Here are some of the typical errors you might encounter:

Missing timeout class

java.lang.NoSuchMethodError: org.mortbay.thread.Timeout

Context menu on project: Properties -> Build Path -> Order and Export: move GWT SDK *above* App Engine SDK (cf. here)

Compilation unit was not seen

com.google.gwt.junit.JUnitFatalLaunchException: The test class 'com.test.client.MyTest' was not found in module 'com.text.Module'; no compilation unit for that type was seen

• The test class needs to be within a package that is configured in the module file (*.gwt.xml). You should create a new source folder (Context menu on project -> New -> Source Folder) and create the whole package structure you use within your project up to “client”, e.g. “com.company.client”, and put your GWT tests in it. This way you make sure that the tests are not going to be deployed to GAE later on.
  

• If you are testing client-side code, your test class needs implement GWTUnit *and* be run as GWTUnitTest *not* as JUnitTest
  

API package not found

java.util.concurrent.ExecutionException: com.google.apphosting.api.ApiProxy$CallNotFoundException: The API package 'urlfetch' or call 'Fetch()' was not found.

The GAE environment has not been initialized.

• Set up the required libraries for testing:
• Context menu on project -> Properties -> Build Path -> Libraries -> Add Variable -> Configure Variables -> New…: Set “SDK_ROOT” as name and search you disk for “appengine-api-stubs.jar” and copy the part before “/lib/impl” into the text box named “Path”
• Select “SDK_ROOT” from the previous dialog, click “Extend…”, unfold lib and impl and select …labs.jar, …stubs.jar, and …api.jar; unfold lib/testing and select …testing.jar — done, whew.
• Add following lines to your test class:
private final LocalServiceTestHelper helper = new LocalServiceTestHelper(new LocalURLFetchServiceTestConfig());
@Before public void setUp() { helper.setUp(); }
@After public void tearDown() { helper.tearDown(); }


• Have a look in com.google.appengine.tools.development.testing to see all the services/APIs available for testing and replace LocalURLFetchServiceTestConfig with the one you need.

I tried to keep it as brief as possible, for more details have a look at this page.

To add themes, you will have to install an extra package (as decribed here) like this:
sudo apt-get install kde-workspace-data
Yes, this will add more data to your disk, but only 10mb this time.

Happy photo-tweaking!

Surprisingly, using com.google.gwt.user.client.Timer seems much faster then relying on com.google.gwt.core.client.Scheduler. I did not yet evaluate the differences in detail but the lower CPU workload speaks for itself.

2. AnimationScheduler

I am not really sure if this helps a lot, but you will find a lot of online documentation explaining that you should use this:
com.google.gwt.animation.client.AnimationScheduler.get(). requestAnimationFrame(callback, element);. The element parameter is optional but said to improve performance as the browser can decide when to render optimally.
So whenever your timer kicks in, and you are ready to render the results, use the AnimationScheduler instead of calling your rendering routines right away. It will only take very few milliseconds (depending on the users screen frame rate—which is 60 Hz or more usually) for the callback to return to your code.

3. Caching

This parts requires much more effort and depends on your software architecture and requirements. The basic idea is not to redraw things that did not change. To do so we can draw each item on a hidden canvas once and then reuse this rendering during every drawing cycle.
As the process is a bit more complicated, I will dedicate it a separate post—comming soon!

4. Save on effects

Yes, effects is what the canvas is all about, but something like a shadow can cost quite some CPU power. So if you run into problems check if some of the effects can be achieved cheaper or even be removed.

1. Link the library project to all relevant projects (“Project Preferences” -> “Java Build Path” -> “Projects” -> “Add…”)
2. Then, add the client-side code of the library (by adding it as a module.) Therefore, edit the gwt.xml of your application and add for example <inherits name="net.svenbuschbeck.gwt.lib.SuperLib "/> where SuperLib is the file name of the gwt.xml file in you library project and before that is the package it lies in.
3. Add the code to the project by linking a source folder. Unfortunately, this is required if you do not want to write an Ant script to compile your application. (If you prefer Ant check this out) I don’t like the idea of using such a script because if you forget to run it each time you make changes, you will end up in confusion—let’s go for the convenient, automatic way then.
   1. Add a folder to your application project; open the “advanced” section in the folder creation dialog, select “Link to alternate location” and pick the source folder (usually “src”) of your library project. (Hint: if you work within a team using a versioning system, you can create a variable for that folder and use this instead. This way, each of your colleagues can put the library project in a different folder and accommodate for that by adjusting the variable )
   2. Right click the folder, “Build Path” -> “Use as Source Folder”. Done.

