从另一个实体引用一个实体

Question

I have two Entities in my Java EAR: User and Password. These, of course, have corresponding tables in the database: Users and Passwords. The passwords table contains the user's id as a foreign key. So my question is how do I pull the password entry from the Passwords table when the User is created from the Users table using purely annotations? I'm at a loss.

I don't want to have to use business logic to access the user's related password entity -- so I need the container to do this for me.

SCCE:

@Entity
@Table(name = "USERS")
public class User{
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "IDENTIFIER", nullable = false)
        private Long identifier;

    @JoinColumn(name = "UserIdentifier")
    @OneToOne
    private Password password;

    // getters, setters, and other User related information such as username
}

@Entity
@Table(name = "PASSWORDS")
public class Password{
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "IDENTIFIER", nullable = false)
        private Long identifier;

    @JoinColumn(name = "USERIDENTIFIER", referencedColumnName = "IDENTIFIER", nullable = false)
    @OneToOne(optional = false, fetch = FetchType.EAGER)
        private User useridentifier;
    //getters, setters, and other password related fields such as the password it's self
}

Answer 1

The entity classes just establish the data and relationships. You retrieve the data from a particular instance in your business logic. You've established the relationships with the @OneToOne annotations on the User and Password properties. What you're missing is a persistent field for the Password entity.

@Entity
@Table(name = "PASSWORDS")
public class Password{
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "IDENTIFIER", nullable = false)
    private Long identifier;

    @JoinColumn(name = "USERIDENTIFIER", referencedColumnName = "IDENTIFIER", nullable = false)
    @OneToOne(mappedBy = "password", optional = false, fetch = FetchType.EAGER)
    private User useridentifier;

    @NotNull
    private String passwordValue;

    // getters/setters go here, since your annotated fields are private 
}

You can then traverse the relationships by retrieving a User instance and navigating to the relationship field. So your business logic might look something like this:

@PersistenceContext
EntityManager em;

Long userIdInput = ...;
String passwordInput = ...;

User user = em.find(User.class, userIdInput);
// compare the input password with the stored password
// retrieve the Password instance by navigating to the relationship field
if (passwordInput.equals(user.getPassword().getPasswordValue()) {
    // passwords match
} else {
    // passwords don't match
}

You've got your usernames as Long values, which is...odd. And you should probably look into how to handle securely handle storing password values in your database, as it's not a good practice to store them unencrypted. But this is gist of JPA.

Edit: The owner of the relationship needs to be set using mappedBy, as pointed out by @neil-stockton.

Answer 2

Turns out the answer was simple enough. I had the right annotations but I needed a few extra things. Below is the relevant code for both entities.

@Entity
@Table(name = "USERS")
public class User{
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "IDENTIFIER", nullable = false)
        private Long identifier;

    @JoinColumn(referencedColumnName = "PASSWORDS.USERIDENTIFIER", name="USERS.IDENTIFIER")
    @OneToOne
    private Password password;

    // getters, setters, and other User related information such as username
}

@Entity
@Table(name = "PASSWORDS")
public class Password{
    @Id
    @Basic(optional = false)
    @NotNull
    @Column(name = "IDENTIFIER", nullable = false)
        private Long identifier;

    @JoinColumn(name = "USERIDENTIFIER", referencedColumnName = "IDENTIFIER", nullable = false)
    @OneToOne(optional = false, fetch = FetchType.EAGER)
        private User useridentifier;
    //getters, setters, and other password related fields such as the password it's self
}

So for future stackers who have one entity that references another entity, you should specify the name and referencedColumnName of the JoinColumn annotation to get the container to pull in the related entity. I was able to pull in the correct password row from the database by doing this. Note that the joincolumn annotation in the user entity must have the table name as well and that the same annotation in the password entity must NOT have the table names. It won't work otherwise -- believe me, I tried it and got a StackOverflow error.