Liquibase diff and changelog options

I need to be able to compare an existing DB schema against a changelog file instead of another schema.   I presume CommandLineUtils.doGenerateChangeLog will store the definitions of all the objects in a schema in a file.  Now I want to do a diff between a DB schema and this change log file instead of another schema.   Is this possible using the java API?   Also,  can one store the changes in a change log file.