Question

我有一个包含几个懒惰初始化集合的类，OneToMany和ManyToMany。 ManyToMany关系是单向的，因此Expert类没有Project集合。

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "project")
private List<Logging> loggings = new ArrayList<Logging>();

@ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
@JoinTable(name = "project_expert", joinColumns = { @JoinColumn(name = "project") }, inverseJoinColumns = { @JoinColumn(name = "expert") })
private List<Expert> experts = new ArrayList<Expert>();

public void add(Logging logging) {
    logging.setProject(this);
    this.loggings.add(logging);
}

public void add(Expert expert) {
    this.experts.add(expert);
}

要将对象添加到OneToMany集合，我会这样做：

Project project = projectService.save(projectForm.getProject());
Logging logging = new Logging();
project.add(logging);
logging = loggingService.save(logging);

这很好用。但是，当我尝试向ManyToMany集合添加元素时，集合似乎在保存父对象时被删除，因此我永远不能添加多个子对象：

@RequestMapping(value="/newexpert", method=RequestMethod.POST)
public String newExpert(@ModelAttribute("projectForm") ProjectForm projectForm, BindingResult result, ModelMap model) {

  Project project = projectService.save(projectForm.getProject());
  Expert expert = projectForm.getExpert();
  if (expert != null) {
      project.add(expert);
  }
  project = projectService.save(project);

  return "redirect:/project/" + project.getId();
}

这就是服务方法的样子（projectRepository扩展PagingAndSortingRepository）：

@Override
@Transactional
public Project save(Project project) {
    return projectRepository.save(project);
}

在日志中我发现了这个：

2014-05-01 17:01:49 DEBUG AbstractCollectionPersister:1174 - Deleting collection: [test.model.Project.experts#1]
2014-05-01 17:01:49 DEBUG SQL:109 - delete from project_expert where project=?
2014-05-01 17:01:49 DEBUG AbstractCollectionPersister:1232 - Done deleting collection

