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| <h1 class="title">Typed literals how-to</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
| |
| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#what-are-typed-literals">What are typed literals?</a></li> |
| <li><a href="#basic-api-operations">Basic API operations</a> |
| <ul> |
| <li><a href="#equality-issues">Equality issues</a></li> |
| </ul> |
| </li> |
| <li><a href="#how-datatypes-are-represented">How datatypes are represented</a> |
| <ul> |
| <li><a href="#error-detection">Error detection</a></li> |
| </ul> |
| </li> |
| <li><a href="#xsd-data-types">XSD data types</a></li> |
| <li><a href="#user-defined-xsd-data-types">User defined XSD data types</a></li> |
| <li><a href="#user-defined-non-xsd-data-types">User defined non-XSD data types</a></li> |
| <li><a href="#a-note-on-xmllang">A note on xml:Lang</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <h2 id="what-are-typed-literals">What are typed literals?</h2> |
| <p>In the original RDF specifications there were two types of literal |
| values defined - plain literals (which are basically strings with |
| an optional language tag) and XML literals (which are more or less |
| plain literals plus a “well-formed-xml” flag).</p> |
| <p>Part of the remit for the 2001 |
| <a href="http://www.w3.org/2001/sw/RDFCore/">RDF Core</a> working group was to |
| add to RDF support for typed values, i.e. things like numbers. |
| These notes describe the support for typed literals in |
| Jena2.</p> |
| <p>Before going into the Jena details here are some informal reminders |
| of how typed literals work in RDF. We refer readers to the RDF core |
| <a href="http://www.w3.org/TR/rdf-mt/">semantics</a>, |
| <a href="http://www.w3.org/TR/rdf-syntax-grammar">syntax</a> and |
| <a href="http://www.w3.org/TR/rdf-concepts/">concepts</a> documents for more |
| precise details.</p> |
| <p>In RDF, typed literal values comprise a string (the lexical form of |
| the literal) and a datatype (identified by a URI). The datatype is |
| supposed to denote a mapping from lexical forms to some space of |
| values. The pair comprising the literal then denotes an element of |
| the value space of the datatype. For example, a typed literal |
| comprising <code>("true", xsd:boolean)</code> would denote the abstract true |
| value <code>T</code>.</p> |
| <p>In the RDF/XML syntax typed literals are notated with syntax such |
| as:</p> |
| <pre><code><age rdf:datatype="http://www.w3.org/2001/XMLSchema#int">13</age> |
| </code></pre> |
| <p>In NTriple syntax the notation is:</p> |
| <pre><code>"13"^^<http://www.w3.org/2001/XMLSchema#int> |
| </code></pre> |
| <p>In Turtle, it can be abbreviated:</p> |
| <pre><code>"13"^^xsd:int |
| </code></pre> |
| <p>This <code>^^</code> notation will appear in literals printed by Jena.</p> |
| <p>Note that a literal is either typed or plain (an old style literal) |
| and which it is can be determined statically. There is no way to |
| define a literal as having a lexical value of, say “13” but leave |
| its datatype open and then infer the datatype from some schema or |
| ontology definition.</p> |
| <p>In the new scheme of things well-formed XML literals are treated as |
| typed literals whose datatype is the special type |
| <code>rdf:XMLLiteral</code>.</p> |
| <h2 id="basic-api-operations">Basic API operations</h2> |
| <p>Jena will correctly parse typed literals within RDF/XML, NTriple |
| and Turtle source files. The same Java object, |
| <a href="/documentation/javadoc/jena/org.apache.jena.core/org/apache/jena/rdf/model/Literal.html"><code>Literal</code></a> |
| will represent “plain” and “typed” literals. Literal now supports |
| some new methods:</p> |
| <ul> |
| <li> |
| <p><code>getDatatype()</code> |
| Returns null for a plain literal or a Java object which represents |
| the datatype of a typed Literal.</p> |
| </li> |
| <li> |
| <p><code>getDatatypeURI()</code> |
| Returns null for a plain literal or the URI of the datatype of a |
| typed Literal.</p> |
| </li> |
| <li> |
| <p><code>getValue()</code> |
| Returns a Java object representing the value of the literal, for |
| example for an xsd:int this will be a java.lang.Integer, for plain |
| literals it will be a String. |
| The converse operation of creating a Java object to represent a |
| typed literal in a model can be achieved using:</p> |
| </li> |
| <li> |
| <p><code>model.createTypedLiteral(value, datatype)</code> |
| This allows the <code>value</code> to be specified by a lexical form (i.e. a |
| String) or by a Java object representing the typed value; the |
