Why does SwingWorker stop unexpectedly?

Question

I wanted to try some ideas using SwingWorker , since I did not use it too much. Instead, I had a problem and I can’t understand what happened.

Here's a short SSCCE that demonstrates this problem (I know people here as SSCCE):

 import javax.swing.SwingUtilities; import javax.swing.SwingWorker; public class SwingWorkerTest { public static void main (String[] args) { SwingUtilities.invokeLater (new Runnable () { @Override public void run () { new MySwingWorker (500).execute (); new MySwingWorker (900).execute (); new MySwingWorker (1200).execute (); } }); } } class MySwingWorker extends SwingWorker<Void, Void> { private int ms; public MySwingWorker (int ms) { this.ms = ms; } @Override protected Void doInBackground() { Thread t = Thread.currentThread (); for (int i = 0; i < 50; i++) { try { Thread.sleep (ms); } catch (InterruptedException e) { e.printStackTrace (); } System.out.println ("I am thread with " + ms + " sleep in iteration " + i + ": " + t.getName () + " (" + t.getId () + ")"); } return null; } } 

So, my program should create 3 SwingWorkers that print a line on the screen, then sleep in the specified number of milliseconds, and then print the line again and sleep again, etc. I create a SwingWorker from a Swing thread, because otherwise they won’t even start.

Thus, the expected result:

 I am thread with 500 sleep in iteration 0: SwingWorker-pool-1-thread-1 (15) I am thread with 900 sleep in iteration 0: SwingWorker-pool-1-thread-2 (16) I am thread with 500 sleep in iteration 1: SwingWorker-pool-1-thread-1 (15) I am thread with 1200 sleep in iteration 0: SwingWorker-pool-1-thread-3 (17) I am thread with 500 sleep in iteration 2: SwingWorker-pool-1-thread-1 (15) I am thread with 900 sleep in iteration 1: SwingWorker-pool-1-thread-2 (16) I am thread with 500 sleep in iteration 3: SwingWorker-pool-1-thread-1 (15) I am thread with 1200 sleep in iteration 1: SwingWorker-pool-1-thread-3 (17) I am thread with 500 sleep in iteration 4: SwingWorker-pool-1-thread-1 (15) I am thread with 900 sleep in iteration 2: SwingWorker-pool-1-thread-2 (16) I am thread with 500 sleep in iteration 5: SwingWorker-pool-1-thread-1 (15) I am thread with 500 sleep in iteration 6: SwingWorker-pool-1-thread-1 (15) I am thread with 900 sleep in iteration 3: SwingWorker-pool-1-thread-2 (16) I am thread with 1200 sleep in iteration 2: SwingWorker-pool-1-thread-3 (17) I am thread with 500 sleep in iteration 7: SwingWorker-pool-1-thread-1 (15) ............. 

etc. for a total of 150 lines (3 work x 50 iterations for each)

Instead, the output I get is:

 I am thread with 500 sleep in iteration 0: SwingWorker-pool-1-thread-1 (15) I am thread with 900 sleep in iteration 0: SwingWorker-pool-1-thread-2 (16) I am thread with 500 sleep in iteration 1: SwingWorker-pool-1-thread-1 (15) 

What is it. Only 3 lines. After that, the program will exit. There is no stacktrace from this try catch .

one). Where is the worker with a sleep of 1200 ms? Actually, if I replace 1200 ms with 1000 ms, then this worker also prints 1 line ... yes, only one. Weird ...

2). Why do workers stop? (and I don't get 150 rows)

I ran this program using JRE 7 Update 11 on Windows 7 64-bit .

PS: I am sure that the error mentioned here has been fixed in this version of the JRE, since I get 2 different values ​​(15 and 16) as a stream identifier printed on the console. That was the first thing I suspected.

Answer 1

I believe you need to show the Swing top-level rendered window to keep the Swing event stream alive. Otherwise, the program will be disabled due to the lack of threads without a daemon.

Edit:
To prove that the SwingWorker stream is a Daemon stream, simply add a line of code to test it:

 System.out.println("I am thread with " + ms + " sleep in iteration " + i + ": " + t.getName() + " (" + t.getId() + ")"); // **** added System.out.println("Daemon?: " + Thread.currentThread().isDaemon()); 

Answer 2

If you look at line 771 of the source code for the SwingWorker class (Java SE 7):

 thread.setDaemon(true); 

You will see that SwingWorker is running in the daemon thread, and in the Java JVM will terminate if all non-daemon threads are terminated.

Answer 3

As already mentioned, you can use the user interface to save streams.

Alternatively, you can use SwingWorker#get (or anything that prevents the main thread from completing) to wait for the threads to finish. By doing so, you will get the result as expected. Here is a modified code that does what you want.

 import javax.swing.SwingUtilities; import javax.swing.SwingWorker; import java.util.concurrent.ExecutionException; public class SwingWorkerTest { public static void main (String[] args) { final MySwingWorker w1 = new MySwingWorker (500), w2 = new MySwingWorker (900), w3 = new MySwingWorker (1200); SwingUtilities.invokeLater (new Runnable () { @Override public void run () { w1.execute (); w2.execute (); w3.execute (); } }); try{ // you can replace the code in this block // by anything that keeps the program from // terminating before the threads/SwingWorkers. w1.get(); w2.get(); w3.get(); }catch(InterruptedException e){ System.err.println("InterruptedException occured"); }catch(ExecutionException e){ System.err.println("InterruptedException occured"); } } } class MySwingWorker extends SwingWorker<Void, Void> { private int ms; public MySwingWorker (int ms) { this.ms = ms; } @Override protected Void doInBackground() { Thread t = Thread.currentThread (); for (int i = 0; i < 50; i++) { try { Thread.sleep (ms); } catch (InterruptedException e) { e.printStackTrace (); } System.out.println ("I am thread with " + ms + " sleep in iteration " + i + ": " + t.getName () + " (" + t.getId () + ")"); } return null; } } 

Answer 4

Have you tried to create any user interface? or put something like new Object().wait(); to the end of main to prevent the main thread from exiting?

I’m not quite sure that this is so, but without any actual display of the user interface, I’m sure that the work-swing is just one more thread, and you haven’t configured any of them as daemon threads, so their main beginning is up. basic finishes and the process comes out?