$ java -Dlogback.configurationFile=missingfile.xml -cp .... MyClass
>> no output is generated

A status dump says:
16:14:01,778 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [missingfile.xml]
16:14:01,779 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [logback.groovy]
16:14:01,779 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [logback-test.xml]
16:14:01,780 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Could NOT find resource [logback.xml]
16:14:01,786 |-INFO in ch.qos.logback.classic.LoggerContext[default] - Setting up default configuration.

First, logback should issue an error saying that the explicitely selected 'missingfile' was not found.
Then I don't thing that it should try to load default config files, since one has been requested by the user.