Surprisingly, the GWT plugin for Eclipse does not honor the project linking, thus all the references need to be made explicit or you will end up with lots of the following: ClassNotFoundException.

com.google.gwt.core.client.JavaScriptException: (NS_ERROR_DOM_SYNTAX_ERR): An invalid or illegal string was specified;

Again, GWT tricked me into thinking I am writing Java code and made me forget about that it is going to be compiled into Javascript. And because of the latter, a division by zero does not throw an DevisionByZeroException, no, it returns NaN even for native data types (there is no differentiation between double and Double in Javascript — there is only the object-version of double, which can be of value Double.NaN).

But also GWT was not prepared to handle Double.NaN. When calling canvas.getContext().drawImage(image,x,y) and one of x and y or both were Double.NaN I got the error message shown above. If you got the same… you know what to do now: check all devisions in your code for potential devisions by zero!!

I tried to add an addAttachHandler to the RootPanel to get informed about the root panel getting detached from the DOM so that I can finalize my application. Surprisingly, that does not work in Chrome (tested Chrome and Firefox only).

But besides that this sounds like a bug to me, I found the “proper” way of doing things before the application ends:

Window.addWindowClosingHandler(new Window.ClosingHandler() {
	@Override public void onWindowClosing(ClosingEvent event) { ... }
});

In the end, I think something like Document.addUnloadHandler would be more suggestive… closing the window or reloading a page is both exiting the application by unloading the DOM — not closing the window.

...
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.Event;

public class Anchor extends com.google.gwt.user.client.ui.Anchor {

	@Override public void setEnabled(boolean enabled) {
		super.setEnabled(enabled);
		if (isAttached()) {
			onDetach();
			if (enabled) {
				sinkEvents(Event.ONCLICK);
			} else {
				unsinkEvents(Event.ONCLICK);
			}
			onAttach();
		}
	}

	@Override protected void onLoad() {
		super.onLoad();
		if (isEnabled()) {
			sinkEvents(Event.ONCLICK);
		} else {
			unsinkEvents(Event.ONCLICK);
		}
	}
}

Note the onLoad method, it sets up the state when the widget gets attached to the DOM the first time. It is required because setEnabled() could have been called before the anchor got attached.

If you encountered the same issue, please vote for this bug report.

There is another fix to the “well known” issues with the ATI Radeon X1400 and Ubuntu 10.04 to 10.10 besides the one posted about one year ago (see here).
The previous fix kept the graphics card working normally but harmed the overall operating system’s stability, so here is another fix based on disabling all 3D features of the device to get rid of the flickering screen.

It’s up to you to choose what you are able to live without
a) The desktop settings panel and “Segmentation fault” errors now and then, or
b) 3D acceleration (does not harm video playback or any other basic OS functionalities)

For solution a) see this.
For b) create this file: /etc/modprobe.d/radeon-kms.conf — and add following content

options radeon modeset=0

Solution found here (in German).

Create a file in /usr/local/bin/skype and insert

#!/bin/bash
LD_PRELOAD=/usr/lib/libv4l/v4l1compat.so /usr/bin/skype

Finally, make it executable by sudo chmod a+x /usr/local/bin/skype
Use this file to start Skype from now on. Done.

But not for me. Skype refused to eat it: ERROR: ld.so: object '/usr/lib/libv4l/v4l1compat.so' from LD_PRELOAD cannot be preloaded: ignored.
Until I finally found this one here.

Just to cut a long story short, here is why: all the other solutions work for 32bit Linux only — but hey, I do not have any of my old laptop’s potential to be wasted — I am running 64bit Ubuntu (tested with version 9.10 and 10.10 64bit). And with a minor tweak, the fix will work for you, too.

Install the video4linux libraries:

 sudo apt-get install lib32v4l-0

And change /usr/local/bin/skype to

LD_PRELOAD=/usr/lib32/libv4l/v4l1compat.so /usr/bin/skype

Its just about the “32” — and one good example more of why you should put meaningful error messages in whatever software!

Thanks Eoin Murphy.