@IdClass使用JPA和Hibernate生成“实例的标识符已更改”

Question

对于使用不区分大小写的数据库模式的JPA实体模型，当我使用@IdClass批注时，始终会出现“实例标识符已更改”异常。对于具有“字符串”主键的对象，当数据库中存在一种情况的字符串并且使用相同的字符串（仅大小写不同）执行查询时，将发生错误。

我看过其他的SO答案，它们的形式是：a）不要修改主键（我不是），b）equals（）/
hashCode（）实现有缺陷。对于’b’，我尝试使用toLowerCase()和，equalsIgnoringCase()但无济于事。[另外，似乎在“更改”发生时，Hibernate代码直接设置属性，而不是调用属性设置器。

这是具体的错误：

Caused by: javax.persistence.PersistenceException: org.hibernate.HibernateException: 
identifier of an instance of db.Company was altered 
 from {Company.Identity [62109154] ACURA}
   to {Company.Identity [63094242] Acura}

问：对于包含公司“ Acura”（作为主键）的不区分大小写的数据库，如何使用@IdClass随后查找其他大写字母？

这是有问题的代码（从一个空的数据库开始）：

public class Main {    
    public static void main(String[] args) {
        EntityManagerFactory emf =
                Persistence.createEntityManagerFactory("mobile.mysql");
        EntityManager em = emf.createEntityManager();

        em.getTransaction().begin();

        Company c1 = new Company ("Acura");
        em.persist(c1);

        em.getTransaction().commit();
        em.getTransaction().begin();

        c1 = em.find (Company.class, new Company.Identity("ACURA"));

        em.getTransaction().commit();
        em.close();
        System.exit (0);    
    }
}

这是“ db.Company”实现：

@Entity
@IdClass(Company.Identity.class)
public class Company implements Serializable {

    @Id
    protected String name;

    public Company(String name) {
        this.name = name;
    }

    public Company() { }

    @Override
    public int hashCode () {
        return name.hashCode();
    }

    @Override
    public boolean equals (Object that) {
        return this == that ||
                (that instanceof Company &&
                        this.name.equals(((Company) that).name));}

    @Override
    public String toString () {
        return "{Company@" + hashCode() + " " + name + "}";
    }

    //

    public static class Identity implements Serializable {
        protected String name;

        public Identity(String name) {
            this.name = name;
        }

        public Identity() { }

        @Override
        public int hashCode () {
            return name.hashCode();
        }

        @Override
        public boolean equals (Object that) {
            return this == that ||
                    (that instanceof Identity &&
                        this.name.equals(((Identity)that).name));
        }

        @Override
        public String toString () {
            return "{Company.Identity [" + hashCode() + "] " + name + "}";
        }
    }
}

注意：我知道@IdClass只有一个主键时不需要使用；上面是问题的最简单的例子。

如我所说，即使将hashCode（）/ equals（）方法区分大小写，我仍然认为该问题仍然存在。但是，建议。

...
INFO: HHH000232: Schema update complete
Hibernate: insert into Company (name) values (?)
Hibernate: select company0_.name as name1_0_0_ from Company company0_ where company0_.name=?
Exception in thread "main" javax.persistence.RollbackException: Error while committing the transaction
    at org.hibernate.jpa.internal.TransactionImpl.commit(TransactionImpl.java:94)
    at com.lambdaspace.Main.main(Main.java:24)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Caused by: javax.persistence.PersistenceException: org.hibernate.HibernateException: identifier of an instance of db.Company was altered from {Company.Identity [62109154] ACURA} to {Company.Identity [63094242] Acura}
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1763)
    at org.hibernate.jpa.spi.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1677)
    at org.hibernate.jpa.internal.TransactionImpl.commit(TransactionImpl.java:82)
    ... 6 more
Caused by: org.hibernate.HibernateException: identifier of an instance of db.Company was altered from {Company.Identity [62109154] ACURA} to {Company.Identity [63094242] Acura}
    at org.hibernate.event.internal.DefaultFlushEntityEventListener.checkId(DefaultFlushEntityEventListener.java:80)
    at org.hibernate.event.internal.DefaultFlushEntityEventListener.getValues(DefaultFlushEntityEventListener.java:192)
    at org.hibernate.event.internal.DefaultFlushEntityEventListener.onFlushEntity(DefaultFlushEntityEventListener.java:152)
    at org.hibernate.event.internal.AbstractFlushingEventListener.flushEntities(AbstractFlushingEventListener.java:231)
    at org.hibernate.event.internal.AbstractFlushingEventListener.flushEverythingToExecutions(AbstractFlushingEventListener.java:102)
    at org.hibernate.event.internal.DefaultFlushEventListener.onFlush(DefaultFlushEventListener.java:55)
    at org.hibernate.internal.SessionImpl.flush(SessionImpl.java:1222)
    at org.hibernate.internal.SessionImpl.managedFlush(SessionImpl.java:425)
    at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.beforeTransactionCommit(JdbcTransaction.java:101)
    at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.commit(AbstractTransactionImpl.java:177)
    at org.hibernate.jpa.internal.TransactionImpl.commit(TransactionImpl.java:77)
    ... 6 more

Answer 1

发生此错误的原因是由于更改了受管实体的实体标识符。

在PersistenceContext的生存期内，任何给定实体都可能只有一个托管实例。为此，您不能更改现有的管理实体标识符。

在您的示例中，即使您开始一个新事务，也必须记住PersistenContext尚未关闭，因此您仍然将一个托管c1实体连接到Hibernate
Session。

当您尝试找到公司时：

c1 = em.find (Company.class, new Company.Identity("ACURA"));

该标识符与当前会话所附公司的标识符不匹配，因此发出查询：

Hibernate: select company0_.name as name1_0_0_ from Company company0_ where company0_.name=?

因为SQL是CASE INSENSITIVE，所以您实际上将选择与当前托管的Company实体（persisted c1）相同的数据库行。

但是同一数据库行只能有一个受管实体，因此Hibernate将重用该受管实体实例，但它将标识符更新为：

new Company.Identity("ACURA");

您可以通过以下测试来检查此假设：

String oldId = c1.name;
Company c2 = em.find (Company.class, new Company.Identity("ACURA"));
assertSame(c1, c2);
assertFalse(oldId.equals(c2.name));

提交第二个事务时，刷新将尝试更新实体标识符（已从“ Acura”更改为“
ACURA”），因此DefaultFlushEntityEventListener.checkId（）方法将失败。

根据JavaDoc的说法，此检查适用于：

确保用户未篡改ID

要解决此问题，您需要删除以下find方法调用：

c1 = em.find (Company.class, new Company.Identity("ACURA"));

您可以检查c1是否已附加：

assertTrue(em.contains(c1));