问题
I just want to know what is the difference between all these annotations. Why are we using these... means they have no effect especially field level and property level.
And what is the purpose of using mixed level annotation like:
@Entity
@Access(AccessType.FIELD)
class Employee {
// why their is a field level access
private int id;
// whats the purpose of transient here
@Transient
private String phnnumber;
// why its a property level access
@Access(AccessType.property)
public String getPhnnumber() {
return "1234556";
}
}
what exactly this class says?
回答1:
By default the access type is defined by the place where you put your identifier annotation (@Id). If you put it on the field - it will be AccessType.FIELD, if you put it on the getter - it will be AccessType.PROPERTY.
Sometimes you might want to annotate not fields but properties (e.g. because you want to have some arbitrary logic in the getter or because you prefer it that way.) In such situation you must define a getter and annotate it as AccessType.PROPERTY.
As far as I remember, if you specify either AccessType.FIELD or AccessType.PROPERTY on any of the entity fields / methods you must specify the default behaviour for the whole class. And that's why you need to have AccessType.FIELD on the class level (despite that AccessType.FIELD is the default value.)
Now, if you wouldn't have @Transient on the phnnumber field, the JPA would provide you with a 3 columns table:
- id,
- phnnumber,
- getphnnumber.
That's because it would use AccessType.FIELD for all of the entity fields (id and phnnumber) and, at the same time, it'd use AccessType.PROPERTY for your getter (getPhnnumber()).
You'll end with phone number mapped twice in the database.
Therefore, the @Transient annotation is required - it means that the entity won't store the value of the field in the underlying storage but the value returned by your getter.
来源：https://stackoverflow.com/questions/13874528/what-is-the-purpose-of-accesstype-field-accesstype-property-and-access