7.2 Parsers | Practical RDF

RDF/XML parsers are usually included as part of a broader API. For instance, Jena has a parser, as do other APIs in other languages. Parsers aren't typically accessed directly, however, because you generally want to do something else with all that data after parsing it into an application-specific stream of data.

However, accessing parsers directly can be handy for a couple of reasons. The primary reason is to validate an RDF/XML document ”a compliant RDF/XML parser should return meaningful error messages and warnings when it encounters erroneous or suspicious RDF/XML. Another reason to run a parser directly is to create another serialization of the RDF/XML, but in a different format, such as a set of N-Triples. When I'm creating a new RDF application, I run my example RDF/XML documents through an N-Triples parser to get the N-Triples; I then use these to help with my coding of the application.

I parse RDF/XML into triples because most RDF APIs provide methods for working with triples, not the higher-level construct view. As an example, instead of creating an RDF container directly, you'll usually have to create all the triples that represent the statements underlying the container. This is demonstrated more clearly in the next several chapters.

In this section, we'll take a quick look at some parsers, beginning with ARP, the parser that forms the core of the well-used RDF Validator.

7.2.1 ARP2

ARP stands for Another RDF/XML Parser. ARP2 is the second generation of this parser, which has been modified to work with the newest RDF specifications. ARP is part of the Jena Toolkit, discussed in Chapter 8, but is also a separate installation in its own right. You can download and install ARP without having to download and install Jena. However, you have to have Java installed, at least JRE 1.4 or above.

ARP is installed with Jena, or you can access it directly at http://www.hpl.hp.com/semweb/arp.htm. If you do download and install ARP as a separate processor, intending it to coexist with a separate installation of Jena, make sure that you don't have the separate ARP in your classpath, or you could have problems working with Jena.

Normally ARP is used within another application, but there is one class that you can access at the command line as a method of testing the viability of your RDF/XML document ”the NTriple class. Once ARP2 is installed, you can run NTriple from the command line thus:

 java com.hp.hpl.jena.rdf.arp.NTriple http://burningbird.net/articles/monsters1.rdf

NTriple produces either a listing of N-Triples from the RDF/XML, or produces errors if there's something wrong with the syntax. A partial sampling of the command-line output from the parser of the file shown in the command line is given in Example 7-1.

Example 7-1. Sample output from triples generated by ARP

 http://burningbird.net/articles/monsters1.htm> <http://www.w3.org/1999/02/22-rdf-syntax- ns#type> <http://burningbird.net/postcon/elements/1.0/Resource> . _:jARP1 <http://purl.org/dc/elements/1.1/title> "Tale of Two Monsters: Legends" . _:jARP1 <http://purl.org/dc/terms/abstract> "\n           When I think of \"monsters\" I  think of the creatures of \n            legends and tales, from the books and movies, and  \n            I think of the creatures that have entertained me for years.\n     \t  " . _:jARP1 <http://purl.org/dc/elements/1.1/description> "\n            Part 1 of four-part  series on cryptozoology, legends, \n            Nessie the Loch Ness Monster and the giant  squid.\n        " . _:jARP1 <http://purl.org/dc/elements/1.1/created> "1999-08-01T00:00:00-06:00" . _:jARP1 <http://purl.org/dc/elements/1.1/creator> "Shelley Powers" . _:jARP1 <http://purl.org/dc/elements/1.1/publisher> "Burningbird Network" . <http://burningbird.net/articles/monsters1.htm> <http://burningbird.net/postcon/elements/ 1.0/Bio> _:jARP1 . _:jARP2 <http://burningbird.net/postcon/elements/1.0/currentStatus> "Active" . _:jARP2 <http://purl.org/dc/terms/valid> "2003-12-01T00:00:00-06:00" . _:jARP2 <http://purl.org/dc/elements/1.1/subject> "legends" . _:jARP2 <http://purl.org/dc/elements/1.1/subject> "giant squid" . _:jARP2 <http://purl.org/dc/elements/1.1/subject> "Loch Ness Monster" . _:jARP2 <http://purl.org/dc/elements/1.1/subject> "Architeuthis Dux" . _:jARP2 <http://purl.org/dc/elements/1.1/subject> "Nessie" . _:jARP2 <http://purl.org/dc/terms/isReferencedBy> "http://www.pibburns.com/cryptozo.htm" . _:jARP2 <http://purl.org/dc/terms/references> "http://www.nrcc.utmb.edu/" . <http://burningbird.net/articles/monsters1.htm> <http://burningbird.net/postcon/elements/ 1.0/Relevancy> _:jARP2 .

Notice that the parser returns annotated text, showing line returns and maintaining the integrity of the text as it found it in the document.

