If you don't know the identifiers of the objects you are looking for, you need a query. Hibernate supports an easy-to-use but powerful object oriented query language (HQL). For programmatic query creation, Hibernate supports a sophisticated Criteria and Example query feature (QBC and QBE). You may also express your query in the native SQL of your database, with optional support from Hibernate for result set conversion into objects.
HQL and native SQL queries are represented with an instance of org.hibernate.Query
.
This interface offers methods for parameter binding, result set handling, and for the execution
of the actual query. You always obtain a Query
using the current
Session
:
List cats = session.createQuery( "from Cat as cat where cat.birthdate < ?") .setDate(0, date) .list(); List mothers = session.createQuery( "select mother from Cat as cat join cat.mother as mother where cat.name = ?") .setString(0, name) .list(); List kittens = session.createQuery( "from Cat as cat where cat.mother = ?") .setEntity(0, pk) .list(); Cat mother = (Cat) session.createQuery( "select cat.mother from Cat as cat where cat = ?") .setEntity(0, izi) .uniqueResult();]] Query mothersWithKittens = (Cat) session.createQuery( "select mother from Cat as mother left join fetch mother.kittens"); Set uniqueMothers = new HashSet(mothersWithKittens.list());
A query is usually executed by invoking list()
, the
result of the query will be loaded completely into a collection in memory.
Entity instances retrieved by a query are in persistent state. The
uniqueResult()
method offers a shortcut if you
know your query will only return a single object. Note that queries that
make use of eager fetching of collections usually return duplicates of
the root objects (but with their collections initialized). You can filter
these duplicates simply through a Set
.
Occasionally, you might be able to achieve better performance by
executing the query using the iterate()
method.
This will only usually be the case if you expect that the actual
entity instances returned by the query will already be in the session
or second-level cache. If they are not already cached,
iterate()
will be slower than list()
and might require many database hits for a simple query, usually
1 for the initial select which only returns identifiers,
and n additional selects to initialize the actual instances.
// fetch ids Iterator iter = sess.createQuery("from eg.Qux q order by q.likeliness").iterate(); while ( iter.hasNext() ) { Qux qux = (Qux) iter.next(); // fetch the object // something we couldnt express in the query if ( qux.calculateComplicatedAlgorithm() ) { // delete the current instance iter.remove(); // dont need to process the rest break; } }
Hibernate queries sometimes return tuples of objects, in which case each tuple is returned as an array:
Iterator kittensAndMothers = sess.createQuery( "select kitten, mother from Cat kitten join kitten.mother mother") .list() .iterator(); while ( kittensAndMothers.hasNext() ) { Object[] tuple = (Object[]) kittensAndMothers.next(); Cat kitten = (Cat) tuple[0]; Cat mother = (Cat) tuple[1]; .... }
Queries may specify a property of a class in the select
clause.
They may even call SQL aggregate functions. Properties or aggregates are considered
"scalar" results (and not entities in persistent state).
Iterator results = sess.createQuery( "select cat.color, min(cat.birthdate), count(cat) from Cat cat " + "group by cat.color") .list() .iterator(); while ( results.hasNext() ) { Object[] row = (Object[]) results.next(); Color type = (Color) row[0]; Date oldest = (Date) row[1]; Integer count = (Integer) row[2]; ..... }
Methods on Query
are provided for binding values to
named parameters or JDBC-style ?
parameters.
Contrary to JDBC, Hibernate numbers parameters from zero.
Named parameters are identifiers of the form :name
in
the query string. The advantages of named parameters are:
named parameters are insensitive to the order they occur in the query string
they may occur multiple times in the same query
they are self-documenting
//named parameter (preferred) Query q = sess.createQuery("from DomesticCat cat where cat.name = :name"); q.setString("name", "Fritz"); Iterator cats = q.iterate();
//positional parameter Query q = sess.createQuery("from DomesticCat cat where cat.name = ?"); q.setString(0, "Izi"); Iterator cats = q.iterate();
//named parameter list List names = new ArrayList(); names.add("Izi"); names.add("Fritz"); Query q = sess.createQuery("from DomesticCat cat where cat.name in (:namesList)"); q.setParameterList("namesList", names); List cats = q.list();
If you need to specify bounds upon your result set (the maximum number of rows
you want to retrieve and / or the first row you want to retrieve) you should
use methods of the Query
interface:
Query q = sess.createQuery("from DomesticCat cat"); q.setFirstResult(20); q.setMaxResults(10); List cats = q.list();
Hibernate knows how to translate this limit query into the native SQL of your DBMS.
