在保持数据完整性的同时对循环依赖进行建模

Posted 2023-02-25

【中文标题】在保持数据完整性的同时对循环依赖进行建模

【英文标题】：Modeling circular dependencies while maintaining data integrity

【发布时间】：2021-10-11 14:22:57

【问题描述】：

我正在设计一个音乐信息系统。我有几个相互连接的实体。 以下是部分域代码。

class Album 
    
    private Set<Track> tracks;
    private boolean published;
    
    public Set<Track> getTracks() 
        return this.tracks;
    
    public boolean isPublished() 
        return this.published;
    

    public void publish() 
        System.out.println("Album.publish() called");
        this.published = true;
        this.tracks.forEach(track -> track.publish());
    



class Track 

    private boolean published;
    private Album album;

    public boolean isPublished() 
        return this.published;
    
    public Album getAlbum() 
        return this.album;
    

    public void publish() 
        // if track is single (this.album == null), set published to true
        // if track is part of an album and the album is NOT published, return;
        // if track is part of an album and the album is published, set published to true
        if(this.album != null && !this.album.isPublished())
            return;
        this.published = true;
    

Track 是一个独立的实体。它可以是单首曲目（即没有专辑）。所以实际上需要album 属性。 一个域规则是，当专辑存档（即未发布）时，其曲目也不能发布，如果专辑已发布，则其任何曲目都可以发布或存档。 问题是当专辑发布时（例如album1.publish()），其曲目的publish()方法也会被调用。但是track1.publish() 根据已有的副本（未发布）检查专辑是否已发布。我该如何解决这个问题？

【问题讨论】：

如果Album 是一个实体，则不应有“副本”——实体具有唯一标识，所有引用都应指向同一个对象。

正确。我应该如何在 Java 中实现它？据我所知，Java 总是按值传递。 @埃里克森

***.com/questions/40480/… 很好地了解 pass-by-value 与 pass-by-ref

是的，将对父 Album 的引用传递给 Track（按值，因为所有引用都在 Java 中传递）。强制数据完整性的一种方法是删除直接为Track 设置Album 的任何API，而是仅在Album 上提供addTrack(Track) 方法，该方法更新轨道集合并设置Album 通过包私有访问。

【参考方案1】：

如果您按行为拆分域模型实体，则可以摆脱所描述的限制

让我们为这些实体创建一些接口：

interface AlbumId
   String asString();
   AlbumId Absent = () -> "NO ALBUM AT ALL";


interface Publication
   void publish() throws Exception;
   void archive() throws Exception;
   boolean published();


interface Track
    TrackId id(); 
    AlbumId albumId(); //smart type (as DDD suggest), therefore, no more nulls

现在您可以通过创建类来执行规则，该类将为您提供可以发布的曲目列表：

public class TracksReadyToPublishOf implements Supplier<Map<TrackId, TrackPublication>> 
    //this class may access to cache and have dosens of other optimizations
    public TracksReadyToPublishOf(AlbumId id)...
    @Override public get()... 

然后您可以在任何地方重复使用您的代码来检查您的规则：

public class TrackPublication implements Publication 
    private final Track track;
    private final Supplier<Map<TrackId, TrackPublication>> allowedTracks;

    //easy for unit testing
    public SmartTrackPublication(Track track, Supplier<Map<TrackId, TrackPublication>> allowedTracks)
        this.track = track;
        this.allowedTracks = allowedTracks;
    

    public SmartTrackPublication(Track track)
        this(track, new TracksReadyToPublishOf(track.albumId());
    

    @Override
    public publish() throws AlbumArchivedException
        if(this.albumId != AlbumId.Absent)
            if(!this.allowedTracks.get().containsKey(this.track.id()))
                throw new AlbumArchivedException();
            
        
        this.allowedTracks.get().get(this.id()).publish();
    

对于专辑发布：

public class AlbumPublication implements Publication

   private final AlbumId id;
   private final Producer<Map<TrackId, TrackPublication>> tracks
   
   private AlbumWithTracks(AlbumId id, Producer<Map<TrackId, TrackPublication>> tracks)
       this.id = id;
       this.tracks = tracks;
   

   public AlbumWithTracks(AlbumId id)
       this(id, new TracksReadyToPublishOf(id))
   
   ...
   @Override publish() throws Exception
      //code for publishing album
         
      for(TrackPublication t : Arrays.asList(
          this.tracks.get() 
      ))
          t.publish(); //track can publish anyway if it presents in list above