Hibernate LazyInitializationException while using

问题:

I have entities like this:

@Entity
@Table(name = "ASSESSMENT")
public class Assessment {

    //All other fields..

    @OneToMany(fetch = FetchType.LAZY, cascade = CascadeType.ALL, mappedBy = "assessment")
    @OrderBy(value = "order ASC")
    private List<AssessmentPart> assessmentParts = new LinkedList<>();

    public List<AssessmentPart> getAssessmentParts() {
        return assessmentParts;
    }

    //All other getters/setters
}

The other one:

@Entity
@Table(name = "ASSESSMENT_PART")
public class AssessmentPart {

    //All other fields

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "ASSESSMENT_ID", nullable = false)
    private Assessment assessment;

    public Assessment getAssessment() {
        return assessment;
    }

    public void setAssessment(Assessment assessment) {
        this.assessment = assessment;
    }

    //All other getters/setters
}

Assessment parts are lazy loaded from assessment entity. I also have spring controller method which is Transactional and loads assessment part from the database:

@Transactional
public void doSomething(String partId, Map<String, Object> model) {

    AssessmentPart assessmentPart = //laods a part with entity manager
    Assessment assessment = assessmentPart.getAssessment(); //Getting the assessments

    model.put("assessmentParts", assessment.getAssessmentParts()); //adding all assessments parts into spring model map
}

Once I've added assessment parts into spring model map, they become available in my JSP page and I am using JSTL to loop through them:

<c:forEach var="assessmentPart" items="${assessmentParts}">
     //Not loading any lazy stuff, just getting an ID of assessment part
</c:forEach>

The exception is thrown from this JSP page:

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: my.package.something.Assessment.assessmentParts, could not initialize proxy - no Session

How is this possible if this collection has already been loaded in a transaction? I am just trying to loop through, hibernate shouldn't be loading anything at this point, because it was already loaded. Why is this weird thing happening?

回答1:

In the view the entitymanager is already closed and as such the elements in your collections fail to retrieve there properties. The code you wrote in the controller doesn't initialize the elements in the collection (it is a LAZY collection) but only the collection is initialized (not the elements inside it).

Either force the entitymanager to remain open by configuring the OpenEntityManagerInViewFilter in your web configuration.

Or change your controller code to include a call to Hibernate.initialize to properly initialize your collection.

@Transactional
public void doSomething(String partId, Map<String, Object> model) {

    AssessmentPart assessmentPart = //laods a part with entity manager
    Assessment assessment = assessmentPart.getAssessment(); //Getting the assessments
    Hibernate.initialize(assesment.getAssesmentParts()); // Init collection
    model.put("assessmentParts", assessment.getAssessmentParts()); //adding all assessments parts into spring model map
}

Either that or create a custom query which forces eager fetching of the collection.

回答2:

Change

model.put("assessmentParts", assessment.getAssessmentParts());

to

Hibernate.initialize(assessment.getAssessmentParts());

to initialize your collection.

The method call assessment.getAssessmentParts() is not initializing your collection. This method call will just return you a collection wrapper and this wrapper you need to pass to the Hibernate.initialize method for manual initialization.

EDIT:

With JPA, you can fetch the AssessmentParts along with the Assessment (wherever needed) using JPQL Fetch Joins, overriding the default behavior:

SELECT a FROM Assessment asmt LEFT JOIN FETCH asmt.assessmentParts WHERE asmt.id = :id

回答3:

Loading AssessmentPart from EntityManager will Load Assessment @ManyToOne(fetch = FetchType.EAGER)

AssessmentPart assessmentPart = //laods a part with entity manager
Assessment assessment = assessmentPart.getAssessment();

Trying to extract AssessmentParts from assessment wont work since it's not @OneToMany(fetch = FetchType.LAZY) and session is already closed

model.put("assessmentParts", assessment.getAssessmentParts());

Get list of AssessmentParts by Assessment from EntityManager in new method should solve the problem.