使用Hibernate / JPA / Spring进行Java延迟加载

Question

I'm having an issue with converting an application to separate REST Server + Web Application rather than having a single application that handles pages + business logic.

I'm having a problem with an org.hibernate.LazyInitializationException being thrown when Jackson is serialising the output.

Below is my JPA configuration and the class in question. I'm aware the lazy initialisation is being caused by the List member, but I don't want this to be populated in the JSON returned. Hence the @JsonIgnore annotation.

PersistenceConfiguration.java

@Configuration
@EnableTransactionManagement
@EnableJpaRepositories(basePackageClasses = {ForumRepository.class})
public class PersistenceConfiguration {

@Bean
public EntityManagerFactory entityManagerFactory() {
    HibernateJpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
    LocalContainerEntityManagerFactoryBean factory = new LocalContainerEntityManagerFactoryBean();

    factory.setJpaVendorAdapter(vendorAdapter);
    factory.setPackagesToScan("com.mypackage.forum.api.models");
    factory.setDataSource(secureDataSource());
    factory.afterPropertiesSet();

    return factory.getObject();
}

@Bean
public JpaTransactionManager transactionManager() {
    EntityManagerFactory factory = entityManagerFactory();
    return new JpaTransactionManager(factory);
}

@Bean
public HibernateExceptionTranslator exceptionTranslator() {
    return new HibernateExceptionTranslator();
}

@Bean
public DataSource secureDataSource() {
    try {
        Context ctx = new InitialContext();
        return (DataSource) ctx.lookup("java:jboss/datasources/ForumDS");
    } catch (Exception e) {
        return null;
    }
}
}

ForumGroup.java

@Entity
public class ForumGroup {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Integer id;

@JsonIgnore
@OneToMany(mappedBy = "group")
private List<Forum> forums;

private String name;
private String description;

public Integer getId() {
    return id;
}

public void setId(Integer id) {
    this.id = id;
}

public List<Forum> getForums() {
    return forums;
}

public void setForums(List<Forum> forums) {
    this.forums = forums;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public String getDescription() {
    return description;
}

public void setDescription(String description) {
    this.description = description;
}

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    ForumGroup that = (ForumGroup) o;

    if (id != null ? !id.equals(that.id) : that.id != null) return false;
    if (name != null ? !name.equals(that.name) : that.name != null) return false;

    return true;
}

@Override
public int hashCode() {
    int result = id != null ? id.hashCode() : 0;
    result = 31 * result + (name != null ? name.hashCode() : 0);
    return result;
}

@Override
public String toString() {
    return "ForumGroup{" +
            "id=" + id +
            ", name='" + name + '\'' +
            ", description='" + description + '\'' +
            '}';
}
}

I presume that I need to configure some sort of Session Manager but not sure the best way to go about doing it in JavaConfig, I've tried various methods via Google with no joy.

Any help would be much appreciated.

Update

I've now managed to resolve this by 1) Adding the @Fetch() annotation to the relevant collections, which means that the collections get populated 2) I have stopped Entities/Models being returned as JSON objects and instead use separate DTOs

Answer 1

org.hibernate.LazyInitializationException happens when call
In your situation , it's seams you select ForumGroup entities and then, outside from session you trying call entity.getForums() . That's why happening LazyInitializationException .

If you need that collection, then load them . You can do that by calling:
entity.getForums().size(); .

You can use anotation, to to load collection by default : @ManyToOne(fetch = FetchType.EAGER)

Answer 2

You just need to fetch the ForumGroup along with its forums list.

This can be achieved with a LEFT JOIN FETCH whenever you retrieve the ForumGroup you are interested in:

"from ForumGroup as fg " +
"left join fetch fg.forums "

You can assign a default fetching strategy, but this is a decision that will affect subsequent queries, so make sure you know the trade-offs:

Use a join fetch mode:

@JsonIgnore
@OneToMany(mappedBy = "group")
@Fetch(FetchMode.JOIN)    
private List<Forum> forums = new ArrayList<Forum>();

Or a sub-select fetch mode:

@JsonIgnore
@OneToMany(mappedBy = "group")
@Fetch(FetchMode.SUBSELECT)
private List<Forum> forums = new ArrayList<Forum>();

And always initialize your collections, never let them assigned to null.