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| <h1 class="title">ARQ - Writing Filter Functions</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="#writing-sparql-value-functions">Writing SPARQL Value Functions</a></li> |
| <li><a href="#registering-functions">Registering Functions</a></li> |
| </ul> |
| </nav> |
| </aside> |
| <article class="flex-column me-lg-4"> |
| <p>Applications can add SPARQL functions to the query engine. This is |
| done by writing a class implementing the right interface, then |
| either registering it or using the fake <code>java:</code> URI scheme to |
| dynamically call the function.</p> |
| <h2 id="writing-sparql-value-functions">Writing SPARQL Value Functions</h2> |
| <p>A SPARQL value function is an extension point of the SPARQL query |
| language that allows URI to name a function in the query |
| processor.</p> |
| <p>In the ARQ engine, code to implement function must implement the |
| interface <code>org.apache.jena.sparql.function.Function</code> although it is |
| easier to work with one of the abstract classes for specific |
| numbers of arguments like |
| <code>org.apache.jena.sparql.function.FunctionBase1</code> for one argument |
| functions. Functions do not have to have a fixed number of |
| arguments.</p> |
| <p>The abstract class <code>FunctionBase</code>, the superclass of |
| <code>FunctionBase1</code> to <code>FunctionBase4</code>, evaluates its arguments and |
| calls the implementation code with argument values (if a variable |
| was unbound, an error will have been generated)</p> |
| <p>It is possible to get unevaluated arguments but care must be taken |
| not to violate the rules of function evaluation. The standard |
| functions that access unevaluated arguments are the logical ‘or’ |
| and logical ‘and’ operations that back <code>||</code> and <code>&&</code> are special |
| forms to allow for the special exception handling rules.</p> |
| <p>Normally, function should be a pure evaluation based on its |
| argument. It should not access a graph nor return different values |
| for the same arguments (to allow expression optimization). Usually, |
| these requirements can be better met with a |
| <a href="library-propfunc.html">property function</a>. Functions can’t bind |
| variables; this would be done in a |
| <a href="library-propfunc.html">property function</a> as well.</p> |
| <p>Example: (this is the max function in the standard ARQ library):</p> |
| <pre><code>public class max extends FunctionBase2 |
| { |
| public max() { super() ; } |
| public NodeValue exec(NodeValue nv1, NodeValue nv2) |
| { |
| return Functions.max(nv1, nv2) ; |
| } |
| } |
| </code></pre> |
| <p>The function takes two arguments and returns a single value. The |
| class NodeValue represents values and supports value-based |
| operations. NodeValue value support includes the XSD datatypes, |
| xsd:decimal and all its subtypes like xsd:integer and xsd:byte, |
| xsd’;double, xsd:float, xsd:boolean, xsd:dateTime and xsd:date. |
| Literals with language tags are also treated as values in |
| additional “value spaces” determined by the language tag without |
| regard to case.</p> |
| <p>The <code>Functions</code> class contains the core XML Functions and Operators |
| operations. Class NodeFunctions contains the implementations of |
| node-centric operations like <code>isLiteral</code> and <code>str</code>.</p> |
| <p>If any of the arguments are wrong, then the function should throw |
| <code>ExprEvalException</code>.</p> |
| <p>Example: calculate the canonical namespace from a URI (calls the |
| Jena operation for the actual work):</p> |
| <pre><code>public class namespace extends FunctionBase1 |
| { |
| public namespace() { super() ; } |
| |
| public NodeValue exec(NodeValue v) |
| { |
| Node n = v.asNode() ; |
| if ( ! n.isURI() ) |
| throw new ExprEvalException("Not a URI: "+FmtUtils.stringForNode(n)) ; |
| String str = n.getNameSpace() ; |
| return NodeValue.makeString(str) ; |
| } |
| } |
| </code></pre> |
| <p>This throws an evaluation exception if it is passed a value that is |
| not a URI.</p> |
| <p>The standard library, in package |
| <code>org.apache.jena.sparql.function.library</code>, contains many examples.</p> |
| <h2 id="registering-functions">Registering Functions</h2> |
| <p>The query compiler finds functions based on the functions URI. |
| There is a global registry of known functions, but any query |
| execution can have its own function registry.</p> |
| <p>For each function, there is a function factory associated with the |
| URI. A new function instance is created for each use of a function |
| in each query execution.</p> |
| <pre><code>// Register with the global registry. |
| FunctionRegistry.get().put("http://example.org/function#myFunction", new MyFunctionFactory()) ; |
| </code></pre> |
| <p>A common case is registering a specific class for a function |
| implementation so there is an addition method that takes a class, |
| wraps in a built-in function factory and registers the function |
| implementation.</p> |
| <pre><code>// Register with the global registry. |
| FunctionRegistry.get().put("http://example.org/function#myFunction", MyFunction.class) ; |
| </code></pre> |
| <p>Another convenience route to function calling is to use the |
| <a href="java-uri.html"><code>java:</code> URI scheme</a>. This dynamically loads the code, |
| which must be on the Java classpath. With this scheme, the function |
| URI gives the class name. There is automatic registration of a |
| wrapper into the function registry. This way, no explicit |
| registration step is needed by the application and queries issues |
| with the command line tools can load custom functions.</p> |
| <pre><code>PREFIX f: <java:app.myFunctions.> |
| ... |
| FILTER f:myTest(?x, ?y) |
| ... |
| FILTER (?x + f:myIntToXSD(?y)) |
| ... |
| </code></pre> |
| <p><a href="index.html">ARQ documentation index</a></p> |
| |
| </article> |
| |
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| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#writing-sparql-value-functions">Writing SPARQL Value Functions</a></li> |
| <li><a href="#registering-functions">Registering Functions</a></li> |
| </ul> |
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