PrintWriter []数组中的NullPointerException-Java [英] NullPointerException in PrintWriter[] array - Java
问题描述
我现在想解决几天的问题。
I have a problem which i am trying to figure out for days now.
简单代码概述（伪代码）：
Simple Code Overview(PseudoCode):
1-当用户连接时，为他创建一个PrintWriter。
1 - When a user connects create a PrintWriter for him.
2-将PrintWriter对象存储到PrintWriter []数组中。
2 - Store the PrintWriter object into a PrintWriter[] array.
3-增加连接的用户数。
3 - Increment the number of users connected.
我的问题是什么？
- 编译器抛出NullPointerException
是什么原因导致NPE？
What caused the NPE?
-
当mainWindow类中创建的对象调用sendMessage（）方法时，PrintWriter数组为空，我不知道是什么原因导致的为空，因为所有变量和数组都具有静态修饰符，并且PrintWriter []数组在构造器中（当用户连接时）获得一个值。
When the sendMessage() method is called by object created in the mainWindow class the PrintWriter array if empty, i don't know what is causing it to be empty since all the the variables and arrays have static modifiers and the PrintWriter[] array gets a value in the constuctor(when a user connects).
Inside ServerHandler构造函数，当我尝试向客户端发送消息时，它工作正常，并且可以看到PrintWriter []数组不为空， t当我从mainWindow实例调用sendMessage（）方法时，数组为空，而userCounter整数变量仍为0。
Inside the ServerHandler constuctor when i try to send a message to the client it works fine and i can see that the PrintWriter[] array is not empty, but when i call the sendMessage() method from the mainWindow instance the array is empty and the userCounter integer variable is still 0.
我试图解决什么？
- 将变量和数组修饰符设置为易变和静态。
- 一遍又一遍地编写所有代码。
服务器类（工作正常）：
Server Class(works fine):
package homeControl;
import java.net.*;
import java.util.Scanner;
import java.io.*;
import java.awt.*;
public class Server{
String sporocilo = null;
int id = 0;
outputHelper o = new outputHelper();
public void sprejmiPovezavo()
{
try
{
ServerSocket svrSock = new ServerSocket(5000);
while(true){
Socket klientSocket = svrSock.accept();
new Thread(new ServerHandler(klientSocket)).start();
}
}catch(IOException ex)
{
ex.printStackTrace();
}
}
public static void main(String[] args){
Server svr = new Server();
svr.sprejmiPovezavo();
}
}
ServerHandler类（此类会导致问题）
ServerHandler class(this class causes the problems)
package homeControl;
import java.net.*;
import java.util.ArrayList;
import java.io.*;
import java.awt.*;
public class ServerHandler implements Runnable{
static PrintWriter pw;
static PrintWriter[] writerHolder = new PrintWriter[10];
static int userCounter;
static int selectedID;
public ServerHandler(Socket socket){
try{
pw = new PrintWriter(socket.getOutputStream());
writerHolder[userCounter] = pw;
// Testing if this writerHolder[] can send the message to the client.
writerHolder[userCounter].println("autoSent - Testint!");
writerHolder[userCounter].flush();
userCounter++;// Increment the number of people connected
//Testing the values of the variables.
System.out.println("ORG!\nuserCounter:" + userCounter + "\nwriterHolder[userCounter]" + writerHolder[0]);
}catch(Exception ex)
{
ex.printStackTrace();
}
}
public ServerHandler()
{
// Empty constructor
// Used so that i don't need to pass a Socket into the class when i create a ServerHandler object from the mainWindow.class...
}
public void sendMessage(int tempID, String messageToBeSent)
{ // Testing the values of variables at the time that this method gets called.
System.out.println("tempID:" + tempID + "\n" + "messageToBeSent:" + messageToBeSent + "\n" + "userCounter:" + userCounter + "\n" + "selectedID:" + selectedID);
// End of test/Actual code:
writerHolder[tempID].println(messageToBeSent);
writerHolder[tempID].flush();
}
public void run()
{
}
}
mainWindows类（调用sendMessage（）方法的地方）
mainWindows class(where the sendMessage() method gets called)
package homeControl;
import java.awt.*;
import java.io.*;
import java.net.Socket;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JMenuBar;
import javax.swing.JMenu;
import javax.swing.JMenuItem;
import javax.swing.JTextField;
import javax.swing.JButton;
public class mainWindow extends JFrame {
private JPanel contentPane;
private JTextField messageTextField;
ServerHandler svr = new ServerHandler(); // Thats why i use the empty constuctor in ServerHandler.class.
outputHelper o = new outputHelper();
int ID;
/**
* 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: 0");
mnId.add(idEnaItem);
JMenuItem idDvaItem = new JMenuItem("ID: 1");
mnId.add(idDvaItem);
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)
{ // Calling the sendMessage() in the ServerHandler.class which should send a message to the Client.
svr.sendMessage(ID,messageTextField.getText());
}
}
});
}
}
我在测试ServerHandler构造函数时的输出PrintWriters：
Output from the ServerHandler constructor when i'm testing the PrintWriters:
ORG！
userCounter：1
userCounter:1
writerHolder [userCounter] java.io.PrintWriter@798c668c
writerHolder[userCounter]java.io.PrintWriter@798c668c
当我手动尝试发送消息时，从SendMessage方法获取输出：
Ouput from the SendMessage method when i manualy try to send a message:
messageToBeSent：dsda
messageToBeSent:dsda
userCounter：0
userCounter:0
selectedID：0
selectedID:0
错误：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at homeControl.ServerHandler.sendMessage(ServerHandler.java:45)
at homeControl.mainWindow$2.actionPerformed(mainWindow.java:84)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
推荐答案
如我所知，这是问题（在服务器中）：
As far as I can tell, this is the problem (in Server):
new Thread(new ServerHandler(klientSocket)).start();
在这里，您正在创建 new ServerHandler 。这与您在 mainWindow 中创建的 ServerHandler 实例完全不同。因此，由 Server 创建的一个将以填充的 PrintWriter 数组结尾，而另一个在 mainWindow 中创建将始终具有一个空元素值数组。
Here you're creating a new ServerHandler. That's completely separate from the instance of ServerHandler which you're creating in mainWindow. So the one created by Server will end up with a populated PrintWriter array... and the one that you're creating in mainWindow will always have an array of null element values.
您应该具有一个 ServerHandler 的em>实例，其中 mainWindow 和 Server 共享
You should have one instance of ServerHandler which both mainWindow and Server share.
这篇关于PrintWriter []数组中的NullPointerException-Java的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
