Java 使用生成器模式的JPA实体图

我有一张这样的汽车桌：

+----+------+--------------+-----------+----------+------+
| Id | Name | Description |   Make    |  Model   | Year |
+----+------+--------------+-----------+----------+------+
|  1 | A    | something1   | Ford      | Explorer | 2010 |
|  2 | B    | something2   | Nissan    | Ultima   | 2005 |
|  3 | C    | something3   | Chevrolet | Malibu   | 2012 |
+----+------+--------------+-----------+----------+------+

@Entity
@Table(schema = "dbo", name = "Car")
public class CarEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

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

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

    @Column(name = "Make")
    private String make;

    @Column(name = "Model")
    private String model;

    @Column(name = "Year")
    private int year;

    //Getters and Setters for all attributes
}

public interface CarRepository {
    CarRepository getCar();

    CarRepository withName();

    CarRepository withDescription();

    CarRepository withMake();

    CarRepository withModel();

    CarRepository withYear();

    List<Car> build(int carId);
}

@Repository
public class JpaCarRepository implements CarRepository {
    private static EntityGraph<CarEntity> eg;

    @Override
    public CarRepository getCar() {
        eg = this.em.createEntityGraph(CarEntity.class);
        eg.addAttributeNodes("id");
        return this;
    }

    @Override
    public CarRepository withName() {
        eg.addAttributeNodes("name");
        return this;
    }

    @Override
    public CarRepository withDescription() {
        eg.addAttributeNodes("description");
        return this;
    }

    //rest of the with methods

    @Override
    public List<Car> build(int carId) {
        List<CarEntity> entities = em.createQuery("SELECT c FROM dbo.Car c WHERE c.id = :carId")
                .setHint("javax.persistence.fetchgraph", eg)
                .getResultList();
        return //convert list of CarEntity to Car and return list;
    }
}

List<Car> cars = repo.getCar().withName().withDescription().build(1);

还有一个汽车实体，看起来像这样：

+----+------+--------------+-----------+----------+------+
| Id | Name | Description |   Make    |  Model   | Year |
+----+------+--------------+-----------+----------+------+
|  1 | A    | something1   | Ford      | Explorer | 2010 |
|  2 | B    | something2   | Nissan    | Ultima   | 2005 |
|  3 | C    | something3   | Chevrolet | Malibu   | 2012 |
+----+------+--------------+-----------+----------+------+

@Entity
@Table(schema = "dbo", name = "Car")
public class CarEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

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

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

    @Column(name = "Make")
    private String make;

    @Column(name = "Model")
    private String model;

    @Column(name = "Year")
    private int year;

    //Getters and Setters for all attributes
}

public interface CarRepository {
    CarRepository getCar();

    CarRepository withName();

    CarRepository withDescription();

    CarRepository withMake();

    CarRepository withModel();

    CarRepository withYear();

    List<Car> build(int carId);
}

@Repository
public class JpaCarRepository implements CarRepository {
    private static EntityGraph<CarEntity> eg;

    @Override
    public CarRepository getCar() {
        eg = this.em.createEntityGraph(CarEntity.class);
        eg.addAttributeNodes("id");
        return this;
    }

    @Override
    public CarRepository withName() {
        eg.addAttributeNodes("name");
        return this;
    }

    @Override
    public CarRepository withDescription() {
        eg.addAttributeNodes("description");
        return this;
    }

    //rest of the with methods

    @Override
    public List<Car> build(int carId) {
        List<CarEntity> entities = em.createQuery("SELECT c FROM dbo.Car c WHERE c.id = :carId")
                .setHint("javax.persistence.fetchgraph", eg)
                .getResultList();
        return //convert list of CarEntity to Car and return list;
    }
}

List<Car> cars = repo.getCar().withName().withDescription().build(1);

