[英]NullPointerException when trying to send a message to a Client Socket
我有一个ServerHandler类,它将创建一个新的PrintWriter并将其存储到PrintWriter数组中。通过为selectedId整数提供应该接收该消息的客户端的ID号,可以发送该消息。 当我在ServerHandler类内调用SendMessage()方法时,它将毫无问题地发送消息,但是当我尝试从包含GUI的类中调用SendMessage(“ some message”)方法时,它给了我NullPointerException。
posljiSporocilo()是实际的SendMessage(),因为某些方法是用我的语言编写的
import java.net.*;
import java.io.*;
import java.awt.*;
public class ServerHandler implements Runnable{
Socket klientSocket;
static int userCounter = 0;
static int selectedId = 0;
BufferedReader reader;
static PrintWriter writer;
static PrintWriter[] writerHolder = new PrintWriter[10];
static outputHelper out = new outputHelper();
public ServerHandler(Socket klientSocket) throws IOException
{
userCounter++;
this.klientSocket = klientSocket;
writer = new PrintWriter(this.klientSocket.getOutputStream());
writerHolder[userCounter] = writer;
InputStreamReader inReader = new InputStreamReader(this.klientSocket.getInputStream());
reader = new BufferedReader(inReader);
out.frameOutput(this.klientSocket.getInetAddress().toString(), "Server sprejel povezavo:");
selectedId = 1;
posljiSporocilo("AutoSent");
当我在实际的类中调用posljiSporocilo()方法时,确实会将消息发送给客户端。
}
public ServerHandler(){
// Do not launch the main Constructor
}
public void run(){
String inMessage = null;
try{
while((inMessage = reader.readLine()) != null){
System.out.println("Server Sprejel: " + inMessage);
}
}catch(IOException ex){
ex.printStackTrace();
}
}
public void selectId(int id){
selectedId = id;
out.frameOutput("ID Changed to: " + selectedId, "ID change");
}
public void posljiSporocilo(String message){
try
{
writerHolder[selectedId].println(message);
writerHolder[selectedId].flush();
}catch(Exception ex)
{
out.frameOutput("ERROR Trying to send a message", "ERR_MESSAGE:");
ex.printStackTrace();
}
}
}
包含GUI内容的类:
package homeControl;
IMPORTS
public class mainWindow extends JFrame {
private JPanel contentPane;
private JTextField messageTextField;
ServerHandler svr = new ServerHandler();
outputHelper o = new outputHelper();
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
mainWindow frame = new mainWindow();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public mainWindow() {
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(100, 100, 450, 300);
JMenuBar menuBar = new JMenuBar();
setJMenuBar(menuBar);
JMenu mnOptions = new JMenu("Options");
menuBar.add(mnOptions);
JMenu mnId = new JMenu("ID");
menuBar.add(mnId);
JMenuItem idEnaItem = new JMenuItem("ID: 1");
mnId.add(idEnaItem);
idEnaItem.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event){
if(event.getActionCommand() != null){
svr.selectId(1);
setTitle("ID changed to 1");
}
}
});
JMenuItem idDvaItem = new JMenuItem("ID: 2");
mnId.add(idDvaItem);
idDvaItem.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event){
if(event.getActionCommand() != null){
svr.selectId(2);
setTitle("ID changed to 2");
}
}
});
contentPane = new JPanel();
contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
setContentPane(contentPane);
contentPane.setLayout(null);
messageTextField = new JTextField();
messageTextField.setBounds(10, 209, 292, 20);
contentPane.add(messageTextField);
messageTextField.setColumns(10);
JButton sendButton = new JButton("Send");
sendButton.setBounds(312, 208, 89, 23);
contentPane.add(sendButton);
sendButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent event){
if(event.getActionCommand() != null)
{
svr.posljiSporocilo("THIS MESSAGE DOES NOT GET SENT");
}
当我从此类调用posljiSporocilo()方法时,它给了我NullPointerException
}
});
}
}
希望我能给您足够的信息,谢谢!
侦错模式SS: http ://imageshack.us/f/543/1vea.png/
您能告诉我为什么要在一开始增加计数器吗? 这将导致writerHolder数组中的第0个元素永远不会填充。
userCounter++;
this.klientSocket = klientSocket;
writer = new PrintWriter(this.klientSocket.getOutputStream());
writerHolder[userCounter] = writer;
我猜您的NPE与这有关。 如果您选择的ID为0,则方法posljiSporocilo中的代码将引发NPE。
但是当我尝试从包含GUI的类中调用SendMessage(“ some message”)方法时,它给了我NullPointerException。
这是很难搞清楚,因为你没有显示该行实际上是扔NPE。 在查看您的代码时,我看到以下代码。 我看到了writeHolder
数组的初始化位置,但我不知道writerHolder[selectedId]
是否可以为null
:
public void posljiSporocilo(String message){
try {
writerHolder[selectedId].println(message);
writerHolder[selectedId].flush();
...
selectedId
可以大于 userCounter
值吗? 如果有多个线程正在调用初始化,则可能应该使用AtomicInteger
而不是非同步的static int
。
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