编辑1：这是save(projectForm.getProject())上发生的事情：

2014-05-01 18:30:04 DEBUG Loader:2136 - Loading entity: [test.model.Project#1]
2014-05-01 18:30:04 DEBUG SQL:109 - select project0_.id as id1_4_1_, ... from Project project0_ left outer join project_expert experts1_ on project0_.id=experts1_.project left outer join Expert expert2_ on experts1_.expert=expert2_.id where project0_.id=?
2014-05-01 18:30:04 DEBUG Loader:951 - Result set row: 0
2014-05-01 18:30:04 DEBUG Loader:1485 - Result row: EntityKey[test.model.Expert#2], EntityKey[test.model.Project#1]
2014-05-01 18:30:04 DEBUG Loader:1305 - Found row of collection: [test.model.Project.experts#1]
2014-05-01 18:30:04 DEBUG TwoPhaseLoad:160 - Resolving associations for [test.model.Project#1]
2014-05-01 18:30:04 DEBUG TwoPhaseLoad:286 - Done materializing entity [test.model.Project#1]
2014-05-01 18:30:04 DEBUG CollectionLoadContext:232 - 1 collections were found in result set for role: test.model.Project.experts
2014-05-01 18:30:04 DEBUG CollectionLoadContext:280 - Collection fully initialized: [test.model.Project.experts#1]
2014-05-01 18:30:04 DEBUG CollectionLoadContext:240 - 1 collections initialized for role: test.model.Project.experts
2014-05-01 18:30:04 DEBUG Loader:2160 - Done entity load
2014-05-01 18:30:04 DEBUG AbstractTransactionImpl:175 - committing
2014-05-01 18:30:04 DEBUG AbstractFlushingEventListener:149 - Processing flush-time cascades
2014-05-01 18:30:04 DEBUG AbstractFlushingEventListener:189 - Dirty checking collections
2014-05-01 18:30:04 DEBUG CollectionEntry:202 - Collection dirty: [test.model.Project.loggings#1]
2014-05-01 18:30:04 DEBUG CollectionEntry:202 - Collection dirty: [test.model.Project.parties#1]
2014-05-01 18:30:04 DEBUG CollectionEntry:202 - Collection dirty: [test.model.Project.experts#1]
2014-05-01 18:30:04 DEBUG Collections:194 - Collection found: [test.model.Project.experts#1], was: [test.model.Project.experts#1] (initialized)
2014-05-01 18:30:04 DEBUG Collections:201 - Collection found: [test.model.Project.loggings#1], was: [test.model.Project.loggings#1] (uninitialized)
2014-05-01 18:30:04 DEBUG Collections:201 - Collection found: [test.model.Project.parties#1], was: [test.model.Project.parties#1] (uninitialized)
2014-05-01 18:30:04 DEBUG AbstractFlushingEventListener:123 - Flushed: 0 insertions, 0 updates, 0 deletions to 3 objects
2014-05-01 18:30:04 DEBUG AbstractFlushingEventListener:130 - Flushed: 0 (re)creations, 3 updates, 0 removals to 3 collections
2014-05-01 18:30:04 DEBUG EntityPrinter:114 - Listing entities:
2014-05-01 18:30:04 DEBUG EntityPrinter:121 - test.model.Expert{id=2, ...}
2014-05-01 18:30:04 DEBUG EntityPrinter:121 - test.model.Project{..., id=1, ...}
2014-05-01 18:30:04 DEBUG EntityPrinter:121 - test.model.Expert{id=1, ...}
2014-05-01 18:30:04 DEBUG SQL:109 - select count(id) from Logging where project_id =?
2014-05-01 18:30:04 DEBUG SQL:109 - select count(id) from Party where project_id =?
2014-05-01 18:30:04 DEBUG AbstractCollectionPersister:1174 - Deleting collection: [test.model.Project.experts#1]
2014-05-01 18:30:04 DEBUG SQL:109 - delete from project_expert where project=?
2014-05-01 18:30:04 DEBUG AbstractCollectionPersister:1232 - Done deleting collection
2014-05-01 18:30:04 DEBUG JdbcTransaction:113 - committed JDBC Connection
2014-05-01 18:30:04 DEBUG JdbcTransaction:126 - re-enabling autocommit
...
2014-05-01 18:30:04 DEBUG AbstractLoadPlanBasedCollectionInitializer:88 - Loading collection: [test.model.Project.loggings#1]
2014-05-01 18:30:04 DEBUG SQL:109 - select loggings0_.project_id as project_5_4_0_, loggings0_.id as id1_2_0_, ... from Logging loggings0_ where loggings0_.project_id=?
2014-05-01 18:30:04 DEBUG ResultSetProcessorImpl:168 - Preparing collection intializer : [test.model.Project.loggings#1]
2014-05-01 18:30:04 DEBUG ResultSetProcessorImpl:127 - Starting ResultSet row #0
2014-05-01 18:30:04 DEBUG CollectionReferenceInitializerImpl:77 - Found row of collection: [test.model.Project.loggings#1]
2014-05-01 18:30:04 DEBUG TwoPhaseLoad:160 - Resolving associations for [test.model.Logging#1]
2014-05-01 18:30:04 DEBUG TwoPhaseLoad:286 - Done materializing entity [test.model.Logging#1]
2014-05-01 18:30:04 DEBUG CollectionLoadContext:232 - 1 collections were found in result set for role: test.model.Project.loggings
2014-05-01 18:30:04 DEBUG CollectionLoadContext:280 - Collection fully initialized: [test.model.Project.loggings#1]
2014-05-01 18:30:04 DEBUG CollectionLoadContext:240 - 1 collections initialized for role: test.model.Project.loggings
2014-05-01 18:30:04 DEBUG AbstractLoadPlanBasedCollectionInitializer:118 - Done loading collection

编辑2：显然，父对象ManyToMany中的projectForm.getProject()集合已初始化为空。怎么可能？我希望它是单元化的，或者由于某种原因而被初始化，不是空的？

Answer 1

在您的服务中创建一个方法，如下所示：

@Transactional
public Project addExperts(Project project, List<Expert> experts){
    List<Expert> projectExperts = project.getExperts();
    for(Expert expert: experts){
        projectExperts.add(expert);
    }

    project.setExperts(projectExperts);
    return project;
}

这将封装在Transaction中获取Project.experts，以便可以正确地获取和初始化延迟加载的集合。通过添加您在控制器中确定的列表，您可以正确保存项目并让级联创建新的专家对象。我很确定这可以解决你的问题。

如果您没有看到创建INSERT个对象的Expert个语句，那就是您的问题。如果数据库中不存在Expert对象，则Hibernate无法将JoinTable对象的引用插入{{1}}，因此它会删除您的集合对象。

Answer 2

我想我可能有罪魁祸首。

您正在将ManyToMany集合初始化为

private List<Expert> experts = new ArrayList<Expert>();

然后您的add()方法以this.experts.add(..)的形式访问该集合。然而，ManyToMany协会是LAZY。那么JPA提供商怎么会知道你不打算用新的覆盖集合覆盖集合呢？

尝试使用集合的getter更改add()，如

public void add(Expert expert) {
    this.getExperts().add(expert);
}

ManyToMany集合不断被删除

2 个答案: