避免通过Hibernate Criteria或HQL查询进行辅助选择或联接

Question

我在优化Hibernate查询以免执行联接或辅助选择时遇到了麻烦。

执行Hibernate查询（条件或hql）时，例如：

return getSession().createQuery(("from GiftCard as card where card.recipientNotificationRequested=1").list();

…，而where子句检查不需要与其他表进行任何联接的属性…，但是Hibernate仍与其他表进行完全联接（或辅助选择，具体取决于我如何设置fetchMode）。

有问题的对象（GiftCard）有几个ManyToOne关联，在这种情况下（但不一定是所有情况），我希望它们被延迟加载。我想要一个解决方案，可以控制执行查询时延迟加载的内容。

以下是GiftCard实体的外观：

@Entity
@Table(name = "giftCards")
public class GiftCard implements Serializable
{
 private static final long serialVersionUID = 1L;

 private String id_;
 private User buyer_;
 private boolean isRecipientNotificationRequested_;


 @Id
 public String getId()
 {
  return this.id_;
 }

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

 @ManyToOne
 @JoinColumn(name = "buyerUserId")
 @NotFound(action = NotFoundAction.IGNORE)
 public User getBuyer()
 {
  return this.buyer_;
 }
 public void setBuyer(User buyer)
 {
  this.buyer_ = buyer;
 }

 @Column(name="isRecipientNotificationRequested", nullable=false, columnDefinition="tinyint")
 public boolean isRecipientNotificationRequested()
 {
  return this.isRecipientNotificationRequested_;
 }

 public void setRecipientNotificationRequested(boolean isRecipientNotificationRequested)
 {
  this.isRecipientNotificationRequested_ = isRecipientNotificationRequested;
 }
}

Answer 1

如前所述

我想要一个 可以控制 执行查询时 延迟加载的内容 的解决方案

如果您有这样的映射

@Entity
public class GiftCard implements Serializable {

    private User buyer;

    @ManyToOne
    @JoinColumn(name="buyerUserId")
    public User getBuyer() {
        return this.buyer;
    }
}

默认情况下，任何
ToOne关系（例如@OneToOne和@ManyToOne）都是FetchType.EAGER，这意味着将始终对其进行提取。但是，这可能不是您想要的。当
我可以控制延迟加载的内容时， 您所说的 内容 可以翻译为“

获取策略”*
。Action
Book中的POJO支持这种模式（Notice方法签名）

public class GiftCardRepositoryImpl implements GiftCardRepository {

     public List<GiftCard> findGiftCardWithBuyer() {
         return sessionFactory.getCurrentSession().createQuery("from GiftCard c inner join fetch c.buyer where c.recipientNotificationRequested = 1").list();
     }

}

因此，根据您的用例，您可以创建自己的 find … With … And … 方法。它会照顾 您想要的东西

但这有一个问题：它不支持通用方法签名。对于每个@Entity存储库，您必须定义自定义 find … With … And
方法。因此，我向您展示了如何定义通用存储库

public interface Repository<INSTANCE_CLASS, UPDATABLE_INSTANCE_CLASS, PRIMARY_KEY_CLASS> {

    void add(INSTANCE_CLASS instance);
    void remove(PRIMARY_KEY_CLASS id);
    void update(PRIMARY_KEY_CLASS id, UPDATABLE_INSTANCE_CLASS updatableInstance);
    INSTANCE_CLASS findById(PRIMARY_KEY_CLASS id);
    INSTANCE_CLASS findById(PRIMARY_KEY_CLASS id, FetchingStrategy fetchingStrategy);
    List<INSTANCE_CLASS> findAll();
    List<INSTANCE_CLASS> findAll(FetchingStrategy fetchingStrategy);
    List<INSTANCE_CLASS> findAll(int pageNumber, int pageSize);
    List<INSTANCE_CLASS> findAll(int pageNumber, int pageSize, FetchingStrategy fetchingStrategy);
    List<INSTANCE_CLASS> findAllByCriteria(Criteria criteria);
    List<INSTANCE_CLASS> findAllByCriteria(Criteria criteria, FetchingStrategy fetchingStrategy);
    List<INSTANCE_CLASS> findAllByCriteria(int pageNumber, int pageSize, Criteria criteria);
    List<INSTANCE_CLASS> findAllByCriteria(int pageNumber, int pageSize, Criteria criteria, FetchingStrategy fetchingStrategy);

