Tim's Validation API (which is in the "ide" cluster in your NetBeans IDE installation directory), in action in an OutlineView:

And in the Properties window:

To achieve the above, you need to create an InPlaceEditor for your Property. (In the example above, I use a JTextField.) Within the InPlaceEditor's constructor, I simply have this: ValidationPanel panel = new ValidationPanel(); ValidationGroup group = panel.getValidationGroup(); group.add( textField, Validators.REQUIRE_NON_EMPTY_STRING, Validators.NO_WHITESPACE, Validators.URL_MUST_BE_VALID);
...which is copied from here:

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