Author: buildbot
Date: Thu Jul 10 08:08:47 2014
New Revision: 915656
Log:
Staging update by buildbot for jena
Added:
websites/staging/jena/trunk/content/documentation/query/writing_propfuncs.html
Modified:
websites/staging/jena/trunk/content/ (props changed)
Propchange: websites/staging/jena/trunk/content/
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+ <h1 class="title">ARQ - Writing Property Functions</h1>
+ <p><strong>ARQ - Writing Property Functions</strong></p>
+<p>See also <a href="writing_functions.html">Writing Filter Functions</a>.</p>
+<p>Applications can add SPARQL property functions to the query engine. This is
done by first implementing the
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/pfunction/PropertyFunction.html";><code>PropertyFunction</code></a>
+ interface, and then either registering that function or using the fake
<code>java:</code> URI scheme to dynamically
+ load the function.</p>
+<p><strong>Writing SPARQL Property Functions</strong></p>
+<p>Similar to SPARQL Filter Functions, a SPARQL Property Function is an
extension point of the SPARQL query language
+ that allows a URI to name a function in the query processor. A key difference
is that Property Functions may
+ generate new bindings.</p>
+<p>Just like
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/function/Function.html";>com.hp.hpl.jena.sparql.function.Function</a>
+ there are various utility classes provided to simplify the creation of a
Property Function. The selection of
+ one depends on the 'style' of the desired built-in. For example,
<code>PFuncSimple</code> is expected to be the predicate
+ of triple patterns <code>?such ex:as ?this</code>, where neither argument is
an <code>rdf:list</code>, and either may be a variable.
+ Alternatively, <code>PFuncAssignToObject</code> assumes that the subject will
be bound, while the object will be a variable.</p>
+<div class="codehilite"><pre><span class="n">PropertyFunction</span>
+ <span class="o">|</span>
+ <span class="o">|--</span><span class="n">PropertyFunctionBase</span>
+ <span class="o">|</span>
+ <span class="o">|--</span><span class="n">PropertyFunctionEval</span>
+ <span class="o">|</span>
+ <span class="o">|--</span><span
class="n">PFuncSimpleAndList</span>
+ <span class="o">|</span>
+ <span class="o">|--</span><span class="n">PFuncSimple</span>
+ <span class="o">|</span>
+ <span class="o">|--</span><span
class="n">PFuncAssignToObject</span>
+ <span class="o">|</span>
+ <span class="o">|--</span><span
class="n">PFuncAssignToSubject</span>
+</pre></div>
+
+
+<p>The choice of extension point determines the function signature that the
developer will need to implement, and
+ primarily determines whether some of the arguments will be
+ <a
href="https://jena.apache.org/documentation/javadoc/jena/com/hp/hpl/jena/graph/Node.html";><code>com.hp.hpl.jena.graph.Node</code></a>s
or
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/pfunction/PropFuncArg.html";><code>com.hp.hpl.jena.sparql.pfunction.PropFuncArg</code></a>s.
+ In the latter case, the programmer can determine whether the argument is a
list as well as how many
+ arguments it consists of.</p>
+<p><strong>Registration</strong></p>
+<p>Every property function is associated with a particular
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/util/Context.html";><code>com.hp.hpl.jena.sparql.util.Context</code></a>.
+ This allows you to limit the availability of the function to be global or
associated with a particular dataset.
+ For example, a custom Property Function may expose an index which only has
meaning with respect to some set
+ of data.</p>
+<p>Assuming you have an implementation of
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/pfunction/PropertyFunctionFactory.html";><code>com.hp.hpl.jena.sparql.pfunction.PropertyFunctionFactory</code></a>
+ (shown later), you can register a function as follows:</p>
+<div class="codehilite"><pre><span class="n">final</span> <span
class="n">PropertyFunctionRegistry</span> <span class="n">reg</span> <span
class="p">=</span> <span class="n">PropertyFunctionRegistry</span><span
class="p">.</span><span class="n">chooseRegistry</span><span
class="p">(</span><span class="n">ARQ</span><span class="p">.</span><span
class="n">getContext</span><span class="p">());</span>
+<span class="n">reg</span><span class="p">.</span><span
class="n">put</span><span class="p">(</span>"<span
class="n">urn</span><span class="p">:</span><span class="n">ex</span><span
class="p">:</span><span class="n">fn</span>#<span
class="n">example</span>"<span class="p">,</span> <span
class="n">new</span> <span class="n">ExamplePropertyFunctionFactory</span><span
class="p">);</span>
+<span class="n">PropertyFunctionRegistry</span><span class="p">.</span><span
class="n">set</span><span class="p">(</span><span class="n">ARQ</span><span
class="p">.</span><span class="n">getContext</span><span class="p">(),</span>
<span class="n">reg</span><span class="p">);</span>
+</pre></div>
+
+
+<p>The only difference between global and dataset-specific registration is
where the <code>Context</code> object comes from:</p>
+<div class="codehilite"><pre><span class="n">final</span> <span
class="n">Dataset</span> <span class="n">ds</span> <span class="p">=</span>
<span class="n">DatasetFactory</span><span class="p">.</span><span
class="n">createMem</span><span class="p">();</span>
+<span class="n">final</span> <span class="n">PropertyFunctionRegistry</span>
<span class="n">reg</span> <span class="p">=</span> <span
class="n">PropertyFunctionRegistry</span><span class="p">.</span><span
class="n">chooseRegistry</span><span class="p">(</span><span
class="n">ds</span><span class="p">.</span><span
class="n">getContext</span><span class="p">());</span>
+<span class="n">reg</span><span class="p">.</span><span
class="n">put</span><span class="p">(</span>"<span
class="n">urn</span><span class="p">:</span><span class="n">ex</span><span
