Redland is a set of C libraries providing a high-level API for the Resource Description Framework (RDF), allowing it to be stored, parsed, serialized, queried, and manipulated. It has an object-based, modular design and comes with detailed reference documentation and examples. Redland supports all RDF vocabularies such as FOAF, RSS 1.0, Dublin Core, DOAP, and OWL, the query languages SPARQL and RDQL, and all RDF syntaxes including Turtle, RDF/XML, RDF/JSON, RSS, Atom, RDFa, and GRDDL.
ServingXML is a markup language for expressing XML pipelines and an extensible Java framework for defining the elements of the language. It defines a vocabulary for expressing flat-XML, XML-flat, flat-flat, and XML-XML transformations in pipelines. The accompanying console app supports reading content as XML files, flat files, SQL queries, or SAX events, and writing it as XML, HTML, PDF, or mail attachments. This software is especially suited for converting flat file or database records to XML, with its support for namespaces, variant record types, multi-valued fields, segments and repeating groups, hierarchical grouping of records, and row-by-row validation with XML Schema. There is also an API for embedding the software in a Java application.
With MetaModel, you use a type-safe SQL-like API for querying any datastore. It is a data access framework providing a common interface for exploration and querying of different types of datastores. It isn't a data mapping framework. Instead, it emphasizes abstraction of metadata and the ability to add data sources at runtime, making MetaModel great for generic data processing applications, but less so for applications modeled around a particular domain.