Default fetch type for one-to-one many-to-one and one-to-many in Hibernate

Question

What is the default fetch type in hibernate mappings   What I got to know after exploring is    for one-to-one it is eager  for one-to-many it is lazy    But after testing it in Eclipse  it was eager for all   Does it depend on whether I am using JPA or Hibernate

User · Answer

To answer your question  Hibernate is an implementation of the JPA standard  Hibernate has its own quirks of operation  but as per the Hibernate docs      By default  Hibernate uses lazy select fetching for collections and lazy proxy fetching for single-valued associations  These defaults make sense for most associations in the majority of applications    So Hibernate will always load any object using a lazy fetching strategy  no matter what type of relationship you have declared  It will use a lazy proxy  which should be uninitialized but not null  for a single object in a one-to-one or many-to-one relationship  and a null collection that it will hydrate with values when you attempt to access it    It should be understood that Hibernate will only attempt to fill these objects with values when you attempt to access the object  unless you specify  fetchType EAGER

User · Answer

It depends on whether you are using JPA or Hibernate   From the JPA 2 0 spec  the defaults are   OneToMany  LAZY ManyToOne  EAGER ManyToMany  LAZY OneToOne  EAGER   And in hibernate  all is Lazy  UPDATE   The latest version of Hibernate aligns with the above JPA defaults

User · Answer

For Single valued associations  i e -One-to-One and Many-to-One - Default Lazy proxy  Proxy lazy loading - This implies a proxy object of your associated entity is loaded  This means that only the id connecting the two entities is loaded for the proxy object of associated entity   Eg  A and B are two entities with Many to one association  ie  There may be multiple A s for every B  Every object of A will contain a reference of B     public class A      int aid        some other A parameters      B b    public class B      int bid         some other B parameters        The relation A will contain columns aid bid    other columns of entity A   The relation B will contain columns bid    other columns of entity B   Proxy implies when A is fetched  only id is fetched for B and stored into a proxy object of B which contains only id  Proxy object of B is a an object of a proxy-class which is a subclass of B with only minimal fields  Since bid is already part of relation A  it is not necessary to fire a query to get bid from the relation B  Other attributes of entity B are lazily loaded only when a field other than bid is accessed   For Collections  i e -Many-to-Many and One-to-Many - Default Lazy true  Please also note that the fetch strategy select join etc  can override lazy  ie  If lazy  true  and fetch  join   fetching of A will also fetch B or Bs In case of collections   You can get the reason if you think about it  Default fetch for single valued association is  join   Default fetch for collections is  select   Please verify the last two lines  I have deduced that logically

User · Answer

I know the answers were correct at the time of asking the question - but since people  like me this minute  still happen to find them wondering why their WildFly 10 was behaving differently  I d like to give an update for the current Hibernate 5 x version   In the Hibernate 5 2 User Guide it is stated in chapter 11 2  Applying fetch strategies      The Hibernate recommendation is to statically mark all associations   lazy and to use dynamic fetching strategies for eagerness  This is   unfortunately at odds with the JPA specification which defines that   all one-to-one and many-to-one associations should be eagerly fetched   by default  Hibernate  as a JPA provider  honors that default    So Hibernate as well behaves like Ashish Agarwal stated above for JPA   OneToMany  LAZY ManyToOne  EAGER ManyToMany  LAZY OneToOne  EAGER    see JPA 2 1 Spec

[java] Default fetch type for one-to-one, many-to-one and one-to-many in Hibernate

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