java之我如何处理 Hibernate 中的身份与平等

zhoujg 2023年11月14日编程语言 154 0

注意:我开发这个 SSSCE 是为了说明我的问题。实际问题要大得多(各个“记录”有更多的字段和更多的数据，而 XML 数据文件有 30K 条记录。)

给定以下 XML 片段:

<?xml version="1.0" encoding="UTF-8"?> 
<manufacturers> 
 
    <manufacturer> 
        <name>BMW</name> 
        <manufacturing-countries> 
            <country code="DE" iso="DEU">Germany</country> 
        </manufacturing-countries> 
        <using-countries> 
            <country code="DE" iso="DEU">Germany</country> 
            <country code="JP" iso="JPN">Japan</country> 
            <country code="US" iso="USA">United States</country> 
            <country code="UK" iso="GBR">Germany</country> 
        </using-countries> 
    </manufacturer> 
 
    <manufacturer> 
        <name>Honda</name> 
        <manufacturing-countries> 
            <country code="JP" iso="JPN">Japan</country> 
            <country code="US" iso="USA">United States</country> 
        </manufacturing-countries> 
        <using-countries> 
            <country code="JP" iso="JPN">Japan</country> 
            <country code="US" iso="USA">United States</country> 
            <country code="UK" iso="GBR">Germany</country> 
        </using-countries> 
    </manufacturer> 
 
</manufacturers>

我有一个基于 JAX-B 的解析器，它将此 XML 读入以下对象 (请注意，对象被设计为不可变的；它们使用构建器模式来构建实际对象):

// @Immutable 
public class Country { 
    private String code; 
    private String name; 
    private String isoCode; 
 
    private Country() { 
        // private, no arg constructor 
    } 
 
    // getters elided 
 
    // builder elided 
}     
 
// @Immutable 
public class Manufacturer { 
 
    private String name; 
    // list of countries where manufacturer builds thier product(s) 
    private List<Country> manufacturingCountries; 
    // list of countries who use manufacturer's product(s) 
    private List<Country> usingCountries; 
 
    private Manufacturer() { 
        // private no-arg constructor 
    } 
    // getters elided 
 
    // builder elided 
}

因此，在我的代码中，我可以轻松地从 XML 文件加载制造商列表:

List<Manufacturer> manufacturers = ManufacturerReader.readManufacturersFromXml(xmlFilePath);

我需要转身将这些信息存入数据库。到目前为止，我已经得到了这个 Hibernate 映射文件:

hibernate .hbm.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 default-access="field"> 
    <class name="Country" 
           table="COUNTRIES"> 
 
        <id 
            name="code" 
            length="2" 
            column="CODE"> 
            <generator class="assigned"/> 
        </id> 
 
        <property 
            name="name" 
            length="40" 
            column="NAME"> 
        </property> 
 
        <property 
            name="isoCode" 
            length="3" 
            column="ISO_CODE"> 
        </property> 
 
    </class> 
 
    <class name="Manufacturer" 
           table="MANUFACTURERS"> 
 
        <id 
            type="string" 
            column="ID"> 
            <generator class="uuid"/> 
        </id> 
 
        <property 
            name="name" 
            length="255" 
            column="NAME"> 
        </property> 
 
        <list 
            name="manufacturingCountries" 
            table="MANUF_MANUF_COUNTRY" 
            cascade="save-update"> 
            <key column="MANUFACTURER_ID"/> 
            <list-index column="POSITION"/> 
            <many-to-many class="Country" column="COUNTRY_CODE"/> 
        </list> 
 
        <list 
            name="usingCountries" 
            table="MANUF_USING_COUNTRY" 
            cascade="save-update"> 
            <key column="MANUFACTURER_ID"/> 
            <list-index column="POSITION"/> 
            <many-to-many class="Country" column="COUNTRY_CODE"/> 
        </list>        
 
    </class> 
</hibernate-mapping>

鉴于解析器不关心相等性，创建的 Country 对象在各个制造商内部和之间重复(例如，有三个“US”Country 对象，两个“JP”Country 对象等)

