Neil的備忘錄

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 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 ResultSets, 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.