If your JDBC driver supports scrollable ResultSet
s, the
Query
interface may be used to obtain a
ScrollableResults
object, which allows flexible
navigation of the query results.
Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " + "order by cat.name"); ScrollableResults cats = q.scroll(); if ( cats.first() ) { // find the first name on each page of an alphabetical list of cats by name firstNamesOfPages = new ArrayList(); do { String name = cats.getString(0); firstNamesOfPages.add(name); } while ( cats.scroll(PAGE_SIZE) ); // Now get the first page of cats pageOfCats = new ArrayList(); cats.beforeFirst(); int i=0; while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add( cats.get(1) ); } cats.close()
Note that an open database connection (and cursor) is required for this
functionality, use setMaxResult()
/setFirstResult()
if you need offline pagination functionality.
You may also define named queries in the mapping document. (Remember to use a
CDATA
section if your query contains characters that could
be interpreted as markup.)
<query name="ByNameAndMaximumWeight"><![CDATA[ from eg.DomesticCat as cat where cat.name = ? and cat.weight > ? ] ]></query>
Parameter binding and executing is done programatically:
Query q = sess.getNamedQuery("ByNameAndMaximumWeight"); q.setString(0, name); q.setInt(1, minWeight); List cats = q.list();
Note that the actual program code is independent of the query language that is used, you may also define native SQL queries in metadata, or migrate existing queries to Hibernate by placing them in mapping files.
Also note that a query declaration inside a <hibernate-mapping>
element requires a global unique name for the query, while a query declaration inside a
<class>
element is made unique automatically by prepending the
fully qualified name of the class, for example
eg.Cat.ByNameAndMaximumWeight
.
A collection filter is a special type of query that may be applied to
a persistent collection or array. The query string may refer to this
,
meaning the current collection element.
Collection blackKittens = session.createFilter( pk.getKittens(), "where this.color = ?") .setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) ) .list() );
The returned collection is considered a bag, and it's a copy of the given collection. The original collection is not modified (this is contrary to the implication of the name "filter", but consistent with expected behavior).
Observe that filters do not require a from
clause (though they may have
one if required). Filters are not limited to returning the collection elements themselves.
Collection blackKittenMates = session.createFilter( pk.getKittens(), "select this.mate where this.color = eg.Color.BLACK.intValue") .list();
Even an empty filter query is useful, e.g. to load a subset of elements in a huge collection:
Collection tenKittens = session.createFilter( mother.getKittens(), "") .setFirstResult(0).setMaxResults(10) .list();
HQL is extremely powerful but some developers prefer to build queries dynamically,
using an object-oriented API, rather than building query strings. Hibernate provides
an intuitive Criteria
query API for these cases:
Criteria crit = session.createCriteria(Cat.class); crit.add( Restrictions.eq( "color", eg.Color.BLACK ) ); crit.setMaxResults(10); List cats = crit.list();
The Criteria
and the associated Example
API are discussed in more detail in Chapter 15, Criteria Queries.
You may express a query in SQL, using createSQLQuery()
and
let Hibernate take care of the mapping from result sets to objects. Note
that you may at any time call session.connection()
and
use the JDBC Connection
directly. If you chose to use the
Hibernate API, you must enclose SQL aliases in braces:
List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10") .addEntity("cat", Cat.class) .list();
List cats = session.createSQLQuery( "SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " + "{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... " + "FROM CAT {cat} WHERE ROWNUM<10") .addEntity("cat", Cat.class) .list()
SQL queries may contain named and positional parameters, just like Hibernate queries. More information about native SQL queries in Hibernate can be found in Chapter 16, Native SQL.