The output from ARP demonstrates one of the dangers of manually creating RDF/XML ”preservation of special characters and whitespace. It's pretty common to break lines or line characters up when you type something manually, but such whitespace will be retained unnecessarily when the RDF/XML is read in by a parser such as ARP. If you create RDF/XML manually, drop whitespace unless it's an integral part of the text.

The NTriple command format is:

 java <class-path> com.hp.hpl.jena.arp.NTriple ( [ -[xstfu]][ -b xmlBase -[eiw]  NNN[,NNN...] ] [ file ] [ url ] )...

Note, though, that with the release of ARP2 that I downloaded (which was alpha), I had to change the classpath to com.hp.hpl.jena.rdf.arp.NTriple . NTriple can work with files on a filesystem or accessed through a URL. The other options for NTriple are given in Table 7-1.

Table 7-1. NTriple options

Option	Description
`-b` `URI` or `-b` `URL`	Set XML base to URI or absolute URL.
`-r`	Document is completely RDF/XML (not embedded) and may not have `rdf:RDF` tags.
`-t`	No triples, errors only.
`-x`	Lax mode, suppress warnings.
`-s`	Strict mode, transform most warnings to errors.
`-u`	Allow unqualified attributes.
`-f`	All errors are final and processing stops when it reaches first.
`-n`	Show line numbers for triples.
`E NNN[,NNN]`	Treat specified warnings as errors.
`-I NNN[,NNN]`	Ignore numbered error/warning conditions.

In particular, if you're working with the new RDF/XML specification constructs, such as rdf:nodeID or rdf:datatype , you may want to test it with ARP, suppressing triples. Example 7-2 shows an RDF/XML document that's actually generated by Jena. It features the newer rdf:nodeID attribute, which breaks down on older parsers. However, I modified the file to change one of the legitimate uses of rdf:resource to rdf:about (in bold).

Example 7-2. RDF/XML document with one error

 <rdf:RDF   xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'   xmlns:NS0='http://burningbird.net/postcon/elements/1.0/'   xmlns:dc='http://purl.org/dc/elements/1.0/'  >   <rdf:Description rdf:nodeID='A0'>     <dc:creator>Shelley Powers</dc:creator>     <dc:publisher>Burningbird</dc:publisher>     <dc:title xml:lang='en'>Tale of Two Monsters: Legends</dc:title>   </rdf:Description>   <rdf:Description rdf:about='http://burningbird.net/articles/monsters1.htm'>     <NS0:related  rdf:about  ='http://burningbird.net/articles/monsters2.htm'/>     <NS0:related rdf:resource='http://burningbird.net/articles/monsters3.htm'/>     <NS0:Bio rdf:nodeID='A0'/>   </rdf:Description> </rdf:RDF>

Running the NTriple application with the -t option returns the following error from this file:

 C:\>java com.hp.hpl.jena.rdf.arp.NTriple -t c:\writing\rdfbook\java\pracRDFThird .rdf Error: file:/c:/writing/rdfbook/java/pracRDFThird.rdf[12:77]: {E201} Syntax error  when processing attribute rdf:about.  Cannot have attribute rdf:about in this context.

As you can see, ARP2 not only finds the error, it also gives you the location of the error and the reason the error occurs.

ARP2 works from the command line only, but if you're more interested in a parser with a GUI frontend, you might want to try out ICS-FORTH's Validating RDF Parser.

7.2.2 ICS-FORTH Validating RDF Parser

The ICS-FORTH Validating RDF Parser (VRP), like ARP, is part of a suite of tools but can also be downloaded separately. In addition, again like ARP, the only requirement to run the tool is a Java Runtime Environment installed, JRE 1.4 or up.

You can download ICS-FORTH's Validating RDF Parser from the following location: http://athena.ics.forth.gr:9090/RDF/. You can also get access to it as part of the RDFSuite.

VRP is a set of Java classes that you can use within your own Java classes. However, the parser also comes with a Swing-based GUI frontend that you can use directly without having to touch any code. To access the GUI for the parser, once you've downloaded and unzipped the file containing the source, you're ready to start using it. Start up the parser by typing the following line:

 java -classpath <path to VRP directory>/classes GUI.VRPGUI

The page that opens has two text input fields, one for an input file and one for recording the results. Below these are a set of checkboxes that switch on specific tests, such as ones for checking the syntax, checking for class hierarchy loops , and so on. Figure 7-3 shows the tool after I validated the test document (at http://burningbird.net/articles/monsters1.rdf ), asking for validation only and having the tool test the syntax and class and property hierarchy loops. VRP can also open an HTML or XHTML document with RDF/XML embedded in it (as described in Chapter 3).

