@ManyToMany無法使用Spring / JPA / REST從子級到父級工作

Question

我對Java / Spring比較陌生。 我已經使用Spring框架創建了一個非常簡單的測試應用程序。 我一直在嘗試@ JsonIgnore / @ JsonManagedReference / @ JsonBackReference。 我嘗試了許多在線建議，但無法獲取@ManyToMany注釋以雙向工作。

我定義了兩個實體“播放列表”和“歌曲”。 它們之間存在ManyToMany關系。

我將播放列表實體定義為：

@Entity
@Table(name="playlist")
public class Playlist implements Serializable {

    /**
     * 
     */
    private static final long serialVersionUID = 1L;


    private Integer playlistId;

    @Id
    @GeneratedValue
    @Column(name="playlist_id")
    public Integer getPlaylistId() {
        return playlistId;
    }

    public void setPlaylistId(Integer playlistId) {
        this.playlistId = playlistId;
    }

    private String name;

    private Set<Song> songs = new HashSet<Song>(0);

    @ManyToMany(cascade={CascadeType.ALL})
    @JoinTable(name = "playlist_song",
            joinColumns = {@JoinColumn(name = "f_playlist_id",  referencedColumnName = "playlist_id")},
            inverseJoinColumns = {@JoinColumn(name = "f_song_id", referencedColumnName = "song_id")}
            )
    @JsonManagedReference
    public Set<Song> getSongs() {
        return songs;
    }

    public void setSongs(Set<Song> songs) {
        this.songs = songs;
    }


    @Column(name="name")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


}

第二實體宋為：

@Entity
@Table(name="song")
public class Song implements Serializable{

    public Song() {
    }

    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    private Integer songIdentifier;
    private String name;
    private Set<Playlist> playlists = new HashSet<Playlist>(0);


    @Id
    @Column(name="song_id")
    @GeneratedValue
    public Integer getSongIdentifier() {
        return songIdentifier;
    }

    public void setSongIdentifier(Integer songIdentifier) {
        this.songIdentifier = songIdentifier;
    }


    @Column(name="name")
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @ManyToMany(mappedBy="songs")
    @JsonBackReference
    public Set<Playlist> getPlaylists() {
        return playlists;
    }

    public void setPlaylists(Set<Playlist> playlists) {
        this.playlists = playlists;
    }

}

將@Service定義為：

@Service
public class SongServiceImpl implements SongServiceDao {

    @Autowired
    private SongRepo songRepo;

    @Override
    public Collection<Song> findAll() {

        Collection <Song> allsongs = (Collection<Song>) songRepo.findAll();
        return allsongs;
    }

}

我定義@Controller為

@RestController
public class M2MController {

    @Autowired
    SongServiceDao songService;

     @RequestMapping(
                value = "/test/songs",
                method = RequestMethod.GET,
                produces = MediaType.APPLICATION_JSON_VALUE)
        public ResponseEntity<Collection<Song>> getSongs() {

            Collection<Song> songs = songService.findAll();

            return new ResponseEntity<Collection<Song>>(songs, HttpStatus.OK);

        }

     @Autowired
     PlaylistServiceDao playlistService;

         @RequestMapping(
                    value = "/test/playlists",
                    method = RequestMethod.GET,
                    produces = MediaType.APPLICATION_JSON_VALUE)
            public ResponseEntity<Collection<Playlist>> getPlaylists() {

                Collection<Playlist> playlists = playlistService.findAll();

                return new ResponseEntity<Collection<Playlist>>(playlists, HttpStatus.OK);

            }

}

我在MySql中定義架構為

-- -----------------------------------------------------
-- Table `m2mDb`.`playlist`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `m2mDb`.`playlist` (
  `playlist_id` INT NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(45) NULL,
  PRIMARY KEY (`playlist_id`))
ENGINE = InnoDB;


-- -----------------------------------------------------
-- Table `m2mDb`.`song`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `m2mDb`.`song` (
  `song_id` INT NOT NULL AUTO_INCREMENT,
  `name` VARCHAR(45) NULL,
  PRIMARY KEY (`song_id`),
  UNIQUE INDEX `songId_UNIQUE` (`song_id` ASC))
ENGINE = InnoDB;


-- -----------------------------------------------------
-- Table `m2mDb`.`playlist_song`
-- -----------------------------------------------------
CREATE TABLE IF NOT EXISTS `m2mDb`.`playlist_song` (
  `f_playlist_id` INT NOT NULL,
  `f_song_id` INT NOT NULL,
  INDEX `toSong_idx` (`f_song_id` ASC),
  INDEX `toPlaylist_idx` (`f_playlist_id` ASC),
  PRIMARY KEY (`f_playlist_id`, `f_song_id`),
  INDEX `secondary` (`f_song_id` ASC, `f_playlist_id` ASC),
  CONSTRAINT `toPlaylist`
    FOREIGN KEY (`f_playlist_id`)
    REFERENCES `m2mDb`.`playlist` (`playlist_id`)
    ON DELETE CASCADE
    ON UPDATE CASCADE,
  CONSTRAINT `toSong`
    FOREIGN KEY (`f_song_id`)
    REFERENCES `m2mDb`.`song` (`song_id`)
    ON DELETE CASCADE
    ON UPDATE CASCADE)
ENGINE = InnoDB;

當我運行它時，我得到以下結果：

/ test /歌曲返回：

[
  {
    "songIdentifier": 1,
    "name": "song one"
  },
  {
    "songIdentifier": 2,
    "name": "song 2"
  },
  {
    "songIdentifier": 3,
    "name": "song 3"
  }
]

/ test / playlists返回：

[
  {
    "playlistId": 1,
    "name": "list 1",
    "songs": [
      {
        "songIdentifier": 2,
        "name": "song 2"
      },
      {
        "songIdentifier": 3,
        "name": "song 3"
      },
      {
        "songIdentifier": 1,
        "name": "song one"
      }
    ]
  },
  {
    "playlistId": 2,
    "name": "list 2",
    "songs": [
      {
        "songIdentifier": 3,
        "name": "song 3"
      },
      {
        "songIdentifier": 1,
        "name": "song one"
      }
    ]
  }

播放列表返回正確的結果。 我希望歌曲也能包含所有播放列表。

如果有人能闡明我將其弄亂的地方，我將不勝感激。

Answer 1

當您使用@JsonManagedReference和@JsonBackReference ，會發生什么情況，即在對Song進行序列化期間，Jackson不會序列化PlayList的列表，因為該PlayList已經在XML的另一個級別進行了序列化。

您可以使用的是@JsonIdentityInfo

至少可以使您擁有JSON來輸出PlayList數組，但要使用與其包含的播放列表關聯的標識符。

[
  {
    "playlistId": 1,
    "name": "list 1",
    "songs": [
      {
        "songIdentifier": 2,
        "name": "song 2"
        "playLists": [1, 2]
      }
    ]
  },