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GWT 2.0 ships with a new HtmlUnit-based testing system that makes testing faster, but isn’t full web-browser under the hood. It can’t perform layout, so any tests that rely on layout measurements won’t succeed.

There are a number of alternate run styles that are useful, however. Manual mode, which outputs a URL that you can connect any browser to, works well for quick testing, but is a pain when you need to keep running it over and over. RemoteWeb and Selenium automate the process of starting and stopping browsers, but require you to start an external server before testing.

There’s another run-style that isn’t well-documented, but I’ve found to be the most useful for testing locally: ExternalBrowser. It requires an executable name in the command-line, the name of the browser to start. On OSX, you can specify the fully-qualified name of the executable beneath the application bundle, or you can use a shell script to launch the browser of your choice.

Place the following script under /usr/bin named safari and make it chmod a+x. This allows you (and ExternalBrowser) to launch Safari from the command-line’s $PATH. The argument to open is the name of any browser that lives under your /Applications/ directory, including subdirectories.

#!/bin/sh
open -W -a Safari $1

Add a Java system property gwt.args to your testing launch configuration in Eclipse, Ant script or other IDE. You can then specify the run style like so:

-Dgwt.args="-runStyle ExternalBrowser:safari"

Now, when you run your unit tests, you’ll see GWT launch Safari and navigate to the appropriate URL on its own. Tests will leave windows up on your screen after they complete, but you can safely close them after the run terminates.