| <code>datatype</code> can be specified by a URI string or a Java object |
| representing the datatype.</p> |
| </li> |
| </ul> |
| <p>In addition there is a built in mapping from standard Java wrapper |
| objects to XSD datatypes (see later) so that the simpler call:</p> |
| <pre><code>model.createTypedLiteral(Object) |
| </code></pre> |
| <p>will create a typed literal with the datatype appropriate for |
| representing that java object. For example,</p> |
| <pre><code>Literal l = model.createTypedLiteral(new Integer(25)); |
| </code></pre> |
| <p>will create a typed literal with the lexical value “25”, of type |
| <code>xsd:int</code>.</p> |
| <p>Note that there are also functions which look similar but do not |
| use typed literals. For example::</p> |
| <pre><code>Literal l = model.createLiteral(25); |
| int age = l.getInt(); |
| </code></pre> |
| <p>These worked by converting the primitive to a string and storing |
| the resulting string as a plain literal. The inverse operation then |
| attempts to parse the string of the plain literal (as an int in |
| this example). These are for backward compatibility with earlier |
| versions of Jena and older datasets. In normal circumstances |
| <code>createTypedLiteral</code> is preferable.</p> |
| <h3 id="equality-issues">Equality issues</h3> |
| <p>There is a well defined notion of when two typed literals should be |
| equal, based on the equality defined for the datatype in question. |
| Jena2 implements this equality function by using the method |
| <code>sameValueAs</code>. Thus two literals (“13”, xsd:int) and (“13”, |
| xsd:decimal) will test as sameValueAs each other but neither will |
| test sameValueAs (“13”, xsd:string).</p> |
| <p>Note that this is a different function from the Java <code>equals</code> |
| method. Had we changed the equals method to test for semantic |
| equality problems would have arisen because the two objects are not |
| substitutable in the Java sense (for example they return different |
| values from a getDatatype() call). This would, for example, have |
| made it impossible to cache literals in a hash table.</p> |
| <h2 id="how-datatypes-are-represented">How datatypes are represented</h2> |
| <p>Datatypes for typed literals are represented by instances of the |
| interface |
| <a href="/documentation/javadoc/jena/org.apache.jena.core/org/apache/jena/datatypes/RDFDatatype.html"><code>org.apache.jena.datatypes.RDFDatatype</code></a>. |
| Instances of this interface can be used to parse and serialized |
| typed data, test for equality and test if a typed or lexical value |
| is a legal value for this datatype.</p> |
| <p>Prebuilt instances of this interface are included for all the main |
| XSD datatypes (see <a href="#xsd">below</a>).</p> |
| <p>In addition, it is possible for an application to define new |
| datatypes and register them against some URI (see |
| <a href="#userdef">below</a>).</p> |
| <h3 id="error-detection">Error detection</h3> |
| <p>When Jena parses a datatype whose lexical value is not legal for |
| the declared datatype is does not immediately throw an error. This |
| is because the RDFCore working group has defined that illegal |
| datatype values are errors but are not syntactic errors so we try |
| to avoid have parsers break at this point. Instead a literal is |
| created which is marked internally as ill-formed and the first time |
| an application attempts to access its value (with <code>getValue()</code>) an |
| error will be thrown.</p> |
| <p>When Jena is reading a file there is also the issue of what to do |
| when it encounters a typed value whose datatype URI is not one that |
| is knows about. The default behaviour is to create a new datatype |
| object (whose value space is the same as its lexical space). Again |
| this behaviour seems in keeping with the working group preference |
| that illegal datatypes are semantic but not syntactic errors.</p> |
| <p>However, both of these behaviours can mean that simple common |
| errors (such as mis-spelling the xsd namespace) may go unnoticed |
| until very late on. To overcome this we have hidden some global |
| switches that allow you to force Jena to report such syntactic |
| errors earlier. These are static Boolean parameters:</p> |
| <pre><code>org.apache.jena.shared.impl.JenaParameters.enableEagerLiteralValidation |
| org.apache.jena.shared.impl.JenaParameters.enableSilentAcceptanceOfUnknownDatatypes |
| </code></pre> |
| <p>They are placed here in an impl package (and thus only visible in |
| the full javadoc, not the API javadoc) because they should not be |
| regarded as stable. We plan to develop a cleaner way of setting |
