lazy loading return null value

问题

i use spring 3.2, spring data and jpa.

i save an Advertisement object, after i save message

i try to access message from Advertisement but it's null

@Entity
public class Advertisement implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

@OneToMany(mappedBy="id", cascade={CascadeType.REMOVE}, fetch=FetchType.LAZY)
private Set<Message> messages;
}

@Entity
public class Message implements Serializable {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

@ManyToOne
private Advertisement advertisement;
}

test unit

Advertisement ads = new Advertisement();
ads = advertisementRepo.save(ads);

assertNotNull(ads);
assertNotNull(ads.getId());

Message message = new Message();
message.setAdvertisement(ads);
message = msgRepo.save(message);

ads = advertisementRepo.findOne(ads.getId());
ads.getMessages(); //return null

why ads.getMessages() don't return messages?

回答1:

The problem is that bidirectional relationships are not transparently managed by JPA. When manipulating one side of a bidirectional relationship, the application has to ensure that the other side is updated accordingly.

This can easily be done by writing setter methods that update the associated entity as well. For example, when setting the Advertisment of a Message, you can add the Message instance to the collection in Advertisment:

@Entity
public class Message implements Serializable {

    ...

    public void setAdvertisement(Advertisement advertisement) {
        this.advertisement = advertisement;
        advertisement.getMessages().add(this);
    }
}

回答2:

Try to save the Advertisement after you assign it a new Message collection:

Advertisement ads = new Advertisement();
ads = advertisementRepo.save(ads);

assertNotNull(ads);
assertNotNull(ads.getId());

Message message = new Message();
message.setAdvertisement(ads);
ads.setMessages(new HashSet<Message>());
ads.getMessages().add(message);
ads = advertisementRepo.save(ads);
message = msgRepo.save(message);

ads = advertisementRepo.findOne(ads.getId());
ads.getMessages(); //return null

回答3:

This is returning null because you are saving the non-owning entity first and then the owning entity. If you save message before ads it should return non-null value.

来源：https://stackoverflow.com/questions/17839976/lazy-loading-return-null-value

标签

Spring

jpa

lazy-loading