延迟加载时出错：com.fasterxml.jackson.databind.JsonMappingException：无法懒惰地初始化角色集合

Question

我有一个集合的实体，我想在懒惰中加载实体但我得到错误：

Caused by: com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: lu.entities.folders.Folders.requests, could not initialize proxy - no Session (through reference chain: lu.entities.folders.Folders.requests)
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:210)
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:177)
    at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:190)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:674)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:156)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:129)
    at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:851)
    at org.jboss.resteasy.plugins.providers.jackson.ResteasyJackson2Provider.writeTo(ResteasyJackson2Provider.java:207)
    at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.writeTo(AbstractWriterInterceptorContext.java:131)
    at org.jboss.resteasy.core.interception.ServerWriterInterceptorContext.writeTo(ServerWriterInterceptorContext.java:60)
    at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.proceed(AbstractWriterInterceptorContext.java:120)
    at org.jboss.resteasy.security.doseta.DigitalSigningInterceptor.aroundWriteTo(DigitalSigningInterceptor.java:145)
    at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.proceed(AbstractWriterInterceptorContext.java:124)
    at org.jboss.resteasy.plugins.interceptors.encoding.GZIPEncodingInterceptor.aroundWriteTo(GZIPEncodingInterceptor.java:100)
    at org.jboss.resteasy.core.interception.AbstractWriterInterceptorContext.proceed(AbstractWriterInterceptorContext.java:124)
    at org.jboss.resteasy.core.ServerResponseWriter.writeNomapResponse(ServerResponseWriter.java:98)
    at org.jboss.resteasy.core.SynchronousDispatcher.writeResponse(SynchronousDispatcher.java:466)
    ... 38 more
Caused by: org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: lu.pgd.ccpd.entities.folders.Folders.requests, could not initialize proxy - no Session
    at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:587)
    at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:204)
    at org.hibernate.collection.internal.AbstractPersistentCollection.readSize(AbstractPersistentCollection.java:148)
    at org.hibernate.collection.internal.PersistentBag.size(PersistentBag.java:261)
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:88)
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:24)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:575)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:666)
    ... 51 more

我的实体喜欢：

@Entity
@Table(name = "FOLDER_FR")
@JsonIdentityInfo(scope=Folders.class, generator = ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Folders implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE)
    @Column(name = "FR_ID")
    private Long id;

    @LazyCollection(LazyCollectionOption.TRUE)
    @JsonIgnore
    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    @JoinTable(name = "M_REQUESTFOLDER_RR", joinColumns = {
            @JoinColumn(name = "RR_FOLDER", referencedColumnName = "FR_ID") }, inverseJoinColumns = {
                    @JoinColumn(name = "RR_REQUEST", referencedColumnName = "RS_ID") })
    private List<Requests> requests = new ArrayList<Requests>();

}

在我的DAO中：

@Override
public Folders retrieveFolder(Long id) throws PersistenceException {
    Folders folders = entityManager.find(Folders.class, id);

    return folders;
}

那么如何在JPA中解决它？当我调用我的方法retrieveFolder时出了什么问题？

非常感谢

Answer 1

您必须初始化收集请求。

您可以调用folders.getRequests（）。iterator（）来触发请求的加载。

但最好的方法是将带有JOIN FETCH的JPQL查询写入将急切加载该集合的请求：

SELECT f FROM Folder f JOIN FETCH f.requests r

或者使用将执行相同操作的EntityGraph。

查看Hibernate文档并搜索JOIN FETCH和EntityGraph。

http://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html