Question

I’m loading contents of a file to JList component. If I do the loading in main thread, everything seems to be OK – contents are loaded. But when I move loading code to separate thread, select an item in a list and try to reload the list, I get random NullPointer or IndexOutOfBounds exceptions. I’m sure this is some kind of Swing threading issue, but can’t determine what.

This is my thread code:

@Override
public void run() {
    List<String> textLines = null;
    textLines = splitter.split(model.getLedMaxChars(), textLoader.loadText(file));
    listener.onTextLoaded(textLines);//listener is in main Swing code
}

Controller is responsible for listening:

@Override
public void onTextLoaded(List<String> textLines) {
    view.fileLoaded(model.getCurrentFile());
    view.setTextLines(textLines);
    view.enableListComponent();
}

And the view updates:

public void setTextLines(List<String> textLines) {
    jList.setListData(textLines.toArray());
}

I’ve tried to leave thread to hang by adding while(true); loop – then everything works OK. If I hit reload without selecting item in a list, everything works too.

Could anyone explain what I’m missing here?

Answer 1

Swing components are usually not thread-safe. This means that only the Swing worker thread should do any modifications:

Runnable worker = new Runnable() {  
  public void run() {
     jList.setListData(textLines.toArray());
  } 
};
SwingUtilities.invokeLater(worker);

See also:

• http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html
• http://java.sun.com/products/jfc/tsc/articles/threads/threads2.html