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6.2. Creating Queries

There are two ways to create a JCR Query object. The first is by supplying a query expression and the name of the query language, and this can be done with the standard JCR API: 
// Obtain the query manager for the session via the workspace ...
javax.jcr.query.QueryManager queryManager = session.getWorkspace().getQueryManager();

// Create a query object ...
String language = ...  // e.g. javax.jcr.query.Query.JCR_SQL2
String expression = ...
javax.jcr.query.Query query = queryManager.createQuery(expression,language);
Before returning the Query , the hierarchical database finds a parser for the language given by the language parameter, and uses this parser to create a language-independent object representation of the query. (Note that any grammatical errors in the expression result in an immediate exception.) This object representation is what JCR 2.0 calls the "Query Object Model", or QOM. After parsing, the hierarchical database embeds the QOM into the Query object.
The second approach for creating a Query object is to programmatically build up the query using the QueryObjectModelFactory . Again, this uses the standard JCR API. Here's a simple example: 
// Obtain the query manager for the session via the workspace ...
javax.jcr.query.QueryManager queryManager = session.getWorkspace().getQueryManager();
javax.jcr.query.qom.QueryObjectModelFactory factory = queryManager.getQOMFactory();

// Create the parts of a query object ...
javax.jcr.query.qom.Source selector = factory.selector(...);
javax.jcr.query.qom.Constraint constraints = ...
javax.jcr.query.qom.Column[] columns = ...
javax.jcr.query.qom.Ordering[] orderings = ...
javax.jcr.query.qom.QueryObjectModel model =
    factory.createQuery(selector,constraints,orderings,columns);

// The model is a query ...
javax.jcr.query.Query query = model;
Of course, the QueryObjectModelFactory can create lots variations of selectors, joins, constraints, and orderings. The hierarchical database fully supports this style of creating queries, and it even offers some very useful extensions (described below).