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FYI: Toolkit and 64-bit linux
I had some problems running IT Mill toolkit development environment, or the hosted mode actually, on 64-bit linux (Ubuntu Hardy in my case). Fortunately, there is a workaround, which is explained below.
To get the hosted mode running on 64-bit linux see comments 39 and 41 of the following GWT ticket http://code.google.com/p/google-web-toolkit/issues/detail?id=134 The problem is actually in the Google Web Toolkit and it is explained in the aforementioned ticket: "The GWT hosted web browser ships with a 32-bit version of the SWT bindings, so the browser does not work in 64-bit Linux."
If you have the same problem the ending of the error message looks something like this ".../itmill-toolkit-linux-5.2.8/gwt/libswt-pi-gtk-3235.so: wrong ELF class: ELFCLASS32 (Possible cause: architecture word width mismatch)"
Hopefully this helps someone other as well.
Jukka Pirinen: I had some problems running IT Mill toolkit development environment, or the hosted mode actually, on 64-bit linux (Ubuntu Hardy in my case).
And just to state the obvious - when you are not adding custom widgets, you do not need to run hosted mode browser and this is not an issue.
Debugging Vaadin in 64-bit works great when there is no customized widget. I run 32-bit ubuntu on virtualbox when I need to debug GWT widget.