Single instance

Do you mean you want to allow only one instance of your applicatoin to run at a time?

If so, then when it starts up, the first step for it to do is to try to open a ServerSocket on some port. If you can open it, that app is not already running (it doesn't already have the socket open). If you can't open it, then the app (or some app) is already running and already has that port.

When the app exits, the socket will automatically be released.

jverda at 2007-7-14 23:47:41 >

Hello,

i maybe have the same problem. I'm developing a Rich Client (eclipse based) with a derby embedded database. The Database allows only one session. To prevent the error of the application, i want that only one instance of the application exists.

i looked around in different forums and only found that following hint:

on start of the application checking the existence of a startup file on the filesystem. If the file isnt there, write the file and start the application. Else finish the application.

I'm not sure, if i'm in the right forum here. Sorry if not. Else i would be interested in the experiences other have.

with respect

maxe

maxea at 2007-7-14 23:47:41 >

> Hello,

>

> i maybe have the same problem. I'm developing a Rich

> Client (eclipse based) with a derby embedded

> database. The Database allows only one session. To

> prevent the error of the application, i want that

> only one instance of the application exists.

What do you mean "prevent the error"? Is an exception thrown when more than one client tries to connect? If so, then just handle it--that means there's more than one app trying to run.

jverda at 2007-7-14 23:47:41 >

> is it right, that the socket mechanism maybe have

> problems with firewalls?

No. You're not trying to open a connection to another host. You're just using a socket as a semaphore--a resource for which the OS will only allow one acquisition at a time.

It might have permission problems if the user running the Java app is not allowed to listen on the socket in question, or if there's a SecurityManager in place that's preventing it.

The code looks like this:

import java.net.ServerSocket;

import java.io.IOException;

import java.net.BindException;

public static void main(String[] args) {

try {

ServerSocket serverSocket = new ServerSocket(somePortThatYouPick);

}

catch (BindException exc) {

// Another process is listening on that port.

// Probably anoher instance of our app.

}

catch (IOException exc) {

// could not listen on that port for some other reason

}

// If we get here, no other app is listening on the port, so

// so no other instance of this app is running.

// When the app ends, the socket will be released

}

jverda at 2007-7-14 23:47:41 >

Hey, thanks for this example, it helped me make my app only run a single instance.

One small correction (and I may be wrong being the n00b programmer that I am), but the comment that says "If we get here, no other app.." will be reached regardless of exceptions thrown. It seems like you'd want to put your conditional code after the ServerSocket declaration.

Anyway, thanks again.

.andisblue.a at 2007-7-14 23:47:41 >

> Also, which socket should I choose? I wouldn't want

> to pick a number that is common to another app. Can

> it be any number?

Up through 1024 (I think) are reserved for well-known services. In general just pick a high number in the tens of thousands. You might want to do something like netstat or cport or something just to see what's used at the moment and increase your odds of guessing at one that's not used elsewhere.

jverda at 2007-7-21 11:11:27 >

> Also, which socket should I choose? I wouldn't want

> to pick a number that is common to another app. Can

> it be any number?

Maybe something like that could be a solution, but I don't know if all that is worth of the result.

1. read the port number p from the configuration files

2. try to create a serversocket at port p

2.1. if 2. succeeds, all is ok. But you must communicate further with clients. goto 3.

2.2. if 2. fails, connect to p as a client, and communicate with it to ensure that the server is an instance of your application

2.2.1. if 2.2. succeeds, close the application. goto 3.

2.2.2. if 2.2. fails, that port is used by another sort of application. Choose random p, write it to the configuration file and goto step 1.

3. end

myavuzselima at 2007-7-21 11:11:27 >

Yeah, like getting instance of MyClass from all local JVM?!!

Since JDK 1.5 and above having something like "jps" to show all process, but they bind to some port too.

Is this a Java limitation? I would prefer someone from Sun Java to answer this.

Regards,

Avatar Ng

Message was edited by:

Avatar_Ng

Avatar_Nga at 2007-7-21 11:11:27 >