Prevent Single Instance of my app In JNLP

Question

HI Guys,
I want to run my app with only one instance for each workstation. I need to know how can i do it. I think i need to set properties, Please help me how can i set properties.
Thanks for your help

Answer 1

A simple way to ensure a single application instance in most Operating System is to create a lock file and open it (for writing) in your application. While your application is running that lock file cannot be deleted because the Operating System doesn't allow it.You can experiment this behavior opening a file for writing and trying to delete that file manually.
But only creating a lock file and opening it for writing is not enough. You should also do 2 things:
1) Check if the file exists. If the file doesn't exist, you know for sure that there is no other application running (use a proper name like 'your application name'.lock);
2) If the file exists, try to delete it. If the deletion of the lock file is not possible that means another application opened it for writing, and so, there is another instance of your application already running.
This solution also works if the application exists abnormally. The deletion of the lock file is not allowed while the application is running. If the application exits (abnormally or not) the deletion of the lock file is possible.
Although it's not mandatory, it is a good practice deleting the lock file after a normal exit of your application (you should close the object that handles the file writing before deleting it - otherwise it's not possible to delete it).That way, you know if the last use of the application was exited ok or not.
Here is the minimum code (without the deletion of the lock file when the application exits normally - I'm not deleting the lock file because it's not mandatory and many programmers thinks that it is. You can delete the lock file when the application exits normally to have additional information of the last use of your app).
try {
File file = new File("app.lock";); // use a proper lock file name.
if (file.exists()) { // if the file doesn't exist you know that there isn't another instance of your app.
System.out.println("Lock exists!");
if (file.delete()) { // test if the app can delete the lock file.
System.out.println("Lock deleted!"); // no other instance was locking the file
else {
System.out.println("An instance of this application is already running."); // the deletion wasn't possible because another instance of your application was locking the file.
System.exit(0);
} catch (Exception nop) {
FileWriter lock;
try {
lock = new FileWriter(app.lock";); // the same lock file name.
} catch (IOException nop) {
IMPORTANT: Do NOT close the lock file after opening it as a FileWriter object - that's the whole point (if you close the lock file, the OS allows the file deletion). If you want, you can close the FileWriter object just before the lock file deletion (if you've decided to delete the lock file after exiting).
Best regards,
Yassuo Toda.
Edited by: Yassuo on Sep 17, 2007 11:20 AM