@IdClass使用JPA和Hibernate生成'实例的标识符'

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注释时,我始终得到"实例的标识符被更改"异常.对于具有"字符串"主键的对象,当数据库中存在一个案例的字符串并且仅使用大小写相同的字符串执行查询时,会发生错误.

我查看了其他SO答案,它们的形式如下:a)不修改主键(我不是)和b)你的equals()/ hashCode()实现有缺陷.对于"B"我已经尝试使用toLowerCase()和equalsIgnoringCase(),但无济于事.[此外,似乎Hibernate代码直接设置属性,而不是在发生'更改'时调用属性设置器.

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DB,使用@IdClass如何随后查找其他大写字母？

这是有问题的代码(从空数据库开始):

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当有一个主键时不需要使用; 以上是问题的最简单的例子.

正如我所说,我相信即使hashCode()/ equals()方法不区分大小写,这个问题仍然存在; 但是,建议采取.

...
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

出现此错误的原因是由于更改了托管实体的实体标识符.

在PersistenceContext的生命周期中,任何给定实体都可以有一个且只有一个托管实例.为此,您无法更改现有的管理实体标识符.

在您的示例中,即使您启动了一个新事务,您也必须记住PersistenContext尚未关闭,因此您仍然有一个c1附加到Hibernate会话的托管实体.

当您试图找到公司时:

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

标识符与附加到当前会话的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=?

由于SQL是CASE INSENSITIVE,因此您实际上将选择与当前托管公司实体(持久性c1)相同的数据库行.

但是,对于同一个数据库行,您只能有一个托管实体,因此Hibernate将重用托管实体实例,但它会将标识符更新为:

new Company.Identity("ACURA");

您可以使用以下测试检查此假设:

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,flush将尝试更新实体标识符(从'Acura'更改为'ACURA'),因此DefaultFlushEntityEventListener.checkId()方法将失败.

据JavaDoc说,这项检查适用于:

make(ing)确定(用户)没有破坏id

要解决此问题,您需要删除此find方法调用:

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

您可以检查c1已经附加的内容:

assertTrue(em.contains(c1));