JPA

Java Persistance API

Entity Types

• @OneToOne
  • One Entity related to one other
• @OneToMany
  • One entity is related to many entities (List, Set, Map, SortedSet, SortedMap)
• @ManyToOne
  • Inverse of OneToMany
• @ManyToMany
  • Many entities are related to many entities
  • Each has a List or Set reference to the other
  • A join table is used to define relationships
• Unidirectional - mapping is only done one way, one side does not know about relationship
• Bidirectional - both entities know about each other (reccommended by hibernate)

The "Owning Side" of a relationship will hold the foreign key in the database.

Fetch Type

• Lazy Fetch Type - Data is not queried until referenced
• Eager Fetch Type - Data is queried up front
• Hibernate 5 supports the JPA 2.1 Fetch Type Defaults:
  • OneToMany - Lazy
  • ManyToOne - Eager
  • ManyToMany - Lazy
  • OneToOne - Eager

Cascade Types

If I delete the parent will the child be deleted as well?

• JPA Cascade Types control how state changes are cascaded from parent objects to child objects
• JPA Cascade Types
  • PERSIST - Save operations will cascade to related entities
  • MERGE - related entities are merged when the owning entitiy is merged
  • REFRESH - related entities are refreshed when the owning entity is refreshed
  • REMOVE - removes all related entities when the owning entity is deleted
  • DETACH - detaches all related entities if a manual detach occurs
  • ALL - Applies all the above cascade options.

By default, no operations are cascaded

Embeddable Types

• JPA/Hibernate support emeddable types
• These are used to define a common set of properties
• For example, a package with a shipping and billing address

Inheritance

• MappedSuperclass - Entities inherit from a super class. A database table IS NOT created for the super class.
• Single Table - (Hibernate Default) - One Table is used for all subclasses
  • This can lead to a lot of unused database columns
• Joined Table - Base class and subclasses have their own tables.
  • Fetching subclass entities require a join to the table of the superclass. (could cause preformance issues)
• Table Per Class - Each subclass has its own table.

Create and Update Timestamps

• For audit purposes it is often a best practice to use timestamps
• JPA supports @Prepersist and @PreUpdate which can be used to support audit timestamps via JPA lifecycle callbacks.
• Hibernate provides @CreationTimestamp and @UpdateTimestamp