我有这样一个存储库层：

+----+------+--------------+-----------+----------+------+
| Id | Name | Description |   Make    |  Model   | Year |
+----+------+--------------+-----------+----------+------+
|  1 | A    | something1   | Ford      | Explorer | 2010 |
|  2 | B    | something2   | Nissan    | Ultima   | 2005 |
|  3 | C    | something3   | Chevrolet | Malibu   | 2012 |
+----+------+--------------+-----------+----------+------+

@Entity
@Table(schema = "dbo", name = "Car")
public class CarEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

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

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

    @Column(name = "Make")
    private String make;

    @Column(name = "Model")
    private String model;

    @Column(name = "Year")
    private int year;

    //Getters and Setters for all attributes
}

public interface CarRepository {
    CarRepository getCar();

    CarRepository withName();

    CarRepository withDescription();

    CarRepository withMake();

    CarRepository withModel();

    CarRepository withYear();

    List<Car> build(int carId);
}

@Repository
public class JpaCarRepository implements CarRepository {
    private static EntityGraph<CarEntity> eg;

    @Override
    public CarRepository getCar() {
        eg = this.em.createEntityGraph(CarEntity.class);
        eg.addAttributeNodes("id");
        return this;
    }

    @Override
    public CarRepository withName() {
        eg.addAttributeNodes("name");
        return this;
    }

    @Override
    public CarRepository withDescription() {
        eg.addAttributeNodes("description");
        return this;
    }

    //rest of the with methods

    @Override
    public List<Car> build(int carId) {
        List<CarEntity> entities = em.createQuery("SELECT c FROM dbo.Car c WHERE c.id = :carId")
                .setHint("javax.persistence.fetchgraph", eg)
                .getResultList();
        return //convert list of CarEntity to Car and return list;
    }
}

List<Car> cars = repo.getCar().withName().withDescription().build(1);

编辑2:因此我通过以下操作修复了上述问题：

@Override
public List<Car> build(int carId) {
    List<CarEntity> entities = em.createQuery("SELECT c FROM dbo.Car c WHERE c.id = :carId")
            .setHint("javax.persistence.fetchgraph", eg)
            .setParameter("id", carId)
            .getResultList();
    return //convert list of CarEntity to Car and return list;
}

@覆盖
公共列表生成（int carId）{
List entities=em.createQuery（“从dbo.Car c中选择c，其中c.id=：carId”）
.setHint（“javax.persistence.fetchgraph”，例如）
.setParameter（“id”，carId）
.getResultList（）；
return//将CarEntity列表转换为Car和return列表；
}

但不幸的是，我认为构建器实际上没有工作，因为当我从查询中删除

withDescription（）

时，我仍然得到一个填充了description字段的实体

---

如何让它只检索我指定的属性？

Your build（）方法返回Car entity对象。因为它返回整个汽车数据对象，所以它拥有数据库中的所有数据字段。如果只想检索指定的属性，则需要将数据对象与已传递的Id一起传递给build（）方法。完成后，您可以更改HQL查询以将属性容纳到其中。我希望我正确理解了你的问题。请告诉我您是否打算这样做。@RahulRaj No build（）返回

Car

业务对象，正如您在

CarEntity

对象上方的代码中看到的那样，该对象是一个单独的对象。build方法获取

CarEntity

对象，然后在返回之前将其转换为业务对象。你能举一个例子说明你的意思吗，因为我没有理解你在数据对象中所说的“传递”的意思。好的，我理解这一部分。我可以知道您是否希望build（）方法检索您指定的属性吗？@RahulRaj我只想使用我用

方法指定的属性检索汽车实体对象。因此，如果我说
repo.getCar（）.withName（）.build（1）
，那么它应该只返回表中id为1的汽车，并且只返回id和name属性。其余属性（description、model、make等）应该为null，因为它们没有被获取。需要澄清的是：setHint（“javax.persistence.fetchgraph”，例如）您传递的这个查询提示是否用于设置查询属性（您只想要由某些列组成的结果）？问这个是因为我以前没有用过entitygraph。让我知道。