java为什么在使用FetchType时会出现LazyInitializationException异常。在一组上懒惰，但在另一组上不懒惰？

Question

我在这里遇到了一个非常奇怪的问题；看起来有些集合确实延迟加载，但其他集合则不延迟加载，但底层类布局完全相同：

在Spring boot Hibernate映射中，我有4个类：

这是最高级别的课程，事件：

@Entity
public class Event {

    // Getters, setters, etc. are omitted.

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "event", cascade = CascadeType.ALL)
    @ElementCollection(targetClass = Question.class)
    private Set<Question> questions = new HashSet<>();

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "event", cascade = CascadeType.ALL)
    @ElementCollection(targetClass = Participant.class)
    private Set<Participant> participants = new HashSet<>();

    @OneToMany(fetch = FetchType.LAZY, mappedBy = "event", cascade = CascadeType.ALL)
    @ElementCollection(targetClass = Run.class)
    private Set<Run> runs = new HashSet<>();

}

以下是链接到此事件根节点类的3个类：

问题：

@Entity
public class Question {

    // Getters, setters, etc. are omitted.

    @ManyToOne(fetch = FetchType.LAZY)
    private Event event;
}

参与者：

@Entity
public class Participant {

    // Getters, setters, etc. are omitted.

    @ManyToOne(fetch = FetchType.LAZY)
    private Event event;
}

跑步：

@Entity
public class Run {

    // Getters, setters, etc. are omitted.

    @ManyToOne(fetch = FetchType.LAZY)
    private Event event;
}

当然，也有与JPA相关的空构造函数、其他构造函数、许多Hibernate/JPA标记和更多的底层关系，但我确信这些并没有带来什么不同。将Set<Run>变量引入事件类时开始出现问题

运行程序，创建类型为Event的对象添加事件，一切正常，但创建新的Run对象并尝试将其添加到Event.runs字段会在JUnit测试中出现以下错误：

打电话给event.getRuns().add(<a new 'Run' object>):

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: com.ps.model.orm.Event.runs, could not initialize proxy - no Session
at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:576)
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:215)
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:555)
at org.hibernate.collection.internal.PersistentSet.add(PersistentSet.java:202)
at com.ps.HibernateTestApplicationTests.testRunCreation(HibernateTestApplicationTests.java:88)
at com.ps.HibernateTestApplicationTests.testAllTheThings(HibernateTestApplicationTests.java:37)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
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:75)
at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:254)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:89)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:193)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

懒惰的实例化在这一点上对我来说似乎是失败的。当我将Event.runs的@OneToMany标记的FetchType.LAZY更改为FetchType.EAGER时，程序按预期执行，但我一辈子都不明白为什么LAZY不在这里工作，而它在其他类中工作，因为它们的结构几乎相同

其他的两个都能工作LAZY和{}，而这一个不行，我真的看不到映射或任何东西的区别，所以即使我现在只是使用EAGER类型，而不是LAZY，我知道这是令人沮丧的，我想知道是否有什么我可能已经看过了

Answer 1

我也有同样的问题。我已经解决了这个问题，在我的网站上添加了以下代码。xml文件。我希望它能帮助你；）

<filter>
    <filter-name>SpringOpenEntityManagerInViewFilter</filter-name>
    <filter-class>org.springframework.orm.jpa.support.OpenEntityManagerInViewFilter</filter-class>
</filter>
<filter-mapping>
    <filter-name>SpringOpenEntityManagerInViewFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Answer 2

stacktrace告诉您问题所在-

could not initialize proxy - no Session

只要确保在会话仍然可用的事务中调用event.getRuns().add(<a new 'Run' object>)
如果使用Spring，只需使用@Transactional注释该方法

像这样的

@Transactional
public void doSomething(){

Event event= // get Event from DB ;
event.getRuns.add(//new Run);
//some other stuff
}