Re: Problem with initializing a class object

Nigel Wade <>
Fri, 09 Feb 2007 09:54:33 +0000
<eqhgcq$68g$> wrote:

On Feb 8, 5:51 am, Nigel Wade <> wrote: wrote:

 > > I am trying to write a more modified version of the infamous

SimpleBrowser by allowing for the WebBrowser object property to
receive a given object.

You've only supplied partial code, so it's only possible to provide guesses


to what is going wrong.


private final WebBrowser webBrowser = new WebBrowser();

What class does this code belong to, where is it's constructor being invoked?

I will attempt to guess at the problem. I think you probably have a race
condition between the thread which is executing the above constructor for
WebBrowser(), and the code below which is explicitly run on the EDT. It looks
like the code below is attempting to set the URL on the WebBrowser before it
has been properly constructed by whichever thread is constructing it.

You might be right, unfortunately I cannot verify it. However, I can
say that the org.jdesktop.jdic.browser.WebBrowser object works when I
do things this way:

     * {@link org.jdesktop.jdic.browser.WebBrowser}
    private WebBrowser browser;

     * Perform setup
    private void setupSimpleBrowser() {

     * Initialize objects
    private void initObjects() {
        WebBrowser.setDebug(false); // SET TO TRUE TO SEE trace()
        browser = new WebBrowser();

     * Initialize components
    private void initComponents() {
        webAddressTextField = new JTextField(51);
        p1 = new JPanel(true);
        p2 = new JPanel(true); // MUST BE SET BEFORE GOING TO
        generateWebBrowser(); // WILL ADD LOCAL WebBrowser INSTANCE
        addToPanel(); // FOR NOW WILL ONLY ADD JPanel p1

     * Generate {@link org.jdesktop.jdic.browser.WebBrowser}
    private void generateWebBrowser() {

        //Use below code to check the status of the navigation
        //or register a listener for the notification events.
                new WebBrowserListener() {
            boolean isFirstPage = true;

            public void initializationCompleted(WebBrowserEvent event)
            public void downloadStarted(WebBrowserEvent event) {;}
            public void downloadCompleted(WebBrowserEvent event) {;}
            public void downloadProgress(WebBrowserEvent event) {;}
            public void downloadError(WebBrowserEvent event) {;}
            public void documentCompleted(WebBrowserEvent event) {
                // Uncomment below code to test getContent()/
                // executeScript() APIs.
                // As the setContent() call will invoke this event,
which falls
                // into a loop, so check if this event is fired by the
                // loaded page.
                if (isFirstPage) {
                    isFirstPage = false;
            public void titleChange(WebBrowserEvent event) {;}
            public void statusTextChange(WebBrowserEvent event) {;}
            public void windowClose(WebBrowserEvent event) {;}


     * Set {@link #browser} with either instantiable {@link} or with {@link #DEFAULT_URL_PATH}
    private void setWebBrowserURL() {
        try {
            URL url = getURL();
            String urlPath = getURLPath();
            if (url != null) {
            } else if (urlPath != null && !urlPath.equals("")) {
                browser.setURL(new URL(urlPath));
            } else {
        } catch (Exception e) {
            try {
            } catch (Exception e2) {

It appears that the crucial element is within initObjects():

webBrowser = new WebBrowser();

In short, you must run the WebBrowser static mehod setDebug() before
you even instantiate a WebBrowser object, otherwise, while the object
will exist regardless, it will not actually set the
parameter unless you have run setDebug() first.


The above is different from your initial code in one very crucial respect. In
the above you create your WebBrowser object in initObjects, which is being run
by the EDT. In your original code you never showed where your WebBrowser was
being created, and if it was not in the EDT that might be the cause of a race

What happens if you reverse the order of the above statements, but keep them
within initObjects()?

The error message in the original post seems to be due to this line of code:
   webBrowser.setURL(new URL(DEFAULT_URL_PATH));
being executed before the construction of webBrowser is complete, when some of
its fields are still null. So the construction of webBrowser and this code must
be being executed in parallel, i.e. on different threads.

I may be wrong of course, but the available evidence points me to that

Nigel Wade, System Administrator, Space Plasma Physics Group,
            University of Leicester, Leicester, LE1 7RH, UK
E-mail :
Phone : +44 (0)116 2523548, Fax : +44 (0)116 2523555

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