25.6.1. Fetching associations

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Related to associations, there are two major fetch strategies:

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• EAGER

• LAZY </div>

  EAGER fetching is almost always a bad choice.

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  Prior to JPA, Hibernate used to have all associations as LAZY by default. However, when JPA 1.0 specification emerged, it was thought that not all providers would use Proxies. Hence, the @ManyToOne and the @OneToOne associations are now EAGER by default.

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  The EAGER fetching strategy cannot be overwritten on a per query basis, so the association is always going to be retrieved even if you don’t need it. More, if you forget to JOIN FETCH an EAGER association in a JPQL query, Hibernate will initialize it with a secondary statement, which in turn can lead to N+1 query issues.

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  So, EAGER fetching is to be avoided. For this reason, it’s better if all associations are marked as LAZY by default.

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  However, LAZY associations must be initialized prior to being accessed. Otherwise, a LazyInitializationException is thrown. There are good and bad ways to treat the LazyInitializationException.

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  The best way to deal with LazyInitializationException is to fetch all the required associations prior to closing the Persistence Context. The JOIN FETCH directive is good for @ManyToOne and OneToOne associations, and for at most one collection (e.g. @OneToMany or @ManyToMany). If you need to fetch multiple collections, to avoid a Cartesian Product, you should use secondary queries which are triggered either by navigating the LAZY association or by calling Hibernate#initialize(proxy) method.

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