Java – even with wrapper classes, swingworker exceptions are lost

I have been trying to solve the usability problem of swingworker. It will throw any exception in the background task, such as on this so thread This thread gives a good description of the problem, but does not discuss restoring the original exception

The applet I have delivered needs to propagate exceptions upward But I haven't caught it yet I am trying to solve this problem by using the simpleswingworker wrapper class in this blog entry This is a fairly small class, but I will reprint it at the end for reference only

The calling code looks like

try {
    // lots of code here to prepare data,finishing with
    SpecialDataHelper helper = new SpecialDataHelper(...stuff...);
    helper.execute();  // this will call get+done on the actual worker
} catch (Throwable e) {
    // used "Throwable" here in desperation to try and get
    // anything at all to match,including unchecked exceptions
    //
    // no luck,this code is never ever used :-(
}

packing:

class SpecialDataHelper extends SimpleSwingWorker {
    public SpecialDataHelper (SpecialData sd) {
        this.stuff = etc etc etc;
    }
    public Void doInBackground() throws Exception {
        OurCodeThatThrowsACheckedException(this.stuff);
        return null;
    }
    protected void done() {
        // called only when successful
        // never reached if there's an error
    }
}

The function of simpleswingworker is that the do() / get() method of the actual swingworker is called automatically This theoretically refutes any exceptions that occur in the background In practice, nothing is caught, and I don't even know why

The simpleswingworker class is used for reference. For brevity, there are no messages:

import java.util.concurrent.ExecutionException;
import javax.swing.SwingWorker;

/**
 * A drop-in replacement for SwingWorker<Void,Void> but will not silently
 * swallow exceptions during background execution.
 *
 * Taken from http://jonathangiles.net/blog/?p=341 with thanks.
 */
public abstract class SimpleSwingWorker {
    private final SwingWorker<Void,Void> worker =
        new SwingWorker<Void,Void>() {
            @Override
            protected Void doInBackground() throws Exception {
                SimpleSwingWorker.this.doInBackground();
                return null;
            }

            @Override
            protected void done() {
                // Exceptions are lost unless get() is called on the
                // originating thread.  We do so here.
                try {
                    get();
                } catch (final InterruptedException ex) {
                    throw new RuntimeException(ex);
                } catch (final ExecutionException ex) {
                    throw new RuntimeException(ex.getCause());
                }
                SimpleSwingWorker.this.done();
            }
    };

    public SimpleSwingWorker() {}

    protected abstract Void doInBackground() throws Exception;
    protected abstract void done();

    public void execute() {
        worker.execute();
    }
}

Solution

Forget your packaging, it's eating abnormally, and swingworker doesn't The following is how to use swingworker and how to handle specific exceptions thrown from background tasks:

class MeaningOfLifeFinder extends SwingWorker<String,Object> {
    @Override
    public String doInBackground() throws SomeException {
        return findTheMeaningOfLife();
    }

    @Override
    protected void done() { // called in the EDT. You can update the GUI here,show error dialogs,etc.
        try { 
            String meaningOfLife = get(); // this line can throw InterruptedException or ExecutionException
            label.setText(meaningOfLife);
        } 
        catch (ExecutionException e) {
            Throwable cause = e.getCause(); // if SomeException was thrown by the background task,it's wrapped into the ExecutionException
            if (cause instanceof SomeException) {
                // TODO handle SomeException as you want to
            }
            else { // the wrapped throwable is a runtime exception or an error
                // TODO handle any other exception as you want to
            }
        }
        catch (InterruptedException ie) {
            // TODO handle the case where the background task was interrupted as you want to
        }
    }
}