One entity may have an attribute which references the other entity. This defines a directionality. In JPA, relationships are only unidirectional, meaning that only one entity can reference the other.
Based on the directionality, one entity’s role is “source” while the other is “target”.
In one-to-one and one-to-many cardinalities, foreign keys are used to implement the relationship. The ownership of the relation is kept by the entity having the foreign key as attribute.
A reference can be single-valued or set-valued (if it has a collection of references). Therefore, there are four possible types of cardinality: one-to-one, one-to-many, many-to-one, many-to-many.
1:N relationships can be achieved in two ways, either by using the JoinColumn annotation or with the mappedBy parameter.
In the first case, JPA uses a bridge table (as for N:M relationships).
Otherwise, JPA is instructed to not create a bridge table, as the relationship is already being mapped by a FK in the opposite entity.
| Signature | Description |
|---|---|
| public ****void persist(Object entity) | Makes an entity instance become part of the persistence context, i.e., managed |
| public <T> T find(Class<T> entityClass, Object primaryKey) | Finds an entity instance by its primary key |
| public ****void remove(Object entity) | Removes an entity instance from the |
| persistence context and thus from the | |
| database | |
| public ****void refresh(Object entity) | Resets the state of entity instance from the |
| content of the database | |
| public ****void flush() | Writes the state of entities to the database |
| as immediately as possible |