| mode switches for Jena and these switches will migrate there in due |
| course, if they prove to be useful.</p> |
| <h2 id="xsd-data-types">XSD data types</h2> |
| <p>Jena includes prebuilt, and pre-registered, instances of |
| <code>RDFDatatype</code> for all of the relevant XSD types:</p> |
| <pre><code>float double int long short byte unsignedByte unsignedShort |
| unsignedInt unsignedLong decimal integer nonPositiveInteger |
| nonNegativeInteger positiveInteger negativeInteger Boolean string |
| normalizedString anyURI token Name QName language NMTOKEN ENTITIES |
| NMTOKENS ENTITY ID NCName IDREF IDREFS NOTATION hexBinary |
| base64Binary date time dateTime duration gDay gMonth gYear |
| gYearMonth gMonthDay |
| </code></pre> |
| <p>These are all available as static member variables from |
| <a href="/documentation/javadoc/jena/org.apache.jena.core/org/apache/jena/datatypes/xsd/XSDDatatype.html"><code>org.apache.jena.datatypes.xsd.XSDDatatype</code></a>.</p> |
| <p>Of these types, the following are registered as the default type to |
| use to represent certain Java classes:</p> |
| <table> |
| <thead> |
| <tr> |
| <th>Java class</th> |
| <th>xsd type</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td>Float</td> |
| <td>float</td> |
| </tr> |
| <tr> |
| <td>Double</td> |
| <td>double</td> |
| </tr> |
| <tr> |
| <td>Integer</td> |
| <td>int</td> |
| </tr> |
| <tr> |
| <td>Long</td> |
| <td>long</td> |
| </tr> |
| <tr> |
| <td>Short</td> |
| <td>short</td> |
| </tr> |
| <tr> |
| <td>Byte</td> |
| <td>byte</td> |
| </tr> |
| <tr> |
| <td>BigInteger</td> |
| <td>integer</td> |
| </tr> |
| <tr> |
| <td>BigDecimal</td> |
| <td>decimal</td> |
| </tr> |
| <tr> |
| <td>Boolean</td> |
| <td>Boolean</td> |
| </tr> |
| <tr> |
| <td>String</td> |
| <td>string</td> |
| </tr> |
| </tbody> |
| </table> |
| <p>Thus when creating a typed literal from a Java <code>BigInteger</code> then |
| <code>xsd:integer</code> will be used. The converse mapping is more adaptive. |
| When parsing an xsd:integer the Java value object used will be an |
| Integer, Long or BigInteger depending on the size of the specific |
| value being represented.</p> |
| <h2 id="user-defined-xsd-data-types">User defined XSD data types</h2> |
| <p>XML schema allows derived types to be defined in which a base type |
| is modified through some facet restriction such as limiting the |
| min/max of an integer or restricting a string to a regular |
| expression. It also allows new types to be created by unioning |
| other types or by constructing lists of other types.</p> |
| <p>Jena provides support for derived and union types but not for list |
| types.</p> |
| <p>These are supported through the <code>XSDDatatype.loadUserDefined</code> |
| method which allows an XML schema datatype file to be loaded. This |
| registers a new <code>RDFDatatype</code> that can be used to create, parse, |
| serialize, test instances of that datatype.</p> |
| <p>There is one difficult issue in here, what URI to give to the user |
| defined datatype? This is not defined by XML Schema, nor RDF nor |
| OWL. Jena2 adopts the position that the defined |
| datatype should have the base URI of the schema file with a |
| fragment identifier given by the datatype name.</p> |
| <p>To illustrate working with the defined types, the following code |
| then tries to create and use two instances of the over 12 type:</p> |
| <pre><code>Model m = ModelFactory.createDefaultModel(); |
| RDFDatatype over12Type = tm.getSafeTypeByName(uri + "#over12"); |
| Object value = null; |
| try { |
| value = "15"; |
| m.createTypedLiteral((String)value, over12Type).getValue(); |
| System.out.println("Over 12 value of " + value + " is ok"); |
| value = "12"; |
| m.createTypedLiteral((String)value, over12Type).getValue(); |
| System.out.println("Over 12 value of " + value + " is OK"); |
| } catch (DatatypeFormatException e) { |
| System.out.println("Over 12 value of " + value + " is illegal"); |
| } |
| </code></pre> |
| <p>which products the output:</p> |
| <pre><code>Over 12 value of 15 is OK |
| Over 12 value of 12 is illegal |
| </code></pre> |
| <h2 id="user-defined-non-xsd-data-types">User defined non-XSD data types</h2> |
| <p>RDF allows any URI to be used as a datatype but provides no |
| standard for how to map the datatype URI to a datatype definition.</p> |
| <p>Within Jena2 we allow new datatypes to be created and registered by |
| using the |
| <a href="/documentation/javadoc/jena/org.apache.jena.core/org/apache/jena/datatypes/TypeMapper.html"><code>TypeMapper</code></a> |
| class.</p> |
| <p>The easiest way to define a new RDFDatatype is to subclass |
