如何从服务方法延迟加载集合？

Question

我正在使用 Hibernate 5.1。3.Final 和 Spring 5.1。我在加载延迟加载的集合时遇到问题。在我的实体中，我有

@OneToMany(fetch = FetchType.LAZY)
@JoinTable(name = "sb_product_object", joinColumns = { @JoinColumn(name = "PRODUCT_ID") }, inverseJoinColumns = { @JoinColumn(name = "MY_OBJECT_ID") })
private List<MyObject> myObjects;

然后我有一个加载对象的服务方法

@Override
@Transactional
public List<MyObject> getMyObjectsByProduct(final String productId) 
{
    final Product product = productService.findById(productId);
    return product.getMyObjects();
}

但是当我尝试遍历对象时，

        final List<MyObject> ret = m_myobjectsvc.getMyObjectsByProduct(productId);
        for (final MyObject assmt : ret)

我收到这个错误...

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: org.mainco.subco.ecom.domain.Product.myObjects, could not initialize proxy - no Session
    at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:582)
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:201)
    at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:561)
    at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:132)
    at org.hibernate.collection.internal.PersistentBag.iterator(PersistentBag.java:277)
    at org.mainco.subco.MyObject.mvc.MyObjectViewControllerIT.testSetDisplayTABIcon(MyObjectViewControllerIT.java:87)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.runners.model.FrameworkMethod.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.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:73)
    at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:83)
    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.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:251)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access[=13=]0(ParentRunner.java:58)
    at org.junit.runners.ParentRunner.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:190)
    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:539)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:761)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:461)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:207)

如何从我的服务方法延迟加载集合？

Answer 1

在启用 Hibernate 时创建会话属性 enable_lazy_load_no_trans

<property name="hibernate.enable_lazy_load_no_trans" value="true"/>

https://vladmihalcea.com/the-hibernate-enable_lazy_load_no_trans-anti-pattern/

Answer 2

在你的情况下，如果你总是迭代你的集合，所以我认为你应该避免使用延迟加载，因为它会导致 N+1 问题并可能影响你的性能。

高级解决方案：您应该在 jpa 2.2 中使用 "entity graph" 来加载您需要的内容。

对于简单的解决方案，只需确保通过在同一事务中调用 collections.size() 来加载集合。

在我看来，没有交易反模式可能是相当危险的。所以避免使用

如何从服务方法延迟加载集合？

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