【发布时间】：2010-01-11 05:06:33
【问题描述】：
我认为我缺少有关 Hibernate 工作原理的一些基本知识，特别是延迟加载。我的问题是调试，因为我不确定这是 Hibernate 问题还是变相的 Spring 问题。我想在进行一些重大重构之前我会在这里问。
我有两个实体。一个以 OneToMany 关系持有另一个的集合。对于我的网页，我希望获取所有第一个实体，然后为每个实体获取一组关联实体并显示它们。
我相信我的问题是：我使用 JpaTemplate 来查找所有实体。这很好用，但是由于延迟加载，我没有得到相关实体的关联集。在我看来（jsp）我想访问这个集合，但它当然是空的，因为它是延迟加载的。现在，我收到一个 LazyInitialization 异常，说明事务已结束。对我来说这是有道理的，当然交易现在应该已经结束了。问题是，如果事务结束，关联的集合怎么会被延迟加载？
实体类：
@Entity
public class LearningEntry implements Serializable {
private Long id;
String imagePath = "";
Set<Sample> samples = null;
//------------------------------
// Constructors
//------------------------------
public LearningEntry(){
imagePath = "";
samples = new HashSet<Sample>();
}
//------------------------------
// Instance Methods
//------------------------------
public void addSample(Sample s){
samples.add(s);
}
public void removeSample(Sample s){
samples.remove(s);
}
//------------------------------
// Setters and Getters
//------------------------------
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
//@Column(name = "wisi_LE_IMAGEPATH", length = 100, nullable = false)
public String getImagePath() {
return imagePath;
}
public void setImagePath(String imagePath) {
this.imagePath = imagePath;
}
// TODO - ONly works with fetch type EAGER
//@OneToMany(fetch = FetchType.EAGER, cascade = {CascadeType.PERSIST, CascadeType.MERGE})
@OneToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
public Set<Sample> getSamples() {
return samples;
}
public void setSamples(Set<Sample> samples) {
this.samples = samples;
}
}
示例实体
@Entity
public class Sample implements Serializable {
private Long id;
Date creationDate;
String audioFileLocation;
Integer votes;
String description;
public Sample(){
creationDate = new Date();
audioFileLocation = "";
votes = 0;
description = "";
}
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getAudioFileLocation() {
return audioFileLocation;
}
public void setAudioFileLocation(String audioFileLocation) {
this.audioFileLocation = audioFileLocation;
}
@Temporal(TemporalType.DATE)
public Date getCreationDate() {
return creationDate;
}
public void setCreationDate(Date creationDate) {
this.creationDate = creationDate;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public Integer getVotes() {
return votes;
}
public void setVotes(Integer votes) {
this.votes = votes;
}
}
DAO 类： LearningEntryDAO
@Transactional
public class JpaLearningEntryDAO implements LearningEntryDAO{
private JpaTemplate jpaTemplate;
public JpaLearningEntryDAO(){
}
public void setJpaTemplate(JpaTemplate jpaTemplate){
this.jpaTemplate = jpaTemplate;
}
@Override
//@Transactional
public void delete(Long leId) {
LearningEntry dp = jpaTemplate.find(LearningEntry.class, leId);
jpaTemplate.remove(dp);
}
@Override
@SuppressWarnings("unchecked")
//@Transactional
public List<LearningEntry> findAll() {
return jpaTemplate.find("from LearningEntry");
}
@Override
//@Transactional
public LearningEntry findById(Long leId) {
return jpaTemplate.find(LearningEntry.class, leId);
}
@Override
//@Transactional
public LearningEntry store(LearningEntry dp) {
return jpaTemplate.merge(dp);
}
@Override
@SuppressWarnings("unchecked")
//@Transactional
public void deleteAll(){
throw new RuntimeException("deleteAll not implemented");
}
}
DAO 示例
@Transactional
public class JpaSampleDAO implements SampleDAO{
private JpaTemplate jpaTemplate;
public JpaSampleDAO(){}
public void setJpaTemplate(JpaTemplate jpaTemplate){
this.jpaTemplate = jpaTemplate;
}
@Override
//@Transactional
public void delete(Long sampleId) {
Sample dp = jpaTemplate.find(Sample.class, sampleId);
jpaTemplate.remove(dp);
}
@Override
@SuppressWarnings("unchecked")
public List<Sample> findAll() {
return jpaTemplate.find("from Sample");
}
@Override
public Sample findById(Long sampleId) {
return jpaTemplate.find(Sample.class, sampleId);
}
@Override
public Sample store(Sample dp) {
return jpaTemplate.merge(dp);
}
@Override
@SuppressWarnings("unchecked")
public void deleteAll(){
throw new RuntimeException("deleteAll not implemented");
}
}
控制器
@RequestMapping(value = "/index.htm", method = RequestMethod.GET)
public ModelAndView sayHello(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
Map<String, Object> model = new HashMap<String, Object>();
List<LearningEntry> le = learningEntryService.getLearningEntries();
model.put("learningEntries", le);
return new ModelAndView("main", model);
}
查看
<section id="content" class="body">
<ol id="posts-list" class="hfeed">
<c:forEach items="${learningEntries}" var="learningEntry">
<li>
<table class="wisiEntry">
<tr>
<td class="pictureCell">
<img class="wisiEntry-pic" src="${learningEntry.imagePath}" />
</td>
<td class="previousNextCell"
<div class="wisiEntry-nextSampleButton">Next</div>
<div class="wisiEntry-previousSampleButton">Previous</div>
<br />
<div class="wisiEntry-addTagButton">Tag</div>
<div class="wisiEntry-addCommentButton">Comment</div>
<br />
<div class="wisiEntry-uploadButton">Upload</div>
</td>
<td>
<!-- ERROR HAPPENS HERE. Samples should not be null -->
<c:forEach items="${learningEntry.samples}" var="sample" varStatus = "status">
<table class="sampleEntry" ${status.first ? '' : 'style = "display:none"'}>
<tr>
<td class="sampleCell">
<p class="description">
${sample.description}
</p>
<audio src="${sample.audioFileLocation}" controls>
Your browser does not support the <code>audio</code> element.
</audio>
</td>
<td class="voteCell">
<img class="upVote" src="/images/upArrow.jpeg" />
<span class="voteNumber">${sample.votes}</span>
<img class="downVote" src="/images/downArrow.jpeg" />
</td>
</tr>
</table>
</c:forEach>
</td>
</tr>
</table>
</li>
</c:forEach>
</ol><!-- /#posts-list -->
</section><!-- /#content -->
【问题讨论】：