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What is the difference between the two if any?
Should one or both be used on an entity?
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In hibernate, 'mutable' is default to 'true' in class and its related collection, it mean the class or collection are allow to add, update and delete. On the other hand, if the mutable is changed to false, it has different meaning in class and its related collection.
The entity class must be annotated with the Entity annotation or denoted in the XML descriptor as an entity. So, if you use annotations for mappings, @Entity is mandated by the specification and Hibernate has to adhere to it.
Immutable classes are classes which state remains unchanged throughout the application lifetime. The set of class instance properties is assigned at creation and property values are never allowed to change. In other words, immutable class instances, once created, can never be modified.
@Entity annotation marks this class as an entity. @Table annotation specifies the table name where data of this entity is to be persisted. If you don't use @Table annotation, hibernate will use the class name as the table name by default.
For entity there's practically no difference. @Immutable gets priority (that is if you have entity that's annotated both as @Immutable and @Entity(mutable = "true") it is going to be treated as immutable).
@Immutable can also be used on collections with pretty much the same semantics. Details are here
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The org.hibernate.annotations.Entity annotation is deprecated and will be removed in a future release of Hibernate.
Hence, you should always use the @Immutabale annotation if you have entities that should never be modified by Hibernate.
The @Immutable annotation tells Hibernate to load entities in read-only mode, hence entity modifications cannot be tracked by the dirty checking mechanism.
However, the @Immutable entities can still be updated via JPQL or Criteria API bulk update queries.
To make sure @Immutabale entities are never modified for bulk update queries, from Hibernate 5.2.17 onwards, you can set the following configuration property:
<property
name="hibernate.query.immutable_entity_update_query_handling_mode"
value="exception"
/>
With this property in place, a bulk update query will end up throwing an exception and the entity update will be prevented.
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