Jena Query Builder - A query builder for Jena.

Table of Contents

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Overview

Query Builder provides implementations of Ask, Construct, Select and Update builders that allow developers to create queries without resorting to StringBuilders or similar solutions. The Query Builder module is an extra package and is found in the jena-querybuilder jar.

Each of the builders has a series of methods to define the query. Each method returns the builder for easy chaining. The example:

SelectBuilder sb = new SelectBuilder()
    .addVar( "*" )
    .addWhere( "?s", "?p", "?o" );

Query q = sb.build() ;

produces

SELECT *
WHERE
  { ?s ?p ?o }

Constructing Expressions

Expressions are primarily used in filter and bind statements as well as in select clauses. All the standard expressions are implemented in the ExprFactory class. An ExprFactory can be retrieved from any Builder by calling the getExprFactory() method. This will create a Factory that has the same prefix mappings and the query. An alternative is to construct the ExprFactory directly, this factory will not have the prefixes defined in PrefixMapping.Extended.

SelectBuilder builder = new SelectBuilder();
ExprFactory exprF = builder.getExprFactory()
    .addPrefix( "cf",
        "http://vocab.nerc.ac.uk/collection/P07/current/CFSN0023/")
builder.addVar( exprF.floor( ?v ), ?floor )

    .addWhere( ?s, "cf:air_temperature", ?v )

Update Builder

The UpdateBuilder is used to create Update, UpdateDeleteWhere or UpdateRequest objects. When an UpdateRequest is built is contains a single Update object as defined by the UpdateBuilder. Update objects can be added to an UpdateRequest using the appendTo() method.

Var subj = Var.alloc( "s" );
Var obj = Var.alloc( "o" );

UpdateBuilder builder = new UpdateBuilder( PrefixMapping.Standard)
    .addInsert( subj, "rdfs:comment", obj )
    .addWhere( subj, "dc:title", obj);

UpdateRequest req = builder.buildRequest();

UpdateBuilder builder2 = new UpdateBuilder()
    .addPrefix( "dc", "http://purl.org/dc/elements/1.1/")
    .addDelete( subj, "?p", obj)
    .where( subj, dc:creator, "me")
    .appendTo( req );

Where Builder

In some use cases it is desirable to create a where clause without constructing an entire query. The WhereBuilder is designed to fit this need. For example to construct the query:

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>

SELECT ?page ?type WHERE
{
    ?s foaf:page ?page .
    { ?s rdfs:label "Microsoft"@en . BIND ("A" as ?type) }
    UNION
    { ?s rdfs:label "Apple"@en . BIND ("B" as ?type) }
}

You could use a WhereBuilder to construct the union queries and add them to a Select or other query builder.

WhereBuilder whereBuilder = new WhereBuilder()
    .addPrefix( "rdfs",  "http://www.w3.org/2000/01/rdf-schema#" )
    addWhere( "?s", "rdfs:label", "'Microsoft'@en" )
    .addBind( "'A'", "?type")
    .addUnion( new WhereBuilder()
        .addPrefix( "rdfs",  "http://www.w3.org/2000/01/rdf-schema#" )
        .addWhere( "?s", "rdfs:label", "'Apple'@en" )
        .addBind( "'B'", "?type")
    );

SelectBuilder builder = new SelectBuilder()
   .addPrefix( "rdfs",  "http://www.w3.org/2000/01/rdf-schema#" )
   .addPrefix( "foaf", "http://xmlns.com/foaf/0.1/" );
   .addVar( "?page")
   .addVar( "?type" )
   .addWhere( "?s", "foaf:page",  "?page" )
   .addWhere( whereBuilder );

The where clauses could be built inline as:

SelectBuilder builder = new SelectBuilder()
  .addPrefixs( PrefixMapping.Standard )
  .addPrefix( "foaf", "http://xmlns.com/foaf/0.1/" );
  .addVar( "?page")
  .addVar( "?type" )
  .addWhere( "?s", "foaf:page",  "?page" )
  .addWhere( new WhereBuilder()
      .addPrefix( "rdfs",  "http://www.w3.org/2000/01/rdf-schema#" )
      .addWhere( "?s", "rdfs:label", "'Microsoft'@en" )
      .addBind( "'A'", "?type")
      .addUnion( new WhereBuilder()
          .addPrefix( "rdfs",  "http://www.w3.org/2000/01/rdf-schema#" )
          .addWhere( "?s", "rdfs:label", "'Apple'@en" )
          .addBind( "'B'", "?type")
      )
  );

Template Usage

In addition to making it easier to build valid queries the QueryBuilder has a clone method. Using this a developer can create as “Template” query and add to it as necessary.

For example using the above query as the “template” with this code:

SelectBuilder sb2 = sb.clone();
sb2.addPrefix( "foaf", "http://xmlns.com/foaf/0.1/" )
   .addWhere( ?s, RDF.type, "foaf:Person") ;

produces

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT *
WHERE
  { ?s ?p ?o .
    ?s <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> foaf:person .
  }

Prepared Statement Usage

The query builders have the ability to replace variables with other values. This can be

SelectBuilder sb = new SelectBuilder()
    .addVar( "*" )
    .addWhere( "?s", "?p", "?o" );

sb.setVar( Var.alloc( "?o" ), NodeFactory.createURI( "http://xmlns.com/foaf/0.1/Person" ) ) ;
Query q = sb.build();

produces

SELECT *
WHERE
  { ?s ?p <http://xmlns.com/foaf/0.1/Person> }