Figure 7-3. ICS-FORTH'sValidating RDF Parser (VRP) standalone application

As you can see from the image, VRP has several input and output options. For instance, I can run the test again, this time checking the Triples, Statements, and Graph options for output. The tool first asks me for permission to overwrite the output file and then runs the tests, printing output to the Results window in the application as well as to the file. The Graph option provides a text description of what would be the RDF directed graph rather than a true graphical representation. A sampling from this file, the classes as defined in the RDFS graph output, are shown in Example 7-3. Note that the information associated with each schema class, such as the isDefinedBy , comment , and label information, isn't showing in the graph, though we know it to be present in the schema.

Example 7-3. VRP graph results describing document's classes

 The classes of the Model:  http://burningbird.net/postcon/elements/1.0/#Movement subClassOf: [] comment: [] label: [] seeAlso: [] isDefinedBy: [] value: [] type: [] http://www.w3.org/1999/02/22-rdf-syntax-ns#Seq subClassOf: [] comment: [] label: [] seeAlso: [] isDefinedBy: [] value: [] type: [] http://burningbird.net/postcon/elements/1.0/#Resource subClassOf: [] comment: [] label: [] seeAlso: [] isDefinedBy: [] value: [] type: []

The version of the tool I used expands the absolute URIs for the classes and properties by converting them to URI fragments , such as #Resource and #Movement , before concatenating them to the URI. The base URI is specified with a trailing slash, just as occurs with the Dublin Core schema. The relative URIs should not have been "corrected" to URI fragments before resolution into absolute URIs. Because of this correction, the schema elements could not resolve correctly (as they did within BrownSauce).

VRP generates Unix-style line-feeds. If you're using the product in Windows, make sure you view the result using a test reader that compensates for this. For example, use Wordpad not Notepad.

This could be why the tool didn't pick up the schema information for the items, or why it may not open related schema documents. Hard to say. One thing the tool does do is correctly resolve the RDF classes in the document, as compared to the RDF properties. This can be very helpful when you're creating an RDF Schema for a vocabulary and do not recall which elements are classes and which are properties.

The checks you can perform on a specific document are:

Syntax check: Checks whether the RDF/XML of the document conforms to the updated RDF/XML specification.
Semantic check: class hierarchy loop: Checks for loops in subclass hierarchy (parent class identifying itself as child of child class and so on).
Semantic check: property hierarchy loop: Same as previous but for properties.
Semantic check: domain/range of subproperties: A property's domain and range are a sum of its subproperties' domains and ranges.
Semantic check: resources of properties: Source/target property values should be instances of domain/range of property.
Semantic check: types of resources: Assigned RDF or XML type.
Extend model: external namespace: Connects to external namespaces to merge in their triples.
Extend model: type inference: Infer type of resource.
Extend model: domain/range inference: Infer domain/range from superproperty.

The semantic check for types of resources failed with all models I tested this against, including Dublin Core, RSS, and FOAF (Friend of a Friend) RDF/XML documents. This check is looking for a specific type information for each resource, something not available in most models. However, the example PostCon vocabulary file (at http://burningbird.net/articles/monsters1.rdf ) did pass all other tests. When I selected the option to include external namespace triples, the model again failed, but the results as a graph were quite interesting.

The results include information from the schema for PostCon, such as the following for the pstcn:movementType property:

 http://burningbird.net/postcon/elements/1.0/movementType range: [http://www.w3.org/2000/01/rdf-schema#Literal] domain: [http://postcon/elements/1.0/Movement] subPropertyOf: [] links:  comment: [Type of Movement (allowable values of Move, Add, Remove)] label: [Movement Type] seeAlso: [] isDefinedBy: [http://burningbird.net/postcon/elements/1.0/] value: [] type: [http://www.w3.org/1999/02/22-rdf-syntax-ns#Property]

Notice the links property and that it has no value. However, later in the document, you'll see the graph for pstcn:movementType :

 http://burningbird.net/postcon/elements/1.0/#movementType range: [] domain: [] subPropertyOf: [] links: (http://www.yasd.com/dynaearth/monsters1.htm, Add) (http://www. dynamicearth.com/articles/monsters1.htm, Move) (http:/burningbird.net/articles/ monsters1.htm, Move)  comment: [] label: [] seeAlso: [] isDefinedBy: [] value: [] type: []

Again, the automatic use of fragment identifiers breaks the information up; however, combine both blocks and you have a relatively good idea of all the dimensions of the PostCon property pstcn:movementType .

In addition to Graph, other VRP outputs are:

Debug: Shows tokens generated by Lexar, the lexicon analyzer
Verbose: Details actions VRP takes
Triples: Model triples
Statements: Model statements
Statistic options: Provides other information such as number of resources, statements, and time taken for some of the processes

I would definitely consider running VRP against an in-progress vocabulary while you're designing your schema and then try it with External Namespaces as a test with some of your models as a check on the schema.