Re: how to prevent a jar executed only once in a machine


Steve W. Jackson wrote:
In article <1151688143.315164.248010@xxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"John_Woo" <john_woo@xxxxxxxxxx> wrote:

Hi,

If my.jar is a swing UI app (or just a simple app), I'm wondering if
it's possible or how, that
this my.jar can be executed only once (no multi-session) in same
machine <or in same folder>?

Can any one tell?

--
Thanks
John
Toronto

There are a couple of common tricks used for preventing multiple
instances of an app from running.

One is to have your app open a ServerSocket on a specific port, which
prevents any other application on the same network interface from doing
the same (it'll already be in use when the second attempt occurs). But
this isn't always a good technique, depending on the port and other
factors.

Another is to create a file opened for output (I use a FileOutputStream)
and then obtain an exclusive lock on it. When the app terminates, the
lock is freed. I close the FileOutputStream at normal termination set
the file to delete when the JVM exits. When a second instance launches,
it may already find the file (in the event of an abnormal termination),
but it can't get a lock if another instance is running. I only use this
technique for restricting an individual user, so that the file is in the
user's home directory, but it can be readily adapted for system-wide use.

= Steve =
--
Steve W. Jackson
Montgomery, Alabama

Thanks lots Steve.
Let's talk about the second aproach further.

I know using:

f = new File("new file");
f.close();

to create a phycical file. but questions :

1. how to obtain an exclusive lock on it? from file API, I can only
see

setReadOnly()

2. if this app is terminated by unexpected way <like in unix, kill
-pid>, how to rm this file?

--
Thanks lots
John
Toronto

.

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