当我尝试使用 Hibernate 保存对象图时，由于重复的(相等的)Country 对象，我得到了 NonUniqueObjectException。

Exception in thread "main" org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session: [Country#JP] 
    at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:190) 
    at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:143) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:210) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:195) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.performSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:117) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93) 
    at org.hibernate.impl.SessionImpl.fireSaveOrUpdate(SessionImpl.java:685) 
    at org.hibernate.impl.SessionImpl.saveOrUpdate(SessionImpl.java:677) 
    at org.hibernate.engine.CascadingAction$5.cascade(CascadingAction.java:252) 
    at org.hibernate.engine.Cascade.cascadeToOne(Cascade.java:392) 
    at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:335) 
    at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:204) 
    at org.hibernate.engine.Cascade.cascadeCollectionElements(Cascade.java:425) 
    at org.hibernate.engine.Cascade.cascadeCollection(Cascade.java:362) 
    at org.hibernate.engine.Cascade.cascadeAssociation(Cascade.java:338) 
    at org.hibernate.engine.Cascade.cascadeProperty(Cascade.java:204) 
    at org.hibernate.engine.Cascade.cascade(Cascade.java:161) 
    at org.hibernate.event.def.AbstractSaveEventListener.cascadeAfterSave(AbstractSaveEventListener.java:475) 
    at org.hibernate.event.def.AbstractSaveEventListener.performSaveOrReplicate(AbstractSaveEventListener.java:353) 
    at org.hibernate.event.def.AbstractSaveEventListener.performSave(AbstractSaveEventListener.java:203) 
    at org.hibernate.event.def.AbstractSaveEventListener.saveWithGeneratedId(AbstractSaveEventListener.java:143) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.saveWithGeneratedOrRequestedId(DefaultSaveOrUpdateEventListener.java:210) 
    at org.hibernate.event.def.DefaultSaveEventListener.saveWithGeneratedOrRequestedId(DefaultSaveEventListener.java:56) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.entityIsTransient(DefaultSaveOrUpdateEventListener.java:195) 
    at org.hibernate.event.def.DefaultSaveEventListener.performSaveOrUpdate(DefaultSaveEventListener.java:50) 
    at org.hibernate.event.def.DefaultSaveOrUpdateEventListener.onSaveOrUpdate(DefaultSaveOrUpdateEventListener.java:93) 
    at org.hibernate.impl.SessionImpl.fireSave(SessionImpl.java:713) 
    at org.hibernate.impl.SessionImpl.save(SessionImpl.java:701) 
    at org.hibernate.impl.SessionImpl.save(SessionImpl.java:697) 
    at Main.main(Main.java:28)

如果没有在从 XML 解析器读取后“手动”删除内存中的这些对象，我该如何解决这个问题？

假设:

必须在我的“保存到数据库逻辑”中进行更改，其中可以包括 hibernate 映射文件。
无法更改国家或制造商对象(它们在上游并且 API 已设置。)
不能改变 XML 阅读器/解析器(同样，上游提供并设置了 API。)
无法更改 XML 架构(同样，上游提供且 API 是设置。)

编辑 1:

我想出的实际传输数据的简单代码:

import java.util.List; 
import org.hibernate.Session; 
import org.hibernate.Transaction; 
 
public class Main { 
    public static void main(String[] args) { 
 
        List<Manufacturer> manuf = ManufacturerXmlReader.readManufacturersFromXml("manufacturers.xml"); 
 
        Session session = HibernateUtil.getSessionFactory().openSession(); 
 
        Transaction tx = session.beginTransaction(); 
 
        for (Manufacturer m : manuf) { 
            session.save(m); 
        } 
 
        tx.commit(); 
        session.close(); 
        HibernateUtil.shutdown(); 
    } 
}

编辑 2:

当更改 session.save() 时至 session.merge() , 我得到

Exception in thread "main" org.hibernate.HibernateException: The class has no identifier property: Manufacturer 
    at org.hibernate.tuple.entity.AbstractEntityTuplizer.getIdentifier(AbstractEntityTuplizer.java:220) 
    at org.hibernate.persister.entity.AbstractEntityPersister.getIdentifier(AbstractEntityPersister.java:3876) 
    at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:233) 
    at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:84) 
    at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:867) 
    at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:851) 
    at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:855) 
    at Main.main(Main.java:16)

编辑 3:

如果我更改 Hibernate 映射文件以给 Manufacturer ID 元素一个 name属性如:

<class name="Manufacturer" 
       table="MANUFACTURERS"> 
 
    <id 
        name="id" 
        type="string" 
        column="ID"> 
        <generator class="uuid"/> 
    </id>

我得到以下信息:

Initial SessionFactory creation failed.org.hibernate.PropertyNotFoundException: field [id] not found on Manufacturer 
Exception in thread "main" java.lang.ExceptionInInitializerError 
    at HibernateUtil.<clinit>(HibernateUtil.java:21) 
    at Main.main(Main.java:11) 
Caused by: org.hibernate.PropertyNotFoundException: field [id] not found on Manufacturer 
    at org.hibernate.property.DirectPropertyAccessor.getField(DirectPropertyAccessor.java:182) 
    at org.hibernate.property.DirectPropertyAccessor.getField(DirectPropertyAccessor.java:174) 
    at org.hibernate.property.DirectPropertyAccessor.getGetter(DirectPropertyAccessor.java:197) 
    at org.hibernate.tuple.PropertyFactory.getGetter(PropertyFactory.java:191) 
    at org.hibernate.tuple.PropertyFactory.buildIdentifierProperty(PropertyFactory.java:67) 
    at org.hibernate.tuple.entity.EntityMetamodel.<init>(EntityMetamodel.java:135) 
    at org.hibernate.persister.entity.AbstractEntityPersister.<init>(AbstractEntityPersister.java:485) 
    at org.hibernate.persister.entity.SingleTableEntityPersister.<init>(SingleTableEntityPersister.java:133) 
    at org.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:84) 
    at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:286) 
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1872) 
    at HibernateUtil.<clinit>(HibernateUtil.java:17) 
    ... 1 more

