This is sort of an esoteric feature, but I just ran across it and it fits nicely into an XPages bag of tricks.

Specifically, as it turns out, there's a way to override the default "which XPage should be used for this document?" behavior in some cases. The most-commonly-known behavior is the "Display XPage Instead" set on the form (or the default form if-and-only-if the document has an empty Form field). This wins in all cases, and is the only way to get an XPage to open for a document when using non-XPage-specific URLs, such as traditional-and-clean-ish "db.nsf/viewname/key" URLs.

There's also a secondary behavior that I knew about before, which is that, if it can't find an explicit XPage for the form, it looks for an XPage that matches the form name. So, for example, "SomeForm" becomes "SomeForm.xsp" (case-sensitive, except the first letter). I used this once upon a time to be able to have XPages for documents stored in remote databases without changing the design of those databases (you can also specify an $XPageAlt field on the remote form without the XPage existing in that DB, by the way).

What I learned today is that there's a way to short-circuit that fallback and specify an alternative via xsp.properties. Specifically, you can add a line that looks like this:

xsp.domino.form.xpage.testform=OtherForm

Note that there are a few notes here:

• This only works for "$$OpenDominoDocument.xsp" URLs, because the XSP runtime is not consulted for "view/key" URLs unless the form explicitly names an XPage.
• This does not override an explicitly-named XPage in the form.
• This does work when no form with the specified name actually exists in the database.
• The form name must be lowercase in the property.
• The form name must also be properly escaped as per the rules for properties files; importantly, this means that spaces in the form name must be preceded by a backslash.
• This does work to specify an XPage for the default form when no form is specified in the document.

Most of the time, this trick won't be needed, since either specifying an XPage name in the URL or designating it in the form note will suffice. However, if you're in a case where you have an app that points to data in remote databases and you don't want to modify the design there but still want to use "$$OpenDominoDocument" URLs, this is an option.