Using Java generics for JPA findAll query with WHERE clause

Question

So  After a 10  year break I m coming back to Java and trying out stuff with JPA and Java generics  I ve created a generics based findAll other  JPA query that basically does  SELECT   FROM source WHERE other id   other id    This is where I m up to  It works  but I m wondering if there s a better  cleaner way to do it  Using ManagedType was hard  and there s not much complete documentation or simple examples around   I ve decided to keep my code as generic as possible  no pun intended  so I use JPA2   This is the root of all Entity Classes  I probably don t need it  but it stops me from having basic mistakes   import java io Serializable   public abstract class DomainObject implements Serializable        private static final long serialVersionUID   1L       public abstract void setId Long id       public abstract Long getId         This is the abstract DAO class  I extend this for the implementation classes as I need to be more specific doing other activities - mostly making sure lazy sets are loaded   public abstract class GenericDAOImpl lt T extends DomainObject  T2 extends DomainObject gt  implements GenericDAO lt T  T2 gt     private Class lt T gt  type    PersistenceContext protected EntityManager entityManager   public GenericDAOImpl Class lt T gt  type        super        this type   type         save and delete classes go here   Override public List lt T gt  findAll T2 where         CriteriaBuilder criteriaBuilder   entityManager getCriteriaBuilder        CriteriaQuery lt T gt  criteriaQuery   criteriaBuilder createQuery type       Root lt T gt  rootQuery   criteriaQuery from type       if  where    null             EntityType lt T gt  entity   entityManager getMetamodel   entity type            SingularAttribute lt   super T    gt  attribute   null          for  SingularAttribute lt   super T    gt  singleAttribute  entity getSingularAttributes                     loop through all attributes that match this class             if  singleAttribute getJavaType   equals where getClass                          winner                  attribute   singleAttribute                  break                                     where t object   object getID           criteriaQuery where criteriaBuilder equal rootQuery get attribute   where              criteriaQuery select rootQuery       TypedQuery lt T gt  query   entityManager createQuery criteriaQuery           need this to make sure we have a clean list         entityManager clear        return query getResultList        Any suggestions  If anything  I want this out there so other people can make use of it

User · Answer

I found this page very useful  https   code google com p spring-finance-manager source browse trunk src main java net stsmedia financemanager dao GenericDAOWithJPA java r 2  public abstract class GenericDAOWithJPA lt T  ID extends Serializable gt         private Class lt T gt  persistentClass         This you might want to get injected by the container     protected EntityManager entityManager        SuppressWarnings  unchecked       public GenericDAOWithJPA                 this persistentClass    Class lt T gt     ParameterizedType  getClass   getGenericSuperclass    getActualTypeArguments   0               SuppressWarnings  unchecked       public List lt T gt  findAll                 return entityManager createQuery  Select t from     persistentClass getSimpleName       t   getResultList

User · Answer

This will work  and if you need where statement you can add it as parameter   class GenericDAOWithJPA lt T  ID extends Serializable gt               public List lt T gt  findAll                 return entityManager createQuery  Select t from     persistentClass getSimpleName       t   getResultList

User · Answer

you can also use a namedQuery named findAll for all your entities and call it in your generic FindAll with entityManager createNamedQuery persistentClass getSimpleName    quot findAll quot   getResultList

User · Answer

Hat tip to Adam Bien if you don t want to use createQuery with a String and want type safety      PersistenceContext  EntityManager em    public List lt ConfigurationEntry gt  allEntries             CriteriaBuilder cb   em getCriteriaBuilder            CriteriaQuery lt ConfigurationEntry gt  cq   cb createQuery ConfigurationEntry class           Root lt ConfigurationEntry gt  rootEntry   cq from ConfigurationEntry class           CriteriaQuery lt ConfigurationEntry gt  all   cq select rootEntry           TypedQuery lt ConfigurationEntry gt  allQuery   em createQuery all           return allQuery getResultList          http   www adam-bien com roller abien entry selecting all jpa entities as

[java] Using Java generics for JPA findAll() query with WHERE clause

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