java - 如何在不出现 "could not initialize proxy - no Session"异常的情况下获取数据

Question

我有一种从数据库检索数据的方法:

public static List<ClassA> getAll() {
    Session session = HibernateUtil.getSessionFactory().openSession();
    Transaction tx = null;
    try {
        tx = session.beginTransaction();
        Criteria criteria = session.createCriteria(ClassA.class);
        List result = criteria.list();
        tx.commit();
        return result;
    } catch (HibernateException ex) {
        transactionRollBack(tx);
        log.error(ex.getMessage());
    } finally {
        session.close();
    }
    return null;
}

使用示例:

List<ClassA> results = getAll();
for (ClassA result : results) {
    System.out.println(result); //"could not initialize proxy - no Session" is thrown here
}

如果我正确理解了 Hibernate 文档，就会发生这种情况，因为 session 已关闭。但是我如何获取数据并关闭 session (因为它需要在一分钟内即将开始的下一个事务之前关闭)？

已更新

这是 ClassA 内容，包括 toString():

@Entity
@Table(name = "classA")
public class ClassA implements Serializable {

    private static final long serialVersionUID = 5163856045361200311L;
    private int id;
    private String name;

    private Set<ClassB> classB = new HashSet<ClassB>(0);

    public ClassA() {}
    public ClassA(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public ClassA(String name) {
        this.name = name;
    }

    public ClassA(ClassA classA) {
        this.id = classA.getId();
        this.name = classA.getName();
    }

    @Id
    @GeneratedValue(strategy = AUTO)
    @Column(name = "id", nullable = false, unique = true)
    public int getId() {
        return id;
    }

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

    @Column(name = "name")
    public String getName() {
        return name;
    }

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

    @OneToMany(fetch = LAZY, mappedBy = "classA_id")
    public Set<ClassB> getClassB() {
        return classB;
    }

    public void setClassB(Set<ClassB> classB) {
        this.classB = classB;
    }

    @Override
    public String toString() {
        return "ClassA{ id=" + id + ", name='" + name + ", classB =" + classB + '}';
    }
}

问题似乎出在 ClassB 上，但我不明白它是什么。这是B类:

@Entity
@Table(name = "classB")
public class ClassB implements Serializable{

    private static final long serialVersionUID = -8481028941663340911L;
    private int id;
    private Date date;
    private ClassA classA_id;
    private int classA_counter;

    public ClassB() {}

    public ClassB(ClassB rec) {
        this.id = rec.getId();
        this.date = rec.getDate();
        this.classA_id = rec.getClassA_id();
        this.classA_counter = rec.classA_counter();
    }

    @Id
    @GeneratedValue
    @Column(name = "id", nullable = false)
    public int getId() {
        return id;
    }

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

    @Temporal(DATE)
    @Column(name = "date")
    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    @ManyToOne(fetch = LAZY)
    @JoinColumn(name = "classA_id", nullable = false)
    public ClassA getCLassA_id() {
        return classA_id;
    }

    public void setClassA_id(ClassA classA_id) {
        this.classA_id = classA_id;
    }

    @Column(name = "classA_counter", columnDefinition = "int default 0")
    public int getClassA_counter() {
        return classA_counter;
    }

    public void setClassA_counter(int classA_counter) {
        this.classA_counter = classA_counter;
    }

    @Override
    public String toString() {
        return "ClassB { id=" + id + ", date=" + date + ", classA_id=" + classA_id.getName() + ", classA_counter=" + classA_counter +'}';
    }
}

Answer 1

问题在于 toString() 方法，该方法由

隐式调用

System.out.println(result);

尝试访问尚未由 getAll() 方法加载的 ClassA 实体的关联。修复 toString) 方法，使其仅访问保证存在的数据。

如果您确实需要在 session 关闭后访问某些关联，请确保已通过查询或调用 Hibernate.initialize(classAInstance.getAssociationThatMustBeLoaded()) 加载此关联.