java - Hibernate:如何处理DAO中的Session以正确设置关联？

Question

我找不到使用 DAO 设置关联的解决方案。需要解释一下。有 3 个实体:银行、客户和账户。银行和客户是双向多对多关联，客户和账户是一对多关联。每个实体的 DAO 都有一个 GenericDAO 接口(interface)来实现。由于应用程序不是多线程，我决定通过在 DAO 接口(interface)实现的每种方法中打开和关闭 session 来实现每个请求 session “模式”。请看一下代码。问题描述如下代码。

POJO 银行

@Entity
@Table(name = "bank")
public class Bank {

    private int bankID;
    private String bankName;
    private Set<Client> setOfClients;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "bank_id")
    public int getBankID() {
        return bankID;
    }

    public void setBankID(int bankID) {
        this.bankID = bankID;
    }

    @Column(name = "bank_name")
    public String getBankName() {
        return bankName;
    }

    public void setBankName(String bankName) {
        this.bankName = bankName;
    }

    @ManyToMany(mappedBy = "setOfBanks", fetch = FetchType.EAGER)
    public Set<Client> getSetOfClients() {
        return setOfClients;
    }

    public void setSetOfClients(Set<Client> setOfClients) {
        this.setOfClients = setOfClients;
    }
}**

客户端 POJO

@Entity
@Table(name = "client")
public class Client {

    private int clientID;
    private String firstName;
    private String secondName;
    private Set<Account> setOfAccounts;
    private Set<Bank> setOfBanks;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "client_id")
    public int getClientID() {
        return clientID;
    }

    public void setClientID(int clientID) {
        this.clientID = clientID;
    }

    @Column(name = "first_name")
    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    @Column(name = "second_name")
    public String getSecondName() {
        return secondName;
    }

    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }

    @OneToMany(mappedBy = "clientID", cascade = {CascadeType.ALL})
    public Set<Account> getSetOfAccounts() {
        return setOfAccounts;
    }

    public void setSetOfAccounts(Set<Account> setOfAccounts) {
        this.setOfAccounts = setOfAccounts;
    }

    @ManyToMany(targetEntity = Bank.class, cascade = {CascadeType.ALL}, fetch = FetchType.EAGER)
    @JoinTable(
            name = "bank_client",
            joinColumns = {@JoinColumn(name = "client_id", nullable = false, referencedColumnName = "client_id")},
            inverseJoinColumns = {@JoinColumn(name = "bank_id", nullable = false, referencedColumnName = "bank_id")}
    )
    public Set<Bank> getSetOfBanks() {
        return setOfBanks;
    }

    public void setSetOfBanks(Set<Bank> setOfBanks) {
        this.setOfBanks = setOfBanks;
    }
}

ClientDAO

public class ClientDAO<Client> implements GenericDAO<Client>{

    private Session session;

    @Override
    public void saveEntity(Client entity) {
        try {
            session = SessionFactoryUtil.getSessionFactory().openSession();
            session.getTransaction().begin();
            session.save(entity);
            session.getTransaction().commit();
        } catch (HibernateException e) {
            session.getTransaction().rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }

    @Override
    public void deleteEntity(Client entity) {
        try {
            session = SessionFactoryUtil.getSessionFactory().openSession();
            session.getTransaction().begin();
            session.delete(entity);
            session.getTransaction().commit();
        } catch (HibernateException e) {
            session.getTransaction().rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }

    }

    @Override
    public void updateEntity(Object entity) {
        try {
            session = SessionFactoryUtil.getSessionFactory().openSession();
            session.getTransaction().begin();
            session.update(entity);
            session.getTransaction().commit();
        } catch (HibernateException e) {
            session.getTransaction().rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }

    @Override
    public Client getEntityByID(int id) {
        Client client = null;
        try {
            session = SessionFactoryUtil.getSessionFactory().openSession();
            session.getTransaction().begin();
            client = (Client) session.get(model.entity.Client.class, id);
        } catch (HibernateException e) {
            session.getTransaction().rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
        return client;
    }

    @Override
    public List<Client> getAllEntities() {
        List<Client> result = null;
        try {
            session = SessionFactoryUtil.getSessionFactory().openSession();
            session.getTransaction().begin();
            result = session.createQuery("from Client").list();
            session.getTransaction().commit();
        } catch (HibernateException e) {
            if (session != null) {
                session.getTransaction().rollback();
            }
            e.printStackTrace();
        } finally {
            session.close();
        }
        return result;
    }
}

BankDAO 有类似的方法实现，但与银行实体规范相关

主要方法

public class Main {
        public static void main(final String[] args) {

            ClientDAO<Client> clientDAO = new ClientDAO<Client>();
            BankDAO<Bank> bankDAO = new BankDAO<Bank>();

            Client client = clientDAO.getEntityByID(11);
            Bank bank = bankDAO.getEntityByID(3);

            Set<Bank> setOfBanks = client.getSetOfBanks();
            setOfBanks.add(bank);

            Set<Client> setOfClients = bank.getSetOfClients();
            setOfClients.add(client);
        }
}

我没有提到有一个中间表，其中包含客户表和银行表的外键值(它需要反射(reflect)它们的多对多关联)。主方法调用的结果应该是中间表中新添加的值(客户表和银行表的外键)。证明该协会已成立。但是当我调用 main-method 时什么也没有发生。据我了解，当我 setOfBanks.add(bank) 和 setOfClients.add(client) 时，对象会添加到与持久对象不相关的集合中，因为 session 已关闭。我不知道如何正确打开和关闭 session 以便在 session 内建立关联。希望我能清楚地描述这个问题。我真的很感激任何回复。

Answer 1

您所实现的看起来更像 session-per-operation anti-pattern比海事组织的每个请求 session 模式。 “每个请求 session ”通常是指客户端-服务器或 Web 应用程序中的一个请求，其中一个请求通常涉及多个数据库操作。

将该模式应用于像您这样的命令行应用程序比较困难，但在您的情况下，最接近请求的可能是从命令行运行应用程序。因此，我同意 nachokk 的观点，您应该在 main 方法的开头打开 session ，并在最后关闭它。