JPA 多对一实体映射答案

【问题标题】：JPA ManyToOne entity mappingJPA 多对一实体映射
【发布时间】：2015-07-02 01:07:39
【问题描述】：

在部署具有使用“OneToMany”映射的实体的 Web 项目时遇到问题。这是第一个实体：

    @Entity 
    @Table(name = "OWNER")
    public class Owner implements Serializable
    {
        @Id
        @Column(name = "ID", nullable = false)
        private double ID;

        @Column(name = "NAME", nullable = false, length = 16)
        private String name;

        @OneToMany(cascade={CascadeType.ALL}, mappedBy="USER_ID", targetEntity=Asset.class)
        private List<Asset> assets;
    }

这是依赖于复合键的第二个实体：

@Entity
@IdClass(AssetKey.class)
@Table(name = "ASSETS")
public class Asset implements Serializable
{
    @Id
    @ManyToOne
    @JoinColumn(name = "USER_ID")
    private double USER_ID;

    @Id
    @Column(name = "ASSET_ID")
    private String asset;
}

这里是关键：

public class ConstraintAnchorAssetImplKey implements Serializable
{
    private double CA_ID;
    private String asset;

    //Also includes overriden hash/equals
}

部署后我收到一个不起眼的 NullPointerException：

12:59:59,841 ERROR [org.jboss.msc.service.fail] (ServerService Thread Pool -- 50) MSC000001: Failed to start service jboss.persistenceunit."MyProj.war#MyProjJPA": org.jboss.msc.service.StartException in service jboss.persistenceunit."MyProj.war#MyProjJPA": java.lang.NullPointerException 
at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1.run(PersistenceUnitServiceImpl.java:103) [jboss-as-jpa-7.4.3.Final-redhat-2.jar:7.4.3.Final-redhat-2]  
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [rt.jar:1.8.0_05]    
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [rt.jar:1.8.0_05]    
at java.lang.Thread.run(Thread.java:745) [rt.jar:1.8.0_05]  
at org.jboss.threads.JBossThread.run(JBossThread.java:122) [jboss-threads-2.1.1.Final-redhat-1.jar:2.1.1.Final-redhat-1] 
Caused by: java.lang.NullPointerException   
at org.hibernate.cfg.Configuration.processFkSecondPassInOrder(Configuration.java:1461)  
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1391)   
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1786)     at org.hibernate.ejb.EntityManagerFactoryImpl.<init>(EntityManagerFactoryImpl.java:96)  
at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:915)    
at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:900)    
at org.hibernate.ejb.HibernatePersistence.createContainerEntityManagerFactory(HibernatePersistence.java:76)     
at org.jboss.as.jpa.service.PersistenceUnitServiceImpl.createContainerEntityManagerFactory(PersistenceUnitServiceImpl.java:200) [jboss-as-jpa-7.4.3.Final-redhat-2.jar:7.4.3.Final-redhat-2]    
at org.jboss.as.jpa.service.PersistenceUnitServiceImpl.access$600(PersistenceUnitServiceImpl.java:57) [jboss-as-jpa-7.4.3.Final-redhat-2.jar:7.4.3.Final-redhat-2]  
at org.jboss.as.jpa.service.PersistenceUnitServiceImpl$1.run(PersistenceUnitServiceImpl.java:99) [jboss-as-jpa-7.4.3.Final-redhat-2.jar:7.4.3.Final-redhat-2]   ... 4 more

OneToMany/ManyToOne 注释引入了此异常。以前的成功是通过简单地分别填充/持久化两个实体来实现的，这当然不理想。

感谢您的建议！

【问题讨论】：

你有没有试过调试代码。
这是关于 web 项目的部署。我对调试休眠没有兴趣，老实说我不确定我是否有能力。我希望这将是我的注释中的一个简单/可识别的错误。

标签： java hibernate jpa persistence

【解决方案1】：

您是否尝试替换：

@OneToMany(cascade={CascadeType.ALL}, mappedBy="USER_ID", targetEntity=Asset.class)
private List<Asset> assets;

与：

@OneToMany(cascade={CascadeType.ALL}, mappedBy="USER_ID", targetEntity=Asset.class)
private Set<Asset> assets = new HashSet<Asset>(0);

或：

@OneToMany(cascade={CascadeType.ALL}, mappedBy="USER_ID", targetEntity=Asset.class)
private List<Asset> assets = new ArrayList<Asset>(0);

希望这可以帮助您解决问题！

【讨论】：