如果我将名称属性/字段更改为 ID 字段，例如

    <id 
        name="name" 
        length="255" 
        column="NAME"> 
        <generator class="assigned"/> 
    </id>

然后我明白了:

org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [Country#JP] 
    at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:435) 
    at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:233) 
    at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:285) 
    at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152) 
    at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1090) 
    at org.hibernate.impl.SessionImpl.internalLoad(SessionImpl.java:1038) 
    at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:630) 
    at org.hibernate.type.EntityType.resolve(EntityType.java:438) 
    at org.hibernate.type.EntityType.replace(EntityType.java:298) 
    at org.hibernate.type.CollectionType.replaceElements(CollectionType.java:508) 
    at org.hibernate.type.CollectionType.replace(CollectionType.java:575) 
    at org.hibernate.type.AbstractType.replace(AbstractType.java:176) 
    at org.hibernate.type.TypeHelper.replaceAssociations(TypeHelper.java:262) 
    at org.hibernate.event.def.DefaultMergeEventListener.copyValues(DefaultMergeEventListener.java:589) 
    at org.hibernate.event.def.DefaultMergeEventListener.mergeTransientEntity(DefaultMergeEventListener.java:389) 
    at org.hibernate.event.def.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:303) 
    at org.hibernate.event.def.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:464) 
    at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:255) 
    at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:84) 
    at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:867) 
    at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:851) 
    at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:855) 
    at Main.main(Main.java:16) 
Exception in thread "main" org.hibernate.ObjectNotFoundException: No row with the given identifier exists: [Country#JP] 
    at org.hibernate.impl.SessionFactoryImpl$2.handleEntityNotFound(SessionFactoryImpl.java:435) 
    at org.hibernate.event.def.DefaultLoadEventListener.load(DefaultLoadEventListener.java:233) 
    at org.hibernate.event.def.DefaultLoadEventListener.proxyOrLoad(DefaultLoadEventListener.java:285) 
    at org.hibernate.event.def.DefaultLoadEventListener.onLoad(DefaultLoadEventListener.java:152) 
    at org.hibernate.impl.SessionImpl.fireLoad(SessionImpl.java:1090) 
    at org.hibernate.impl.SessionImpl.internalLoad(SessionImpl.java:1038) 
    at org.hibernate.type.EntityType.resolveIdentifier(EntityType.java:630) 
    at org.hibernate.type.EntityType.resolve(EntityType.java:438) 
    at org.hibernate.type.EntityType.replace(EntityType.java:298) 
    at org.hibernate.type.CollectionType.replaceElements(CollectionType.java:508) 
    at org.hibernate.type.CollectionType.replace(CollectionType.java:575) 
    at org.hibernate.type.AbstractType.replace(AbstractType.java:176) 
    at org.hibernate.type.TypeHelper.replaceAssociations(TypeHelper.java:262) 
    at org.hibernate.event.def.DefaultMergeEventListener.copyValues(DefaultMergeEventListener.java:589) 
    at org.hibernate.event.def.DefaultMergeEventListener.mergeTransientEntity(DefaultMergeEventListener.java:389) 
    at org.hibernate.event.def.DefaultMergeEventListener.entityIsTransient(DefaultMergeEventListener.java:303) 
    at org.hibernate.event.def.DefaultMergeEventListener.entityIsDetached(DefaultMergeEventListener.java:464) 
    at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:255) 
    at org.hibernate.event.def.DefaultMergeEventListener.onMerge(DefaultMergeEventListener.java:84) 
    at org.hibernate.impl.SessionImpl.fireMerge(SessionImpl.java:867) 
    at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:851) 
    at org.hibernate.impl.SessionImpl.merge(SessionImpl.java:855) 
    at Main.main(Main.java:16)

请您参考如下方法:

您可以将 merge 与 Hibernate 结合使用，我认为它非常适合您的情况，请看这个问题:Hibernate : Downside of merge() over update()

您可以将 merge 与一个 transient 对象(应该是来自 XML 的对象)和第二个对象一起使用，该对象是附加到 session 的对象(您从 Hibernate 配置的 session 中获得的对象)。如果您之前没有在 session 中显式加载它，调用 merge 将加载它(实际上 Hibernate 检查 1 级缓存)。

然后 Hibernate 获取临时对象，检查与附加对象的差异并将更改应用到附加对象；请注意， transient 字段不会合并。

Merge 返回附加的对象，因此您可以在仍在 Hibernate session 中的同时安全地继续对其进行更改。

所以在你的代码中:

Manufacturer manufacturerDB = session.merge(manufacturerFromXML);

manufacturerDB 将代表您的制造商数据库，您在其中合并了来自 XML 值的值。

如果您不想修改它，另一个简单的解决方案是遍历制造商，然后遍历国家并将每个国家/地区的值(int the manufacturerXML)替换为 countryDB 值(来自 Hibernate by session.load( ...)，您不必担心多次调用相同的 ID，因为 Hibernate 具有 1 级缓存，来自 session 的相同国家/地区是单个值，这要归功于缓存，因此您不会有 NonUnique... 异常)，它将起作用。

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