不完全理解延遲加載

Question

我有一種情況，當我希望它在一對多時沒有發生延遲加載。很顯然，我對此很陌生。當我調用s.getSongComposers（）時，爲什麼SongComposers不會被抓取？

這裏是我的DAO類：

public class SongDAOImpl implements SongDAO { 

    @PersistenceContext 
    private EntityManager em; 

    @Override 
    @SuppressWarnings(value = { "unchecked" }) 
    public List<Song> getAllSongsOnAlbum(int albumID) { 
     Query q = em.createQuery("SELECT s FROM Song s WHERE s.album.albumID = :albumid ORDER BY s.songName"); 
     q.setParameter("albumid", albumID); 
     List<Song> list = (List<Song>) q.getResultList(); 
     for (Song s: list) { 
      s.getSongComposers(); 
     } 
     return list; 
    } 

} 

這裏是我的歌實體：

@Entity 
@Table(name="songs") 
public class Song { 

    @Id 
    @GeneratedValue 
    @Column(name="SongID") 
    private int songID; 

    @Column(name="SongName") 
    private String songName; 

    @Column(name="Length") 
    private int length; 

    @ManyToOne 
    @JoinColumn(name="AlbumID", referencedColumnName="AlbumID") 
    private Album album; 

    @OneToMany(mappedBy="song") 
    private List<SongComposer> songComposers; 


     .... all getters/setters ...... 
     } 

這裏是我的SongComposer實體：

@Entity 
@Table(name="song_composer") 
public class SongComposer { 


    @Id 
    @GeneratedValue 
    @Column(name="SongComposerID") 
    private int songComposerID; 


    @ManyToOne 
    @JoinColumn(name="SongID", referencedColumnName="SongID") 
    private Song song; 


    @ManyToOne 
    @JoinColumn(name="ComposerID", referencedColumnName="ComposerID") 
    private Composer composer; 


     ..... getters/setters ....... 

} 

Answer 1

你必須明確地做到這一點。試試這個查詢：

SELECT DISTINCT s FROM Song s JOIN FETCH s.songComposers WHERE s.album.albumID = :albumid ORDER BY s.songName"