The MappingCharFilterFactory takes a path to a file as a parameter. The Javadoc for the MappingCharFilterFactory shows the declaration to put in schema.xml.
<charFilter class="solr.MappingCharFilterFactory" mapping="mapping.txt"/>In the source code for the MappingCharFilterFactory we can find the following line.
mapping = args.get("mapping");Looking through the class hierarchy, all token filter factories, tokenizer factories, and char filter factories are descendants of AbstractAnalysisFactory where args is declared as a protected variable. All you have to do access the parameters passed from schema.xml is access the args map. args can also be accessed via the getArgs() function.