}

但是， 有时
，您不希望通用存储库接口定义所有方法。解决方案：创建一个AbstractRepository类，它将实现一个虚拟存储库。例如，Spring
Framework 大量使用这种模式 接口>> AbstractInterface

public abstract class AbstractRepository<INSTANCE_CLASS, UPDATABLE_INSTANCE_CLASS, PRIMARY_KEY_CLASS> implements Repository<INSTANCE_CLASS, UPDATABLE_INSTANCE_CLASS, PRIMARY_KEY_CLASS> {

    public void add(INSTANCE_CLASS instance) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public void remove(PRIMARY_KEY_CLASS id) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public void update(PRIMARY_KEY_CLASS id, UPDATABLE_INSTANCE_CLASS updatableInstance) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public INSTANCE_CLASS findById(PRIMARY_KEY_CLASS id) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public INSTANCE_CLASS findById(PRIMARY_KEY_CLASS id, FetchingStrategy fetchingStrategy) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAll() {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAll(FetchingStrategy fetchingStrategy) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAll(int pageNumber, int pageSize) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAll(int pageNumber, int pageSize, FetchingStrategy fetchingStrategy) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAllByCriteria(Criteria criteria) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAllByCriteria(Criteria criteria, FetchingStrategy fetchingStrategy) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAllByCriteria(int pageNumber, int pageSize, Criteria criteria) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public List<INSTANCE_CLASS> findAllByCriteria(int pageNumber, int pageSize, Criteria criteria, FetchingStrategy fetchingStrategy) {
        throw new UnsupportedOperationException("Not supported yet.");
    }

}

因此，您的GiftCardRepository可以重写为（请参见扩展而不是工具），并且 可以覆盖您真正想要的内容

public class GiftCardRepository extends AbstractRepository<GiftCard, GiftCard, String> {

    public static final GIFT_CARDS_WITH_BUYER GIFT_CARDS_WITH_BUYER = new GIFT_CARDS_WITH_WITH_BUYER();
    public static final GIFT_CARDS_WITHOUT_NO_RELATIONSHIP GIFT_CARDS_WITHOUT_NO_RELATIONSHIP = new GIFT_CARDS_WITHOUT_NO_RELATIONSHIP();

    public List<GiftCard> findAll(FetchingStrategy fetchingStrategy) {
        sessionFactory.getCurrentSession().getNamedQuery(fetchingStrategy.toString()).list();
    }


    /**
      * FetchingStrategy is just a marker interface
      * public interface FetchingStrategy {}
      *
      * And AbstractFetchingStrategy allows you to retrieve the name of the Fetching Strategy you want, by overriding toString method
      * public class AbstractFetchingStrategy implements FetchingStrategy {
      *
      *     @Override
      *     public String toString() {
      *         return getClass().getSimpleName();
      *     } 
      *
      * }
      * 
      * Because there is no need to create an instance outside our repository, we mark it as private
      * Notive each FetchingStrategy must match a named query
      */
    private static class GIFT_CARDS_WITH_BUYER extends AbstractFetchingStrategy {}    
    private static class GIFT_CARDS_WITHOUT_NO_RELATIONSHIP extends AbstractFetchingStrategy {}
}

现在，我们将命名查询外部化为多行且可读且可维护的xml文件

// app.hbl.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <query name="GIFT_CARDS_WITH_BUYER">
        <![CDATA[
            from 
                GiftCard c
            left join fetch 
                c.buyer
            where
                c.recipientNotificationRequested = 1
        ]]>
    </query>
    <query name="GIFT_CARDS_WITHOUT_NO_RELATIONSHIP">
        <![CDATA[
            from 
                GiftCard
        ]]>
    </query>
</hibernate-mapping>

因此，如果您想与买家一起找回GiftCard，只需致电

Repository<GiftCard, GiftCard, String> giftCardRepository;

List<GiftCard> giftCardList = giftCardRepository.findAll(GiftCardRepository.GIFT_CARDS_WITH_WITH_BUYER);

并要获取没有任何关系的GiftCard，只需致电

List<GiftCard> giftCardList = giftCardRepository.findAll(GiftCardRepository.GIFT_CARDS_WITHOUT_NO_RELATIONSHIP);

或使用import static

import static packageTo.GiftCardRepository.*;

和

List<GiftCard> giftCardList = giftCardRepository.findAll(GIFT_CARDS_WITHOUT_NO_RELATIONSHIP);

希望对您有用！