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| <h1 class="title">ARQ - Application API</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="#key-classes">Key Classes</a></li> |
| <li><a href="#select-queries">SELECT queries</a> |
| <ul> |
| <li><a href="#passing-a-result-set-out-of-the-processing-loop">Passing a result set out of the processing loop.</a></li> |
| <li><a href="#example-formatting-a-result-set">Example: formatting a result set</a></li> |
| <li><a href="#example-processing-results">Example: Processing results</a></li> |
| </ul> |
| </li> |
| <li><a href="#construct-queries">CONSTRUCT Queries</a></li> |
| <li><a href="#describe-queries">DESCRIBE Queries</a></li> |
| <li><a href="#ask-queries">ASK Queries</a></li> |
| <li><a href="#formatting-xml-results">Formatting XML results</a></li> |
| <li><a href="#datasets">Datasets</a></li> |
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| <article class="flex-column me-lg-4"> |
| <p>The application API is in the package <code>org.apache.jena.query</code>.</p> |
| <p>Other packages contain various parts of the system (execution |
| engine, parsers, testing etc). Most applications will only need to |
| use the main package. Only applications wishing to programmatically |
| build queries or modify the behaviour of the query engine need to |
| use the others packages directly.</p> |
| <h2 id="key-classes">Key Classes</h2> |
| <p>The package <code>org.apache.jena.query</code> is the main application |
| package.</p> |
| <ul> |
| <li><code>Query</code> - a class that represents the application query. It is |
| a container for all the details of the query. Objects of class |
| Query are normally created by calling one of the methods of |
| <code>QueryFactory</code> methods which provide access to the various parsers.</li> |
| <li><code>QueryExecution</code> - represents one execution of a query.</li> |
| <li><code>QueryExecutionFactory</code> - a place to get <code>QueryExecution</code> instances.</li> |
| <li><code>DatasetFactory</code> - a place to make datasets.</li> |
| <li>For SELECT queries: |
| <ul> |
| <li><code>QuerySolution</code> - A single solution to the query.</li> |
| <li><code>ResultSet</code> - All the QuerySolutions. An iterator.</li> |
| <li><code>ResultSetFormatter</code> - turn a ResultSet into various forms; |
| into json, text, or as plain XML.</li> |
| </ul> |
| </li> |
| </ul> |
| <h2 id="select-queries">SELECT queries</h2> |
| <p>The basic steps in making a SELECT query are outlined in the |
| example below. A query is created from a string using the |
| <code>QueryFactory</code>. The query and model or RDF dataset to be queried |
| are then passed to <code>QueryExecutionFactory</code> to produce an instance |
| of a query execution. <code>QueryExecution</code> objects are <code>java.lang.AutoCloseable</code> |
| and can be used in try-resource. Result are handled in a loop and finally the |
| query execution is closed.</p> |
| <pre><code> import org.apache.jena.query.* ; |
| Model model = ... ; |
| String queryString = " .... " ; |
| Query query = QueryFactory.create(queryString) ; |
| try (QueryExecution qexec = QueryExecutionFactory.create(query, model)) { |
| ResultSet results = qexec.execSelect() ; |
| for ( ; results.hasNext() ; ) |
| { |
| QuerySolution soln = results.nextSolution() ; |
| RDFNode x = soln.get("varName") ; // Get a result variable by name. |
| Resource r = soln.getResource("VarR") ; // Get a result variable - must be a resource |
| Literal l = soln.getLiteral("VarL") ; // Get a result variable - must be a literal |
| } |
| } |
| </code></pre> |
| <p>It is important to cleanly close the query execution when finished. |
| System resources connected to persistent storage may need to be |
| released.</p> |
| <p>A <code>ResultSet</code> supports the Java iterator interface so the |
| following is also a way to process the results if preferred:</p> |
| <pre><code> Iterator<QuerySolution> results = qexec.execSelect() ; |
| for ( ; results.hasNext() ; ) |
| { |
| QuerySolution soln = results.next() ; |
| . . . |
| } |
| </code></pre> |
| <p>The step of creating a query and then a query execution can be |
| reduced to one step in some common cases:</p> |
| <pre><code> import org.apache.jena.query.* ; |
| Model model = ... ; |
| String queryString = " .... " ; |
| try (QueryExecution qexec = QueryExecutionFactory.create(queryString, model)) { |
| ResultSet results = qexec.execSelect() ; |
| . . . |
| } |
| </code></pre> |
| <h3 id="passing-a-result-set-out-of-the-processing-loop">Passing a result set out of the processing loop.</h3> |
| <p>A <code>ResultSet</code> is an iterator and can be traversed only once. What is more, much of query execution |
| and result set processing is handled internally in a streaming fashion. The <code>ResultSet</code> returned |
| by <code>execSelect</code> is not valid after the <code>QueryExecution</code> is closed, |
| whether explicitly or by |
| try-resources as the <code>QueryExecution</code> implements <code>AutoCloseable</code>.</p> |
| <p>A result set may be materialized - this is then usable outside</p> |
| <pre><code> try (QueryExecution qexec = QueryExecutionFactory.create(queryString, model)) { |
| ResultSet results = qexec.execSelect() ; |
| results = ResultSetFactory.copyResults(results) ; |
| return results ; // Passes the result set out of the try-resources |
| } |
| </code></pre> |
| <p>The result set from <code>ResultSetFactory.copyResults</code> is a <code>ResultSetRewindable</code> which has a |
| <code>reset()</code> operation that positions the iterator at the start of the result again.</p> |
