@IdClass 使用 JPA 和 Hibernate 生成“实例的标识符已更改”

对于使用不区分大小写的数据库架构的 JPA 实体模型，当我使用 @IdClass 注释时，我始终会收到“实例的标识符已更改”异常。对于具有“字符串”主键的对象，当数据库中存在一种大小写的字符串并且使用仅大小写不同的相同字符串执行查询时，会发生错误。

我看过其他答案，它们的形式是：a）不要修改主键（我不是）和b）你的 equals()/hashCode() 实现有缺陷。对于“b”我尝试使用toLowerCase() and 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）的不区分大小写的数据库，使用 @IdClass 我随后如何找到其他大写字母？

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”的实现：

@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：我知道使用@IdClass当只有一个主键时不需要；上面是这个问题最简单的例子。

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；不过，已采纳建议。

...
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

此错误的原因是由于更改了托管实体的实体标识符。

在 PersistenceContext 的生命周期内，任何给定实体只能有一个托管实例。为此，您无法更改现有的托管实体标识符。

在您的示例中，即使您启动一个新事务，您也必须记住 PersistenContext 尚未关闭，因此您仍然有一个托管的c1附加到 Hibernate 会话的实体。

当您尝试寻找该公司时：

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

该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 不区分大小写，因此您实际上将选择与当前托管 Company 实体相同的数据库行（持久化的c1).

但是同一数据库行只能有一个托管实体，因此 Hibernate 将重用托管实体实例，但会将标识符更新为：

new Company.Identity("ACURA");

您可以通过以下测试来检查该假设：

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

当第二个事务提交时，刷新将尝试更新实体标识符（从“Acura”更改为“ACURA”），因此DefaultFlushEntityEventListener.checkId() https://docs.jboss.org/hibernate/orm/4.3/javadocs/org/hibernate/event/internal/DefaultFlushEntityEventListener.html#checkId%28java.lang.Object,%20org.hibernate.persister.entity.EntityPersister,%20java.io.Serializable,%20org.hibernate.engine.spi.SessionImplementor%29方法将会失败。

根据 JavaDoc，此检查用于：

确保用户没有破坏 id

要修复它，您需要删除此 find 方法调用：

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

你可以检查一下c1已经附上：

assertTrue(em.contains(c1));