptionPane.showMessageDialog(window,"Clientul nu se poate conecta."+ex.getMessage().toString());
				}				
			}			
		});

在这里你打电话给所需的ctor并开始一个线程，ctor，初始化流，因此通话很好。



PS：你的用户界面很乱，做一些功课。

Here you call the required ctor and start a thread, the ctor, initializes the streams and hence the calls are fine.

PS: Your UI is messy, do some homework.

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