| <p>This can also be used when the results are going to be used in a loop that modifies |
| the data. It is not possible to update the model or dataset while looping |
| over the results of a <code>SELECT</code> query.</p> |
| <p>The models returned by <code>execConstruct</code> and <code>execDescribe</code> are valid |
| after the <code>QueryExecution</code> is closed.</p> |
| <h3 id="example-formatting-a-result-set">Example: formatting a result set</h3> |
| <p>Instead of a loop to deal with each row in the result set, the |
| application can call an operation of the <code>ResultSetFormatter</code>. This |
| is what the command line applications do.</p> |
| <p>Example: processing results to produce a simple text presentation:</p> |
| <pre><code> ResultSetFormatter fmt = new ResultSetFormatter(results, query) ; |
| fmt.printAll(System.out) ; |
| </code></pre> |
| <p>or simply:</p> |
| <pre><code> ResultSetFormatter.out(System.out, results, query) ; |
| </code></pre> |
| <h3 id="example-processing-results">Example: Processing results</h3> |
| <p>The results are objects from the Jena RDF API and API calls, which |
| do not modify the model, can be mixed with query results |
| processing:</p> |
| <pre><code> for ( ; results.hasNext() ; ) |
| { |
| // Access variables: soln.get("x") ; |
| RDFNode n = soln.get("x") ; // "x" is a variable in the query |
| // If you need to test the thing returned |
| if ( n.isLiteral() ) |
| ((Literal)n).getLexicalForm() ; |
| if ( n.isResource() ) |
| { |
| Resource r = (Resource)n ; |
| if ( ! r.isAnon() ) |
| { |
| ... r.getURI() ... |
| } |
| } |
| } |
| </code></pre> |
| <p>Updates to the model must be carried out after the query execution |
| has finished. Typically, this involves collecting results of |
| interest in a local datastructure and looping over that structure |
| after the query execution has finished and been closed.</p> |
| <h2 id="construct-queries">CONSTRUCT Queries</h2> |
| <p><code>CONSTRUCT</code> queries return a single RDF graph. As usual, the query |
| execution should be closed after use.</p> |
| <pre><code>Query query = QueryFactory.create(queryString) ; |
| QueryExecution qexec = QueryExecutionFactory.create(query, model) ; |
| Model resultModel = qexec.execConstruct() ; |
| qexec.close() ; |
| </code></pre> |
| <h2 id="describe-queries">DESCRIBE Queries</h2> |
| <p><code>DESCRIBE</code> queries return a single RDF graph. |
| <a href="extension.html#describeHandlers">Different handlers</a> for the |
| <code>DESCRIBE</code> operation can be loaded by added by the application.</p> |
| <pre><code>Query query = QueryFactory.create(queryString) ; |
| QueryExecution qexec = QueryExecutionFactory.create(query, model) ; |
| Model resultModel = qexec.execDescribe() ; |
| qexec.close() ; |
| </code></pre> |
| <h2 id="ask-queries">ASK Queries</h2> |
| <p>The operation Query.execAsk() returns a boolean value indicating |
| whether the query pattern matched the graph or dataset or not.</p> |
| <pre><code>Query query = QueryFactory.create(queryString) ; |
| QueryExecution qexec = QueryExecutionFactory.create(query, model) ; |
| boolean result = qexec.execAsk() ; |
| qexec.close() ; |
| </code></pre> |
| <h2 id="formatting-xml-results">Formatting XML results</h2> |
| <p>The <code>ResultSetFormatter</code> class has methods to write out the |
| <a href="http://www.w3.org/TR/rdf-sparql-XMLres/">SPARQL Query Results XML Format</a>. |
| See ResultSetFormatter.outputAsXML method.</p> |
| <h2 id="datasets">Datasets</h2> |
| <p>The examples above are all queries on a single model. A SPARQL |
| query is made on a dataset, which is a default graph and zero or |
| more named graphs. Datasets can be constructed using the |
| <code>DatasetFactory</code>:</p> |
| <pre><code>String dftGraphURI = "file:default-graph.ttl" ; |
| List namedGraphURIs = new ArrayList() ; |
| namedGraphURIs.add("file:named-1.ttl") ; |
| namedGraphURIs.add("file:named-2.ttl") ; |
| |
| Query query = QueryFactory.create(queryString) ; |
| |
| Dataset dataset = DatasetFactory.create(dftGraphURI, namedGraphURIs) ; |
| try(QueryExecution qExec = QueryExecutionFactory.create(query, dataset)) { |
| ... |
| } |
| </code></pre> |
| <p>Already existing models can also be used:</p> |
| <pre><code>Dataset dataset = DatasetFactory.create() ; |
| dataset.setDefaultModel(model) ; |
| dataset.addNamedModel("http://example/named-1", modelX) ; |
| dataset.addNamedModel("http://example/named-2", modelY) ; |
| try(QueryExecution qExec = QueryExecutionFactory.create(query, dataset)) { |
| ... |
| } |
| </code></pre> |
| <p><a href="index.html">ARQ documentation index</a></p> |
| |
| </article> |
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| <li><a href="#key-classes">Key Classes</a></li> |
| <li><a href="#select-queries">SELECT queries</a> |
| <ul> |
| <li><a href="#passing-a-result-set-out-of-the-processing-loop">Passing a result set out of the processing loop.</a></li> |
| <li><a href="#example-formatting-a-result-set">Example: formatting a result set</a></li> |
| <li><a href="#example-processing-results">Example: Processing results</a></li> |
| </ul> |
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| <li><a href="#construct-queries">CONSTRUCT Queries</a></li> |
| <li><a href="#describe-queries">DESCRIBE Queries</a></li> |
| <li><a href="#ask-queries">ASK Queries</a></li> |
| <li><a href="#formatting-xml-results">Formatting XML results</a></li> |
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| } |
| })() |
| </script> |
| |
| </body> |
| </html> |
