Is it a CSV file or a proper Excel file? There is a big difference in handling.
If it is a flat file (CSV), then you’ll define a flat file schema using Developer to parse the file. The flat file adapter will use this schema to convert the flat file to an IS document.
For the “app schema” define an IS schema, either by hand or from an XML schema, that describes the app document.
Then you can map the IS document created from the flat file to another IS document that has the app schema structure. Then use the XML services to convert that to an XML string. Then you can transport it to whereever it needs to go.
You most likely won’t need any Java services to do any of this.