JPA:所有实体都有一个DAO,每个实体有一个DAO？

Question

首先,抱歉我的英语.

我为朋友制作了一个访问SQLite数据库的JPA(EclipseLink)桌面应用程序.

我已经在Eclipse中创建了数据库和实体.但我还创建了一个名为class的类UniversalDAO,它是所有实体用来访问和持久化数据库的实用程序类:

package model.DAO;

import java.util.ArrayList;

import javax.persistence.*;

import model.entities.Entities;


public class UniversalDAO {


    private static EntityManagerFactory emf = Persistence.createEntityManagerFactory("TheDatabase");
    private static EntityManager em = emf.createEntityManager();

    private UniversalDAO (){}


    public static void close(){
        em.close();
        emf.close();
    }

    public static Entities getOne(Class<? extends Entities> table, Object primaryKey) {

        return em.find(table, primaryKey);
    }

    public static ArrayList<Entities> getAll(Class<? extends Entities> table) {

        ArrayList<Entities> ret = new ArrayList<>();

        for(Object obj : em.createQuery("SELECT o FROM " + table.getName() + " o").getResultList())
            ret.add((Entities) obj);

        return ret;
    }

    public static ArrayList<Entities> getWithCondition(Class<? extends Entities> table, String condition) {

        ArrayList<Entities> ret = new ArrayList<>();

        for(Object obj : em.createQuery("SELECT o FROM " + table.getName() + " o WHERE " + condition).getResultList())
            ret.add((Entities) obj);

        return ret;
    }

    public static void insert(Entities row) {
        em.getTransaction().begin();

        em.persist(row);

        em.flush();

        em.getTransaction().commit();
    }

    public static void update(Entities row) {

        em.getTransaction().begin();

        em.merge(row);

        em.flush();

        em.getTransaction().commit();
    }

    public static void delete(Class<? extends Entities> table, Object primaryKey) {

        em.getTransaction().begin();

        Entities row = em.find(table, primaryKey);

        em.remove(row);

        em.flush();

        em.getTransaction().commit();
    }
}

为了分组所有的entites并在这个类中使用它我创建了一个名为的空接口Entities.

这是其中一个实体的样子:

package model.entities;

import java.util.ArrayList;

import javax.persistence.*;


@Entity
@Table(name="emails")
public class EntityEmail implements Entities {

    @Id
    @Column(name="id_email")
    @GeneratedValue(strategy=GenerationType.SEQUENCE)
    private int idEmail;

    @Column(name="email")
    private String email;

    @Column(name="description")
    private String description;

    @ManyToMany(fetch=FetchType.EAGER)
    @JoinTable(name="people_emails",
               joinColumns=@JoinColumn(name="id_email", referencedColumnName="id_email"),
               inverseJoinColumns=@JoinColumn(name="id_person", referencedColumnName="id_person"))
    private ArrayList<EntityPerson> people;

    public EntityEmail() {

    }   

    public int getIdEmail() {
        return this.idEmail;
    }
    public void setIdEmail(int idEmail) {
        this.idEmail = idEmail;
    }   

    public String getEmail() {
        return this.email;
    }
    public void setEmail(String email) {
        this.email = email;
    }

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

    public ArrayList<EntityPerson> getPeople() {
        return people;
    }
    public void setPeople(ArrayList<EntityPerson> people) {
        this.people = people;
    }   
}

就像你可以欣赏我不是一个专业人士,我仍然有很多东西需要学习.所以,我想知道这种方法是否正确,或者我是否应该为每个实体都有一个DAO.

Answer 1

看起来你试图发明Generic DAO模式.如果是这样,你基本上是正确的方式.

通用DAO模式的工作原理如下:

• 创建一个所有DAO将扩展的通用基类:

  public abstract class GenericDao<E, ID extends Serializable> {
      ...
      // Implement common operations that are relevant to all entities here:
      public E findById(ID id) { ... }
      public void save(E entity) { ... }
      // etc
      ...
  }
  

• 通过扩展创建具体的DAO实现GenericDao:

  public class EntityEmailDao extends GenericDao<EntityEmail, Integer> {
      // This class may contain operations that are relevant to specific entity:
      public E findByEmail(String email) { ... }
  }
  

• 由于GenericDao是通用的,因此您无需使实体扩展任何通用接口

这个模式有很多现有的实现,例如,看看这里.