ADR-008: Database fetch strategy
We are using JPA on this project (See ADR-004).
JPA allows to use different fetching strategies: Lazy fetching or Eager fetching
For lazy fetching to work, you need to be in an open transaction, which you are normally not in the web layer.
Spring has a workaround for this in the form of the spring.jpa.open-in-view property, but this is an anti-pattern
that should be avoided.
For this reason, the Open Session In View has been disabled on this project.
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Due to disabling the Open Session In View, you might get a LazyInitializationException.
Don’t enable it again, but follow the recommendations below to deal with it.
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Always using eager fetching on everything is not desirable from a performance view.
You would retrieve too much data always.
A good middle ground is using JPQL "join fetch" statements:
Declare the associations as lazy on the entity.
When a query needs information from the associations, explicitly requests this via JOIN FETCH.
Example of setting the fetch type to Lazy:
@Entity
public class Customer extends User {
@OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true) (1)
private final Set<Address> addresses;
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@OneToMany is LAZY by default |
Example of using JOIN FETCH in a JPQL query:
@Query("SELECT c FROM Customer c LEFT JOIN FETCH c.addresses WHERE c.id = :id")
Optional<Customer> findCustomerByIdEagerly(@Param("id") UserId userId);
This will get the matching customer with his addresses set fully initialized, thus avoiding the N+1 query problem.
Duplicates due to JOIN FETCH
It can happen that you get duplicates of your entity (which will usually be an aggregate root) when using JOIN FETCH.
To archieve this, you need to pass a query hint to Hibernate like this:
@QueryHints(@QueryHint(name = "hibernate.query.passDistinctThrough", value = "false"))
@Query("SELECT DISTINCT c FROM Concert c LEFT JOIN FETCH c.studentMatches sm LEFT JOIN FETCH c.student s WHERE c.endDateTime >= :now ORDER BY c.startDateTime")
LinkedHashSet<Concert> findUpcomingConcerts(@Param("now") OffsetDateTime now);
To avoid having to type that property each time, there is the @NoDistinctInSqlQueryHints meta-annotation that can be used:
@NoDistinctInSqlQueryHints
@Query("SELECT DISTINCT c FROM Concert c LEFT JOIN FETCH c.studentMatches sm LEFT JOIN FETCH c.student s WHERE c.endDateTime >= :now ORDER BY c.startDateTime")
LinkedHashSet<Concert> findUpcomingConcerts(@Param("now") OffsetDateTime now);
The used return type should be Set (if order is not important) or LinkedHashSet (if order is important).
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Open sesion in view is disabled via spring.jpa.open-in-view in application.properties
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Associations in entities should be Lazy
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@OneToMany is Lazy by default
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@ManyToOne is Eager by default, so use @ManyToOne(fetch = FetchType.LAZY)
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Use JOIN FETCH when information from the associations is needed (If you get a LazyInitializationException, it is needed).