class="p">:</span><span class="n">fn</span>#<span
class="n">example</span>"<span class="p">,</span> <span
class="n">new</span> <span class="n">ExamplePropertyFunctionFactory</span><span
class="p">);</span>
+<span class="n">PropertyFunctionRegistry</span><span class="p">.</span><span
class="n">set</span><span class="p">(</span><span class="n">ds</span><span
class="p">.</span><span class="n">getContext</span><span class="p">(),</span>
<span class="n">reg</span><span class="p">);</span>
+</pre></div>
+
+
+<p>Note that
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/pfunction/PropertyFunctionRegistry.html";><code>com.hp.hpl.jena.sparql.pfunction.PropertyFunctionRegistry</code></a>
+ has other <code>put</code> methods that allow registration by passing a
<code>Class</code> object, as well.</p>
+<p><strong>Implementation</strong></p>
+<p>The implementation of a Property Function is actually quite straight
forward once one is aware of the tools
+ at their disposal to do so. For example, if we wished to create a Property
Function that returns no results
+ regardless of their arguments we could do so as follows:</p>
+<div class="codehilite"><pre><span class="n">public</span> <span
class="n">class</span> <span class="n">ExamplePropertyFunctionFactory</span>
<span class="n">implements</span> <span
class="n">PropertyFunctionFactory</span> <span class="p">{</span>
+ <span class="p">@</span><span class="n">Override</span>
+ <span class="n">public</span> <span class="n">PropertyFunction</span>
<span class="n">create</span><span class="p">(</span><span
class="n">final</span> <span class="n">String</span> <span
class="n">uri</span><span class="p">)</span>
+ <span class="p">{</span>
+ <span class="k">return</span> <span class="n">new</span> <span
class="n">PFuncSimple</span><span class="p">()</span>
+ <span class="p">{</span>
+ <span class="p">@</span><span class="n">Override</span>
+ <span class="n">public</span> <span class="n">QueryIterator</span>
<span class="n">execEvaluated</span><span class="p">(</span><span
class="n">final</span> <span class="n">Binding</span> <span
class="n">parent</span><span class="p">,</span> <span class="n">final</span>
<span class="n">Node</span> <span class="n">subject</span><span
class="p">,</span> <span class="n">final</span> <span class="n">Node</span>
<span class="n">predicate</span><span class="p">,</span> <span
class="n">final</span> <span class="n">Node</span> <span
class="n">object</span><span class="p">,</span> <span class="n">final</span>
<span class="n">ExecutionContext</span> <span class="n">execCxt</span><span
class="p">)</span>
+ <span class="p">{</span>
+ <span class="k">return</span> <span
class="n">QueryIterNullIterator</span><span class="p">.</span><span
class="n">create</span><span class="p">(</span><span
class="n">execCtx</span><span class="p">);</span>
+ <span class="p">}</span>
+ <span class="p">};</span>
+ <span class="p">}</span>
+<span class="p">}</span>
+</pre></div>
+
+
+<p><code>Node</code> and <code>PropFuncArg</code> objects allow the developer
to reflect on the state of the arguments, and choose what
+ bindings to generate given the intended usage of the Property Function. For
example, if the function expects a
+ list of three bound arguments for the object of the property, then it can
throw a <code>ExprEvalException</code>
+ (or derivative) to indicate incorrect use. It is the responsability of the
developer to identify what parts
+ of the argument are bound, and to respond appropriately.</p>
+<p>For example, if <code>?a ex:f ?b</code> were a triple pattern in a query,
it could be called with <code>?a</code> bound, <code>?b</code> bound,
+ or neither. It may make sense to return new bindings that include
<code>?b</code> if passed a concrete value for <code>?a</code>,
+ or conversely to generate new bindings for <code>?a</code> when passed a
concrete <code>?b</code>. If both <code>?a</code> and <code>?b</code> are bound,
+ and the function wishes to confirm that the pairing is valid, it can return
the existing binding. If there are
+ no valid solutions to return, then an empty solution may be presented.</p>
+<p>There are several extremely useful implementations of
<code>QueryIterator</code> within the Jena library that make it
+easy to support typical use cases.</p>
+<p>Of particular note:</p>
+<ul>
+<li><a
href="https://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/engine/iterator/QueryIterNullIterator.html";><code>QueryIterNullIterator</code></a>
- to indicate that there are no valid solutions/bindings for the given
values</li>
+<li><a
href="https://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/engine/iterator/QueryIterSingleton.html";><code>QueryIterSingleton</code></a>
- to provide a single solution/binding for the given values</li>
+<li><a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/engine/iterator/QueryIterPlainWrapper.html";><code>QueryIterPlainWrapper</code></a>
- to provide multiple solutions/bindings for the given values</li>
+</ul>
+<p>The second two cases require instances of <code>Binding</code> objects
which can be obtained through static methods of
+ <a
href="http://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/engine/binding/BindingFactory.html";><code>BindingFactory</code></a>.
+ Creation of <code>Binding</code> objects will also require references to <a
href="https://jena.apache.org/documentation/javadoc/arq/com/hp/hpl/jena/sparql/core/Var.html";><code>Var</code></a>
+ and <a
href="https://jena.apache.org/documentation/javadoc/jena/com/hp/hpl/jena/graph/NodeFactory.html";><code>NodeFactory</code></a></p>
+<p>Note that it can make a lot of sense to generate the
<code>Iterator<Binding></code> for <code>QueryIterPlainWrapper</code> by
means of
+ Jena's <code>ExtendedIterator</code>. This can allow domain-specific value to
be easily mapped to <code>Binding</code> objects in
+ a lazy fashion.</p>
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