How do I avoid NullPointerException with OneToOne mapping against a parent entity class?

问题

I have searched and found similar issues, but they don't quite seem to be the same problem as

Why am I getting this NullPointer exception?
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ANN-613 - NPE when mappedBy property is wrong on a @OneToOne
ANN-558 - @OneToMany(mappedBy="") can not recognize properties in parent classes
Hibernate Users - NPE with @Id on @OneToOne

I have a few entities mapped like this:

Person
  |
  +--User

I want to add a new entity PersonPartDeux with a OneToOne mapping to Person. The resulting mapping should look something like this:

Person + PersonPartDeux
  |
  +--User

When I do so, a NullPointerException is thrown while trying to load the mapping:

java.lang.NullPointerException
    at org.hibernate.cfg.OneToOneSecondPass.doSecondPass(OneToOneSecondPass.java:135)

How do I specify the mapping so I can avoid this exception?

Here's my code:

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public abstract class Person implements Serializable
{
    @Id
    @GeneratedValue
    public Long id;

    @Version
    public int version = 0;

    public String name;

    @OneToOne(cascade = CascadeType.ALL)
    @PrimaryKeyJoinColumn
    public PersonPartDeux personPartDeux;
}

@Entity
public class PersonPartDeux implements Serializable
{
    @Id
    @GeneratedValue(generator = "person-primarykey")
    @GenericGenerator(
        name = "person-primarykey",
        strategy = "foreign",
        parameters = @Parameter(name = "property", value = "person")
    )
    public Long id = null;

    @Version
    public int version = 0;

    @OneToOne(optional=false, mappedBy="person")
    public Person person;

    public String someText;
}

@Entity
@PrimaryKeyJoinColumn(name = "person_Id")
public class User extends Person
{
    public String username;
    public String password;
}

As for why I'm bothering, I need both the inheritance and the OneToOne mapping to solve different known issues in my application.

回答1:

Attach the Hibernate source to your project, so you can click thru or 'Open Type' (Ctrl-Shift-T in Eclipse) to view the OneToOneSecondPass source.

Seeing the source, will give you a clear indication as to what needs to be specified.

In my source (Hibernate 4.1.7), line 135 is

propertyHolder.addProperty( prop, inferredData.getDeclaringClass() );

However you're probably using an earlier version.

Looking at the mappings, I'm suspicious of the @OneToOne definition -- mappedBy="person".

@OneToOne(optional=false, mappedBy="person")
public Person person;

What does it usefully mean, to map an association property by itself? Hibernate already knows the property is a OneToOne -- you just told it that.

Pointing the underpinning mapping/ FK of the property, at itself.. probably isn't actually telling Hibernate any correct or useful information.

Here's an example from the HB dosc, perhaps showing better how to do what you want:

@Entity
class MedicalHistory implements Serializable {
  @Id Integer id;

  @MapsId @OneToOne
  @JoinColumn(name = "patient_id")
  Person patient;
}

@Entity
class Person {
  @Id @GeneratedValue Integer id;
}

Source: http://docs.jboss.org/hibernate/annotations/3.5/reference/en/html_single/ (3.5 docs off JBoss site.)

Cheers, hope this helps.

来源：https://stackoverflow.com/questions/12683026/how-do-i-avoid-nullpointerexception-with-onetoone-mapping-against-a-parent-entit

标签

java

hibernate

nullpointerexception

one-to-one