Eager and Lazy
When you call find() to retrieve an entity, JPA will eagerly (immediately) load data into scalar types, but defer loading data into collection types until the data is needed.
- Called lazy loading, this deferred loading is enabled by default for performance and memory conservation.
By default one-to-one and many-to-one relationships are eagerly loaded and one-to-many and many-to-many are lazily loaded.
- Override the default behavior by adding
fetch=FetchType.EAGERorLAZYto your cardinality annotation.
@ManyToMany(mappedBy="films", fetch=FetchType.EAGER)
private List<Actor> actors;
Overriding Loading Strategy¶
Be careful when overriding the default loading strategy since it may affect performance.
If all of your entities are set to eagerly load their associations, and you query a list of all of the entities of a given type, the number of entities loaded could be immense depending on the size of your data. As a result, it is better for performance to set the FetchType to LAZY in most circumstances.
Later we will see that you can override the loading strategy on an individual query, which may be more appropriate.