JPA：在fied和getter上设置@Id之间的区别

Question

我使用EclipseLink，结果很奇怪。请考虑下面的代码：

此代码：

@Entity 
@Table(name = "someTable") 
public class SomeClass{ 
    @Id// PAY ATTENTION TO THIS LINE 
    private String id; 

    public String getId() { 
     return id; 
    } 

    public void setId(String id) { 
     this.id = id; 
    } 

    @Column (name = "somecol")// PAY ATTENTION TO THIS LINE 
    private String name; 

    public String getName() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 
} 

此代码也可以工作：

@Entity 
@Table(name = "someTable") 
public class SomeClass{ 
    @Id// PAY ATTENTION TO THIS LINE 
    private String id; 

    public String getId() { 
     return id; 
    } 

    public void setId(String id) { 
     this.id = id; 
    } 

    //@Column (name = "somecol")// PAY ATTENTION TO THIS LINE 
    private String name; 

    public String getName() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 
} 

此代码也可以工作：

@Entity 
@Table(name = "someTable") 
public class SomeClass{ 

    private String id; 

    @Id// PAY ATTENTION TO THIS LINE 
    public String getId() { 
     return id; 
    } 

    public void setId(String id) { 
     this.id = id; 
    } 

    //@Column (name = "somecol")// PAY ATTENTION TO THIS LINE 
    private String name; 

    public String getName() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 
} 

此代码没有按” T工作：

@Entity 
@Table(name = "someTable") 
public class SomeClass{ 

    private String id; 

    @Id // PAY ATTENTION TO THIS LINE 
    public String getId() { 
     return id; 
    } 

    public void setId(String id) { 
     this.id = id; 
    } 

    @Column (name = "somecol")// PAY ATTENTION TO THIS LINE 
    private String name; 

    public String getName() { 
     return name; 
    } 

    public void setName(String name) { 
     this.name = name; 
    } 
} 

我得到以下异常：

Exception Description: Entity class [class SomeClass] has no primary key specified. It should define either an @Id, @EmbeddedId or an @IdClass. If you have defined PK using any of these annotations then make sure that you do not have mixed access-type (both fields and properties annotated) in your entity class hierarchy. 
    at org.eclipse.persistence.exceptions.ValidationException.noPrimaryKeyAnnotationsFound(ValidationException.java:1425) 
    at org.eclipse.persistence.internal.jpa.metadata.accessors.classes.EntityAccessor.validatePrimaryKey(EntityAccessor.java:1542) 
    at org.eclipse.persistence.internal.jpa.metadata.accessors.classes.EntityAccessor.processMappingAccessors(EntityAccessor.java:1249) 
    at org.eclipse.persistence.internal.jpa.metadata.accessors.classes.EntityAccessor.process(EntityAccessor.java:699) 
    at org.eclipse.persistence.internal.jpa.metadata.MetadataProject.processStage2(MetadataProject.java:1808) 
    at org.eclipse.persistence.internal.jpa.metadata.MetadataProcessor.processORMMetadata(MetadataProcessor.java:573) 
    at org.eclipse.persistence.internal.jpa.deployment.PersistenceUnitProcessor.processORMetadata(PersistenceUnitProcessor.java:607) 
    at org.eclipse.persistence.internal.jpa.EntityManagerSetupImpl.predeploy(EntityManagerSetupImpl.java:1948) 
    at org.eclipse.persistence.internal.jpa.deployment.JPAInitializer.callPredeploy(JPAInitializer.java:100) 
    at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactoryImpl(PersistenceProvider.java:104) 
    at org.eclipse.persistence.jpa.PersistenceProvider.createEntityManagerFactory(PersistenceProvider.java:188) 
    at org.eclipse.gemini.jpa.ProviderWrapper.createEntityManagerFactory(ProviderWrapper.java:128) 
    at org.eclipse.gemini.jpa.proxy.EMFServiceProxyHandler.createEMF(EMFServiceProxyHandler.java:151) 
    at org.eclipse.gemini.jpa.proxy.EMFServiceProxyHandler.syncGetEMFAndSetIfAbsent(EMFServiceProxyHandler.java:127) 
    at org.eclipse.gemini.jpa.proxy.EMFServiceProxyHandler.invoke(EMFServiceProxyHandler.java:73) 
    at com.sun.proxy.$Proxy8.createEntityManager(Unknown Source) 

为什么不过去的代码工作的？如何解释它？

Answer 1

这是因为有像@Access这样的东西，如果您想使用混合模式，您必须在实体和字段级别上指定它。有两个值AccessType.PROPERTY和AccesType.FIELD。

默认访问类型由您放置标识符注释的位置（@Id）定义。如果你把它放在场上 - 这将是AccessType.FIELD，如果你把它放在吸气 - 它将是AccessType.PROPERTY。 - 编辑，未由JPA定义。

如果你想注解不是字段，但属性（字段中仍有@Id），你必须定义一个getter并将其注释为AccessType.PROPERTY。 （反之亦然，对于吸气剂@Id）。