Reading RDF in Apache Jena

This page details the setup of RDF I/O technology (RIOT) for input introduced in Jena 2.10.

See Writing RDF for details of the RIOT Writer system.

Full details of operations are given in the javadoc.

API

Much of the functionality is accessed via the Jena Model API; direct calling of the RIOT subsystem isn't needed. A resource name with no URI scheme is assumed to be a local file name.

Applications typically use at most RDFDataMgr to read RDF datasets.

The major classes in the RIOT API are:

Class Comment
RDFDataMgr Main set of functions to read and load models and datasets
StreamManager Handles the opening of typed input streams
StreamRDF Interface for the output of all parsers
RDFLanguages Registered languages
RDFParserRegistry Registered parser factories

Determining the RDF syntax

The syntax of the RDF file is determined by the content type (if an HTTP request), then the file extension if there is no content type. Content type text/plain is ignored; it is assumed to be type returned for an unconfigured http server. The application can also pass in a declared language hint.

The string name traditionally used in model.read is mapped to RIOT Lang as:

Jena reader RIOT Lang
"TURTLE" TURTLE
"TTL" TURTLE
"Turtle" TURTLE
"N-TRIPLES" NTRIPLES
"N-TRIPLE" NTRIPLES
"NT" NTRIPLES
"RDF/XML" RDFXML
"N3" N3
"JSON-LD" JSONLD
"RDF/JSON" RDFJSON
"RDF/JSON" RDFJSON

The following is a suggested Apache httpd .htaccess file:

AddType  text/turtle             .ttl
AddType  application/rdf+xml     .rdf
AddType  application/n-triples   .nt

AddType  application/ld+json     .jsonld
AddType  application/owl+xml     .owl

AddType  text/trig               .trig
AddType  application/n-quads     .nq

AddType  application/trix+xml    .trix
AddType  application/rdf+thrift  .trdf

Example 1 : Common usage

In this example, a file in the current directory is read as Turtle.

Model model = ModelFactory.createDefaultModel() ;
model.read("data.ttl") ;

If the syntax is not as the file extension, a language can be declared:

model.read("data.foo", "TURTLE") ;

Example 2 : Using the RDFDataMgr

In versions of Jena priot to 2.10.0, the FileManager provided some of this functionality. It was more basic, and not properly web enabled. The RDFDataMgr superceeds the FileManager. "load*" operations create an in-memory container (model, or dataset as appropriate); "read" operations add data into an existing model or dataset.

// Create a model and read into it from file 
// "data.ttl" assumed to be Turtle.
Model model = RDFDataMgr.loadModel("data.ttl") ;

// Create a dataset and read into it from file 
// "data.trig" assumed to be TriG.
Dataset dataset = RDFDataMgr.loadDataset("data.trig") ;

// Read into an existing Model
RDFDataMgr.read(model, "data2.ttl") ;

Logging

The parsers log to a logger called org.apache.jena.riot. To avoid WARN messages, set this in log4j.properties to ERROR.

StreamManager and LocationMapper

By default, the RDFDataMgr uses the global StreamManager to open typed InputStreams. This is available to applications via RDFDataMgr.open as well as directly using a StreamManager.

The StreamManager is choosen based on the Context object for the operation, but normally this defaults to the global Context available via Context.get(). The constant RDFDataMgr.streamManagerSymbol, which is http://jena.apache.org/riot/streamManager is used.

Specialized StreamManagers can be configured with specific locators for data:

  • File locator (with own current directory)
  • URL locator
  • Class loader locator
  • Zip file locator

Configuring a StreamManager

The StreamManager can be reconfigured with different places to look for files. The default configuration used for the global StreamManager is a file access class, wihere the current directory is that of the java process, a URL accessor for reading from the web, and a class loader-based accessor. Different setups can be built and used either as the global set up,

There is also a LocationMapper for rewiting file names and URLs before use to allow placing known names in different places (e.g. having local copies of import http resources).

Configuring a LocationMapper

Location mapping files are RDF, usually written in Turtle although an RDF syntax can be used.

@prefix lm: <http://jena.hpl.hp.com/2004/08/location-mapping#>

[] lm:mapping
   [ lm:name "file:foo.ttl" ;      lm:altName "file:etc/foo.ttl" ] ,
   [ lm:prefix "file:etc/" ;       lm:altPrefix "file:ETC/" ] ,
   [ lm:name "file:etc/foo.ttl" ;  lm:altName "file:DIR/foo.ttl" ]
   .

There are two types of location mapping: exact match renaming and prefix renaming. When trying to find an alternative location, a LocationMapper first tries for an exact match; if none is found, the LocationMapper will search for the longest matching prefix. If two are the same length, there is no guarantee on order tried; there is no implied order in a location mapper configuration file (it sets up two hash tables).

In the example above, file:etc/foo.ttl becomes file:DIR/foo.ttl because that is an exact match. The prefix match of file:/etc/ is ignored.

All string tests are done case sensitively because the primary use is for URLs.

Notes:

  • Property values are not URIs, but strings. This is a system feature, not an RDF feature. Prefix mapping is name rewriting; alternate names are not treated as equivalent resources in the rest of Jena. While application writers are encouraged to use URIs to identify files, this is not always possible.
  • There is no check to see if the alternative system resource is equivalent to the original.

A LocationMapper finds its configuration file by looking for the following files, in order:

  • file:location-mapping.rdf
  • file:location-mapping.ttl
  • file:etc/location-mapping.rdf
  • file:etc/location-mapping.ttl

This is a specified as a path - note the path separator is always the character ';' regardless of operating system because URLs contain ':'.

Applications can also set mappings programmatically. No configuration file is necessary.

The base URI for reading models will be the original URI, not the alternative location.

Debugging

Using log4j, set the logging level of the classes:

  • org.apache.jena.riot.stream.StreamManager
  • org.apache.jena.riot.stream.LocationMapper

Advanced examples

Example code may be found in jena-arq/src-examples.

Iterating over parser output

One of the capabilities of the RIOT API is the ability to treat parser output as an iterator, this is useful when you don't want to go to the trouble of writing a full sink implementation and can easily express your logic in normal iterator style.

To do this you use one of the subclasses of PipedRDFIterator in conjunction with a PipedRDFStream.

This PipedRDFStream provides an implementation of StreamRDF which allows it to consume parser output and this is consumed by the PipedRDFIterator implementation. This has some advantages over a direct StreamRDF implementation since it allows the parser production of data to run ahead of your consumption of data which may result in better overall throughput.

The only complication is that you need to ensure that the thread feeding the PipedRDFStream and the consumer of the iterator are on different threads as otherwise you can run into a deadlock situation where one is waiting on data from the other which is never started.

See RIOT example 6 which shows an example usage including a simple way to push the parser onto a different thread to avoid the possible deadlock.

Filter the output of parsing

When working with very large files, it can be useful to process the stream of triples or quads produced by the parser so as to work in a streaming fashion.

See RIOT example 4

Add a new language

The set of languages is not fixed. A new languages, together with a parser, can be added to RIOT as shown in RIOT example 5