| BaseDatatype and define implementations for parse, unparse and |
| isEqual.</p> |
| <p>For example here is the outline of a type used to represent |
| rational numbers:</p> |
| <pre><code>class RationalType extends BaseDatatype { |
| public static final String theTypeURI = "urn:x-hp-dt:rational"; |
| public static final RDFDatatype theRationalType = new RationalType(); |
| |
| /** private constructor - single global instance */ |
| private RationalType() { |
| super(theTypeURI); |
| } |
| |
| /** |
| * Convert a value of this datatype out |
| * to lexical form. |
| */ |
| public String unparse(Object value) { |
| Rational r = (Rational) value; |
| return Integer.toString(r.getNumerator()) + "/" + r.getDenominator(); |
| } |
| |
| /** |
| * Parse a lexical form of this datatype to a value |
| * @throws DatatypeFormatException if the lexical form is not legal |
| */ |
| public Object parse(String lexicalForm) throws DatatypeFormatException { |
| int index = lexicalForm.indexOf("/"); |
| if (index == -1) { |
| throw new DatatypeFormatException(lexicalForm, theRationalType, ""); |
| } |
| try { |
| int numerator = Integer.parseInt(lexicalForm.substring(0, index)); |
| int denominator = Integer.parseInt(lexicalForm.substring(index+1)); |
| return new Rational(numerator, denominator); |
| } catch (NumberFormatException e) { |
| throw new DatatypeFormatException(lexicalForm, theRationalType, ""); |
| } |
| } |
| |
| /** |
| * Compares two instances of values of the given datatype. |
| * This does not allow rationals to be compared to other number |
| * formats, Lang tag is not significant. |
| */ |
| Public Boolean isEqual(LiteralLabel value1, LiteralLabel value2) { |
| return value1.getDatatype() == value2.getDatatype() |
| && value1.getValue().equals(value2.getValue()); |
| } |
| } |
| </code></pre> |
| <p>To register and use this type you simply need the call:</p> |
| <pre><code>RDFDatatype rtype = RationalType.theRationalType; |
| TypeMapper.getInstance().registerDatatype(rtype); |
| ... |
| // Create a rational literal |
| Literal l1 = m.createTypedLiteral("3/5", rtype); |
| </code></pre> |
| <p>Note that whilst any serialization of RDF containing such user |
| defined literals will be perfectly legal a client application has |
| no standard way of looking up the datatype URI you have chosen. |
| This has to be done “out of band” as they say.</p> |
| <h2 id="a-note-on-xmllang">A note on xml:Lang</h2> |
| <p>Plain literals have an xml:Lang tag as well as a string value. Two |
| plain literals with the same string but different Lang tags are not |
| equal.</p> |
| <p>XML Schema states that xml:Lang is not meaningful on xsd |
| datatypes.</p> |
| <p>Thus for almost all typed literals there is no xml:Lang tag.</p> |
| <p>At the time of last call the RDF specifications allowed the special |
| case that <code>rdf:XMLLiteral</code>s could have a Lang tag that would be |
| significant in equality testing. Thus in preview releases of Jena2 |
| the createTypedLiterals calls took an extra Lang tag argument.</p> |
| <p>However, at the time of writing that specification has been changed |
| so that Lang tags will never be significant on typed literals |
| (whether this means that xml:Lang is not significant on XMLLiterals |
| or means that XMLLiteral will cease to be a typed literal is not |
| completely certain).</p> |
| <p>For this reason we have removed the Lang tag from the |
| createTypedLiterals calls and deprecated the createLiteral call |
| which allowed both wellFormedXML and Lang tag to be specified.</p> |
| <p>We do not expect to need to change the API even if the working |
| group decision changes again, the most we might expect to do would |
| be to undeprecate the 3-argument version of createLiteral.</p> |
| |
| </article> |
| |
| <aside class="text-muted align-self-start mb-3 mb-xl-5 p-0 d-none d-xl-flex flex-column sticky-top"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#what-are-typed-literals">What are typed literals?</a></li> |
| <li><a href="#basic-api-operations">Basic API operations</a> |
| <ul> |
| <li><a href="#equality-issues">Equality issues</a></li> |
| </ul> |
| </li> |
| <li><a href="#how-datatypes-are-represented">How datatypes are represented</a> |
| <ul> |
| <li><a href="#error-detection">Error detection</a></li> |
| </ul> |
| </li> |
| <li><a href="#xsd-data-types">XSD data types</a></li> |
| <li><a href="#user-defined-xsd-data-types">User defined XSD data types</a></li> |
| <li><a href="#user-defined-non-xsd-data-types">User defined non-XSD data types</a></li> |
| <li><a href="#a-note-on-xmllang">A note on xml:Lang</a></li> |
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