java - JPA 延迟获取不起作用并抛出 LazyInitializationException

Question

我是 spring 数据 JPA 的新蜜蜂，所以我试图用它做点什么，这样我就可以知道获取模式，但它抛出了一个异常，请看一下代码。

public class State {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Integer id;

  String name;

  @OneToMany(mappedBy = "state", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
  Set<Constituency> constituencies ;

  public void fetchLazyCollection() {
    getConstituencies().size();
  }
}

public class Constituency {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Integer id;

  String name;

  @ManyToOne
  @JoinColumn(name = "state_id")
  State state;
}

因此，如您所见，有两个类 国家和 选区来自 国家至 选区存在一对多映射，反之亦然存在多对一映射。
如果我在这里犯了任何愚蠢的错误，请告诉我。

@Test
public void delhiShouldHaveTwoConstituency(){
  State delhi = new State();
  delhi.setName("New Delhi");

  Constituency northWest = new Constituency();
  northWest.setName("North West");
  northWest.setState(delhi);

  Constituency southDelhi = new Constituency();
  southDelhi.setName("South Delhi");
  southDelhi.setState(delhi);

  Set<Constituency> constituencies = new HashSet<Constituency>();
  constituencies.add(northWest);
  constituencies.add(southDelhi);

  delhi.setConstituencies(constituencies);

  stateRepository.save(delhi);

  List<State> states= stateRepository.findByName("New Delhi");
  states.get(0).fetchLazyCollection();
  assertThat(delhi.getConstituencies().size(), is(2));
}

现在，我有一个测试，它正在保存一个具有两个选区的州，然后我尝试使用以下方法再次检索它们:

List<State> states= stateRepository.findByName("New Delhi");

根据我的理解，我假设这条语句会执行，选区在 国家在下一条语句执行之前不会被初始化，该语句将调用 fetchLazyCollection 但作为 fetchLazyCollection 调用它抛出异常

无法初始化代理 - 无 session

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.prateekj.model.State.constituencies, could not initialize proxy - no Session
at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:567)
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:187)
at org.hibernate.collection.internal.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:138)
at org.hibernate.collection.internal.PersistentSet.size(PersistentSet.java:156)
at com.prateekj.model.State.fetchLazyCollection(State.java:35)
at com.prateekj.repositories.StateRepositoryTest.shouldDoIt(StateRepositoryTest.java:53)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:47)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:44)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:88)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:238)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:63)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:236)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:53)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:229)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
at org.junit.runners.ParentRunner.run(ParentRunner.java:309)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
at org.junit.runner.JUnitCore.run(JUnitCore.java:160)
at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:65)

正如我假设的那样，Java 中的每个单元测试都维护自己的与数据库交互的 session ，可能是我错了，有人能告诉我我在这里做错了什么吗？
任何帮助将不胜感激。

Answer 1

您会收到 LazyInitializationException因为在你走出stateRepository.findByName("New Delhi")之后, hibernate Session关闭，并且您无法在正在运行的 Hibernate 之外获取其他关联 Session .
添加 @Transactional到您的测试将解决此问题，您不必调用:

states.get(0).fetchLazyCollection();

最好在原始实体查询中获取您需要的惰性关联:

select s 
from State s 
left join fetch s.constituencies 
where s.name = :stateName