CODEWITHSUNDEEP
jakarta-eejpa
user1727072
user1727072

Reputation: 319

Why does a JPA query return different objects

Im using JPA in my Java ee application. I have a table A mapped to a class A. Now when I query the entity (e.g. by its id) it always returns a different object. Example:

public A getAById(int id) {
    A obj = (A) em.createNamedQuery("getAById")
            .setParameter("id", id).getSingleResult();
    return obj;
}

public void test(){
    getAById(1)==getAById(1) //is false
}

Is there a way to tell JPA not to always create new instances when querying from a database but to return an existing object if it hast alreay been queried?

//EDIT This article helped me a lot: http://en.wikibooks.org/wiki/Java_Persistence/Caching#Example_JPA_2.0_Cacheable_annotation

Upvotes: 1

Views: 3081

Answers (3)

Chris
Chris

Reputation: 21145

JavaEE containers wrap the JPA providers EntityManager in a proxy, so the behavior you see is likely because your container gets a new EM outside of a transaction. Your two getAById(1) calls are going to two different EntityManagers underneath the covers. You can get around this by wrapping them in a single transaction, forcing the container to use the same entity manager for the life of the transaction.

Upvotes: 2

vels4j
vels4j

Reputation: 11298

Is there a way to tell JPA not to always create new instances when querying from a database but to return an existing object if it hast already been queried?

JPA never creates new entity for select queries. See the following example

Query qry = em.createNamedQuery("SELECT r FROM BusinessUnit r WHERE r.buKey = :buKey");
Object result1 = qry.setParameter("buKey", "32").getSingleResult();
qry = em.createNamedQuery("SELECT r FROM BusinessUnit r WHERE r.buKey = :buKey");
Object result2 = qry.setParameter("buKey", "32").getSingleResult();
System.out.println(result1 == result2);

For the above, I get result1 == result2 true, since hashCode method is overwritten in entity class.

If your entity class is overwritten the equals method, you can test with equals method instead of comparing with object reference.

public void test(){
    getAById(1).equals(getAById(1)) ;
}

Upvotes: 3

spiritwalker
spiritwalker

Reputation: 2257

if there is already an existing entity instance in the persistence context, yes, but you need to use EntityManager operations such entityManager.find(entity.class, id); Doing this will ask entitymanager to find entity from associated PersistenceContext first, if it cannot find it it will query from DB. Once the entity instance is first time loaded into associated PersistenceContext, the rest query against same entity ID will not trigger a DB query and always reuse the one cached in PersistenceContext. This is JPA　first level cache.

Using JPQL will not fit your requirement, since JPQL will bypass PersistenceContext and always communicate with DB directly, which will end up with getting different java object(same entity in terms of primary key).

Upvotes: -2

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