NullpointerException

[Medes Agri]

Hi,

I have created a server application (very simple) when i run it by using JCreator it goes well and it wates untill a client connect on it.

for the first time i used NetBeans and I made my first Java Application I added my Server to the project and removed the main method of the server.

the main method of the Main class of my project looks like this.

public static void main(String[] args)
{
int port = 7500;
MyServer MS = new MyServer(port);
MS.accept();
}

when i compile it , it goes well and it does not generate any error but when i run my project i get following error:

Exception in thread "main" java.lang.NullPointerException
at MyPackage.MyServer.accept(MyServer.java:41)


thanks for your help

[Ernest Friedman-Hill]

The error message says that the problem is in the accept() method of the MyServer class, at line 41 of MyServer.java. Let's see that method, and tell us which is line 41; the problem will probably be obvious.

[Medes Agri]

yes,

line 41 is s = MyServerSocket.accept();

s is socket.

[Ernest Friedman-Hill]

OK, well, the error means that MyServerSocket is null -- i.e., the actual ServerSocket object is not being created. Where does it get initialized? Is the code that initializes it still going to run in your new configuration?

[Medes Agri]

Thanks,

here is MyServer


public class MyServer
{
private ServerSocket MyServerSocket = null;
ArrayList words = new ArrayList();
ArrayList numbers = new ArrayList();

public MyServer(int port)
{
try
{
MyServerSocket = new ServerSocket(port);
}catch(IOException e){}
}

public void accept()
{
try
{
System.out.println("Server 1");
Socket s;
int ThreadId = 0;
while(true)
{
s = MyServerSocket.accept();
ThreadId++;


and I have called MyServer as i wrote in my first topic

[Henry Wong]

public MyServer(int port) {
  try {
    MyServerSocket = new ServerSocket(port);
  }catch(IOException e){}
}

Don't just ignore the IO exception. If something goes wrong with the instantiation -- such as another app is already using that port. The MyServerSocket will still be null.

Henry
[ July 10, 2006: Message edited by: Henry Wong ]

[Medes Agri]

thank you,

I changed to catch(Exception e)instead.
and moved MyServerSocket = new ServerSocket(port); to the accept method

but now it generates


FAILED TO HOST ON 7500
[ July 10, 2006: Message edited by: Medes Agri ]

[Ernest Friedman-Hill]

This:

}catch(IOException e){}


Is about the worst thing you can do, especially as a beginner. An exception is a message that tells you about something important that's happening; an empty catch block tears that message up without even looking at it. At the very least, you want to say

} catch (IOException e) { e.printStackTrace(); }

In any case, since that variable is null, I can guarantee you that this will print something out, and it's going to tell you exactly what the problem is!

[Jody Brown]

Changing the Exception type wont alter a thing - there is still a fundamental reason why your attempt to create an instance of ServerSocket is failing. in your catch block, try the following, and post the results it displays here for us to take a look at:

  catch(IOException e){ e.printStackTrace(); }  

[John Bartlett]

Hi there,

I believe what Henry means, is that you need to handle the exception if it is thrown. All you are doing is throwing the exception you are not actually saying what you want done incase of the exception.

Perhaps something like this:

    
 
 try
   {
     ss = new ServerSocket(4448);
     System.out.println("Server listening on port 4448...");
   }
   catch (IOException e)
   {
     System.out.println("Error Couldnt create Socket!");
     System.exit(1);
   }
        
 

I may be wrong, but worth a try i suppose?

Regards,

John

[Medes Agri]

I used catch(IOException) {e.printStackTrace();

and the result is

init:
deps-jar:
Compiling 3 source files to C:\JavaProjects\OneClientTwoServer\build\classes

Note: Recompile with -Xlint:unchecked for details.
compile:
run:
java.net.BindException: Address already in use: JVM_Bind
Server 1
at java.net.PlainSocketImpl.socketBind(Native Method)
at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:359)
at java.net.ServerSocket.bind(ServerSocket.java:319)
at java.net.ServerSocket.<init>(ServerSocket.java:185)
at java.net.ServerSocket.<init>(ServerSocket.java:97)
at oneclienttwoserver.ServerOne.<init>(ServerOne.java:22)
at oneclienttwoserver.Main.main(Main.java:30)
FAILED TO HOST ON 7500
BUILD SUCCESSFUL (total time: 0 seconds)

how it could use port 7500 when it generated exceptions always
[ July 10, 2006: Message edited by: Medes Agri ]

[Ernest Friedman-Hill]

OK, well, there you go. Only one program can listen on a given port at a time, but the error says another program is already using your port 7500:

java.net.BindException: Address already in use: JVM_Bind

Perhaps you just have another copy of your code still running.

[John Bartlett]

yes, you are already running something on that port. probably another copy of the program. Perhaps you shud try using a different port?

John