This application uses the spring rest framework to build a web application that implements the 3rd party rest api specification.
It implements two separate adapter instances.
1. salesforcessl - a relational instance that provides connectivity to salesforce rest api
2. proxyfileservice - a file system instance that returns file objects from a specified root directory. 

A useful tutorial on building a Hello World Rest application using the spring framework is available here.
https://spring.io/guides/gs/rest-service/

SFController is the class that implements the rest end points.
The responses are returned as JSON by the framework, but we use standard java classes to represent them internally. 
CatalogResult.ListingResult : lists of schemas, table-names, or file-names.
CatalogResult.CatalogItem: represents a single item from a catalog.
CatalogResult.CollectionSchema: schema of a single table. It is nothing but an arraylist of ObjectSchemas.
QueryResult: represents the type for a QueryResult. It has a metadata field of type CollectionSchema, an arraylist of items i.e records of type Map<String,String> and an array list of LinkDescriptor to represent relationship links (only used for paged query results).


"/"
getDescription0() is the end point for the adapter metadata. The static adapter meta-data is provided from the file provider-instances.json

{instanceName}/{version}/Validate
This implements the method that validates user specified credentials. It is implemented by refreshing the authentication token with salesforce.

{instanceName}/{version}/metadata-catalog)
metadataCatalogRootQuery returns the top schemas. There is only one top schema "RESTADAPTER_SCHEMA" surfaced by this adapter.
The returned object type is ListingResult

"{instanceName}/{version}/metadata-catalog/{schemaName:.+}"
metadataCatalogListObjects is the method that returns a listing of objects under a specified schema.  
The returned object type is ListingResult.

 "{instanceName}/{version}/metadata-catalog/object-schema"
metadataCatalogGetObjectSchema() is the method that returns a description
of the schema of an object. The returned object type is CollectionSchema

"{instanceName}/{version}/metadata-catalog/list"
metadataCatalogGetFolderListing() is the method that returns the listing of folders under optionally a specified folder name (or the root folder if no containing folder is specified). The returned object type is ListingResult.

"{instanceName}/{version}"
downloadFile() is the method that returns a stream of records for a specified file from the file based instance. The returned type is a regular stream.

"{instanceName}/{version}/Query"
getQueryResult() is the method that executes a query with Salesforce and returns the result as a QueryResult object.