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| <h1 class="title">ARQ - Writing Property Functions</h1> |
| |
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| <main class="d-flex flex-xl-row flex-column"> |
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| <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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/function/Function.html">org.apache.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> |
| <pre><code>PropertyFunction |
| | |
| |--PropertyFunctionBase |
| | |
| |--PropertyFunctionEval |
| | |
| |--PFuncSimpleAndList |
| | |
| |--PFuncSimple |
| | |
| |--PFuncAssignToObject |
| | |
| |--PFuncAssignToSubject |
| | |
| |--PFuncListAndSimple |
| | |
| |--PFuncListAndList |
| </code></pre> |
| <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="/documentation/javadoc/jena/org.apache.jena.core/org/apache/jena/graph/Node.html"><code>org.apache.jena.graph.Node</code></a>s or |
| <a href="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/pfunction/PropFuncArg.html"><code>org.apache.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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/util/Context.html"><code>org.apache.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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/pfunction/PropertyFunctionFactory.html"><code>org.apache.jena.sparql.pfunction.PropertyFunctionFactory</code></a> |
| (shown later), you can register a function as follows:</p> |
| <pre><code>final PropertyFunctionRegistry reg = PropertyFunctionRegistry.chooseRegistry(ARQ.getContext()); |
| reg.put("urn:ex:fn#example", new ExamplePropertyFunctionFactory()); |
| PropertyFunctionRegistry.set(ARQ.getContext(), reg); |
| </code></pre> |
| <p>The only difference between global and dataset-specific registration is where the <code>Context</code> object comes from:</p> |
| <pre><code>final Dataset ds = DatasetFactory.createGeneral(); |
| final PropertyFunctionRegistry reg = PropertyFunctionRegistry.chooseRegistry(ds.getContext()); |
| reg.put("urn:ex:fn#example", new ExamplePropertyFunctionFactory()); |
| PropertyFunctionRegistry.set(ds.getContext(), reg); |
| </code></pre> |
| <p>Note that |
| <a href="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/pfunction/PropertyFunctionRegistry.html"><code>org.apache.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> |
| <pre><code>public class ExamplePropertyFunctionFactory implements PropertyFunctionFactory { |
| @Override |
| public PropertyFunction create(final String uri) |
| { |
| return new PFuncSimple() |
| { |
| @Override |
| public QueryIterator execEvaluated(final Binding parent, final Node subject, final Node predicate, final Node object, final ExecutionContext execCtx) |
| { |
| return QueryIterNullIterator.create(execCtx); |
| } |
| }; |
| } |
| } |
| </code></pre> |
| <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 responsibility 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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/engine/iterator/QueryIterSingleton.html"><code>QueryIterSingleton</code></a> - to provide a single solution/binding for the given values</li> |
| <li><a href="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/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="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/engine/binding/BindingFactory.html"><code>BindingFactory</code></a>. |
| Creation of <code>Binding</code> objects will also require references to <a href="/documentation/javadoc/arq/org.apache.jena.arq/org/apache/jena/sparql/core/Var.html"><code>Var</code></a> |
| and <a href="/documentation/javadoc/jena/org.apache.jena.core/org/apache/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> |
| <p><strong>Graph Operations</strong></p> |
| <p>Additional operations on the current, or another, Graph can be achieved through the Execution Context. |
| Once retrieved the Graph can be operated upon directly, queried or wrapped in a Model, if preferred.</p> |
| <pre><code> // Retrieve current Graph. |
| Graph graph = execCxt.getActiveGraph(); |
| |
| // Wrap Graph in a Model. |
| Model model = ModelFactory.createModelForGraph(graph); |
| </code></pre> |
| <p>Access another graph:</p> |
| <pre><code> // Retrieve DatasetGraph of current Graph. |
| DatasetGraph datasetGraph = execCxt.getDataset(); |
| |
| // Retrieve a different Graph in the Dataset. |
| Node otherGraphNode = NodeFactory.createURI("http://example.org/otherGraph"); |
| Graph otherGraph = datasetGraph.getNamedGraph(otherGraphNode); |
| |
| // Access the other graph |
| ExtendedIterator<Triple> iter = otherGraph.find(...); |
| </code></pre> |
| |
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