There are a number of RDF processing implementations around. One of the most widely known is the Jena toolkit. Jena started off as a project at the HP Labs in Bristol, and was open source from the outset. HP ceased active development of Jena in 2009, and in 2010 it was adopted as an incubation project by the Apache Software Foundation.
In these notes, I will refer to the 2.6.x releases, which pre-date the adoption by ASF.
Jena is easy to install, and documentation is also available.
Although Jena has been developed as a set of Java libraries, I will focus on using it via its command-line utilities. See the Jena API docs for a good overview of processing RDF from within Java.
The 2.6.2 version of Jena is installed on DICE in the following directory:
/usr/share/java/jena/bin
To check the status of Jena on the desktop you are using, use the ls -l command on the above directory. The output you get should look something like this:
dice:~> ls -l /usr/share/java/jena/bin
total 72
-rwxr-xr-x 1 root root 164 Jun 22 2009 arq
-rwxr-xr-x 1 root root 167 Jun 22 2009 arq_version
-rwxr-xr-x 1 root root 143 Jun 18 2009 jena_path
-rwxr-xr-x 1 root root 1059 Oct 15 10:48 make_classpath
-rwxr-xr-x 1 root root 163 Jun 18 2009 n3
-rwxr-xr-x 1 root root 165 Jun 22 2009 qexpr
-rwxr-xr-x 1 root root 166 Jun 22 2009 qparse
-rwxr-xr-x 1 root root 165 Jun 22 2009 qtest
-rwxr-xr-x 1 root root 165 Jun 22 2009 query
-rwxr-xr-x 1 root root 167 Jun 18 2009 rdfcat
-rwxr-xr-x 1 root root 171 Jun 18 2009 rdfcompare
-rwxr-xr-x 1 root root 168 Jun 18 2009 rdfcopy
-rwxr-xr-x 1 root root 169 Jun 18 2009 rdfparse
-rwxr-xr-x 1 root root 164 Jun 22 2009 rset
-rwxr-xr-x 1 root root 170 Jun 18 2009 schemagen
-rwxr-xr-x 1 root root 166 Jun 22 2009 sparql
-rwxr-xr-x 1 root root 166 Jun 22 2009 update
-rwxr-xr-x 1 root root 168 Aug 14 14:02 version
From the command line, set the Jena root directory:
dice:~> export JENAROOT=/usr/share/java/jena
Jena should now work. You can test it by using the –help option to one of the programs, such as rdfcat:
dice:~> /usr/share/java/jena/bin/rdfcat --help
Usage: java jena.rdfcat (option|input)*
Concatenates the contents of zero or more input RDF documents.
Options: -out N3 | N-TRIPLE | RDF/XML | RDF/XML-ABBREV
-n expect subsequent inputs in N3 syntax
-x expect subsequent inputs in RDF/XML syntax
-t expect subsequent inputs in N-TRIPLE syntax
-[no]include include rdfs:seeAlso and owl:imports
input can be filename, URL, or - for stdin
Recognised aliases for -n are: -n3 -ttl or -N3
Recognised aliases for -x are: -xml -rdf or -rdfxml
Recognised aliases for -t are: -ntriple
Output format aliases: x, xml or rdf for RDF/XML, n, n3 or ttl for N3, t or ntriple for N-TRIPLE
See the Javadoc for jena.rdfcat for additional details.
You can make this all a bit more convenient by adding the following lines to your .brc (bash shell configuration) file:
export JENAROOT=/usr/share/java/jena
export PATH="${JENAROOT}/bin:${PATH}"
The first line means that the environment variable JENAROOT will be set automatically for you whenever you log in to a DICE machine. The second line means that you don’t need to use the full pathname for Jena executables; instead of /usr/share/java/jena/bin/rdfcat you can just use the short command rdfcat.
Download the latest stable version of Jena (namely, 2.6.4) from Jena download on Sourceforge.
Unzip the archive, which should create a sub-directory Jena-2.6.4 in your home directory. Using the command line, you can do this as follows:
: unzip jena-2.6.4.zip
From the command line, set the Jena root directory:
: export JENAROOT=~/Jena-2.6.4
Add execute permissions to all the files in the bin directory:
: chmod u+x $JENAROOT/bin/*
Jena should now work in the same way as it does on DICE.
One of the most basic things we can do is parse an RDF file to check that it is well-formed, and also convert it between N3 (similar to Turtle) and RDF/XML format. This can be done with rdfcat. As mentioned above, you can get help for this on the command line. Slightly more detailed information can be found in the Javadoc for rdfcat
To give an example, let’s suppose we want to process this RDF file in N3 format:
@prefix : <http://inf.ed.ac.uk/ont#> .
@prefix dbp: <http://dbpedia.org/property/> .
@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
:ebagel dc:title "Elephant and Bagel" .
:ebagel dbp:locatedIn :central .
:ebagel :rating "4"^^xsd:decimal .
:ebagel :lovedBy :bea .
:ebagel :lovedBy :amy .
:aroast dc:title "Artisan Roast" .
:aroast dbp:locatedIn:eastEnd .
:aroast :rating "5"^^xsd:decimal .
:aroast :lovedBy :stu .
:aroast :lovedBy :rod .
:pyard dc:title "Peter's Yard" .
:pyard dbp:locatedIn:central .
:pyard :rating "4"^^xsd:decimal .
:pyard :lovedBy :amy .
:hacraft dc:title "Himalaya Art & Craft" .
:hacraft dbp:locatedIn:southSide .
:hacraft :rating "3"^^xsd:decimal .
:hacraft :lovedBy :rod .
:vittoria dc:title "Vittoria" .
:vittoria dbp:locatedIn:central .
:vittoria :rating "4"^^xsd:decimal .
:vittoria :lovedBy :stu .
:vittoria :hasCuisine :italian .
To convert the file to RDF/XML format, do the following:
dice:~> rdfcat -out xml cafes.n3
You can also convert in the other direction. For example, suppose that you created a FOAF file foaf.rdf in RDF/XML. Then the following command will convert it to N3 and save it in an appropriate file:
dice:~> rdfcat -out n3 foaf.rdf > foaf.n3
We can also merge several graphs into one. Suppose we are given :
@prefix : <http://inf.ed.ac.uk/ont#> .
@prefix foaf: <http://xmlns.com/foaf/0.1/> .
:stu foaf:knows :bea ;
foaf:name "Stuart" .
:bea foaf:knows :amy ;
foaf:name "Beatrice" .
:amy foaf:name "Amy" .
Then we can merge the two graphs to form a new file merged.n3 as follows:
rdfcat -out n3 cafes.n3 knows.n3 > merged.n3