Is it possible to have immutable fields in Hibernate/JPA?

问题

In our application, we need to have fields that are assignable only once.

At first we thought of encapsulating the fields and making the setters private. However, some questions arouse:

Without a public setter, is Hibernate still able to map the field from the database?
Can I strip out the setter and make the field mutable only in the entity constructor?
Finally, is there any standard JPA way to make a field immutable?

Thanks in advance.

回答1:

Ad. 1: I believe JPA uses plain private fields for both read and write if annotations are placed on fields and not on getters. Recently I discovered that Hibernate as an underlying JPA provider does not even need get*() and set*() methods at all. This was truly enlightening solution since from the beginning I thought Hibernate needs accessors. So the answer is: you don't need setters as far as Hibernate is concerned.
Ad. 2: However please note that Hibernate still needs no-arg constructor, otherwise it will fail to load entities with a descriptive exception. This is also a JPA requirement.
Ad. 3: No, there isn't. Remember that your collections would also had to be immutable.

回答2:

Try

@Column(updatable = false)

And make your setter private. (Leave your getter public if you want)

I think this is the best practice.

P.S.: JPA uses field access if you annotate your fields and uses getter/setter access if you annotate your getter method.

回答3:

In JPA 2.0 you have two ways to define what attributes should be persisted:

Access(FIELD) - the fields name are persisted,
Access(PROPERTY) - the properties name are persisted.

If no Access(-) annotation is used, the decision what access will be used depends on where you put your @Id annotation. If you put it next to your field - Access(FIELD) will be used. If you put it next to your accessor - Access(PROPERTY) will be used.

Therefore, if you use Access(FIELD) you don't have to have an appropriate JavaBeans-style accessor for particular field. You can have a private field named 'myField' and a public setter for it named 'public setBlahBlah(-)'. The JPA will persist just the 'myField'.

回答4:

Try

@Column(updatable = false)

From javadoc:

Whether the column is included in SQL UPDATE statements generated by the persistence provider.

回答5:

You can mark an entity with @Entity(mutable=false) or @Immutable annotations for the framework to make use of this fact for performance gain in caching and such. (Hibernate)

Then you can use an immutable wrapper class like this:

public class ImmutableStuff {
    private final FooField barValue;

    public ImmutableStuff(Stuff stuff) {
        barValue = stuff.barValue;
    }

    public FooField getBarValue(){
        return barValue;
    }
}

来源：https://stackoverflow.com/questions/7941161/is-it-possible-to-have-immutable-fields-in-hibernate-jpa

标签

java

hibernate

jpa-2.0

immutability