Question

I have a password saving program that saves passwords to a text file. The user can add or delete passwords and the data will be re-written to the text file. To prevent the program from being really slow, I create a new thread every time the program needs to save. I have a class that extends Thread and will write all data to the text file specified. The problem I am having is when I try to save all data before the user wants to quit. When the quit button is pressed I create a new thread to save the data and then call the System.exit() function to quit.

Thread t = new SaveThread();
t.start();
System.exit(0);

I have noticed that the program will quit before the thread completes. Is there any way that I can tell java to only quit when my thread has stopped running?

Answer 1

You have three options:

1. Call t.join() after you start the thread
2. Use t.run() instead of t.start()
3. Add your save thread as a shutdown hook via Runtime.addShutdownHook.

Option 1

This one is easy:

Thread t = new SaveThread();
t.start();
t.join();
System.exit(0);

Option 2

Also easy:

Thread t = new SaveThread();
t.run();
System.exit(0);

Option 3

Also easy, but is done when your application is starting. In your app initialization code, do this:

Runtime.getRuntime().addShutdownHook(new SaveThread());

And it will run automatically when the application is closing.