12.4.12. About Dynamic Instantiation

There is a particular expression type that is only valid in the select clause. Hibernate calls this "dynamic instantiation". JPQL supports some of this feature and calls it a "constructor expression".

Example 12.11. Dynamic Instantiation Example - Constructor

select new Family( mother, mate, offspr )
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

select new Family( mother, mate, offspr )
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

Copy to Clipboard

Toggle word wrap

So rather than dealing with the Object[] here we are wrapping the values in a type-safe java object that will be returned as the results of the query. The class reference must be fully qualified and it must have a matching constructor.

The class here need not be mapped. If it does represent an entity, the resulting instances are returned in the NEW state (not managed!).

This is the part JPQL supports as well. HQL supports additional "dynamic instantiation" features. First, the query can specify to return a List rather than an Object[] for scalar results:

Example 12.12. Dynamic Instantiation Example - List

select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

Copy to Clipboard

Toggle word wrap

The results from this query will be a List<List> as opposed to a List<Object[]>

HQL also supports wrapping the scalar results in a Map.

Example 12.13. Dynamic Instantiation Example - Map

select new map( mother as mother, offspr as offspr, mate as mate )
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

select new map( max(c.bodyWeight) as max, min(c.bodyWeight) as min, count(*) as n )
from Cat cxt"/>

select new map( mother as mother, offspr as offspr, mate as mate )
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

select new map( max(c.bodyWeight) as max, min(c.bodyWeight) as min, count(*) as n )
from Cat cxt"/>

Copy to Clipboard

Toggle word wrap

The results from this query will be a List<Map<String,Object>> as opposed to a List<Object[]>. The keys of the map are defined by the aliases given to the select expressions.

Report a bug

Voltar ao topo

Este conteúdo não está disponível no idioma selecionado.

12.4.12. About Dynamic Instantiation

Aprender

Experimente, compre e venda

Comunidades

Sobre a documentação da Red Hat

Tornando o open source mais inclusivo

Sobre a Red Hat

Theme

Red Hat legal and privacy links

Red Hat legal and privacy links