Question

我是一个用于弹出数据JPA的新蜜蜂，所以我试图用它来制作一些东西，这样我就可以知道如何获取模式，但它会抛出异常，请查看代码

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;
}

所以，你可以看到有两个类州和选区，从州到选区一个存在多对映射，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，它正在保存一个有两个选区的州，然后我试图再次使用它来检索它们：

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

根据我的理解，我假设当这个陈述将执行时，州中的选区不会获得初始化，直到下一个语句执行即将调用< strong> fetchLazyCollection 但是当 fetchLazyCollection 调用它时抛出异常

无法初始化代理 - 无会话

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)

正如我所假设的，java中的每个单元测试都维护着自己与数据库交互的会话，可能是我错了，有谁能告诉我我在这里做了什么错？任何帮助将不胜感激。

Answer 1

您得到LazyInitializationException因为stateRepository.findByName("New Delhi")是打开/关闭事务/会话的人，因此您无法在正在运行的Hibernate Session之外获取其他关联。

在测试中添加@Transactional可解决此问题，您无需致电：

states.get(0).fetchLazyCollection();

如果你总是希望用状态获取Constituencies，你应该有类似的东西：

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

Answer 2

延迟加载在持久化上下文中工作。当您的交易完成后，您无法执行延迟提取。您可以扩展事务范围，以便在事务中获取集合，或创建将为您返回集合的查询。另一种方法是在一个stareRepository方法中手动获取集合，例如List<State> states = stateRepository.findByNameAndFetchCollection("New Delhi");，其中事务应该仍处于活动状态。

Answer 3

这是因为您在事务（会话）之外获取延迟集合。所以你有两种方法：

在加载实体的同一会话/事务中获取您的集合
设置hibernate属性（对于SessionFactory），它允许非事务性加载lazies：

＆lt; prop key =＆＃34; hibernate.enable_lazy_load_no_trans＆＃34;＆gt; true＆lt; / prop＆gt;

Answer 4

将延迟标记设置为true，如下所示

@OneToMany(mappedBy = "objectId", cascade = CascadeType.ALL, orphanRemoval = true , fetch = FetchType.EAGER)

JPA懒惰提取不起作用并抛出LazyInitializationException

4 个答案: