OneToOne (Java EE 5)

@Target(value={METHOD, FIELD})
@Retention(value=RUNTIME)

此注释定义对另一个带有一对一多样性的实体的单值关联。一般不必显式指定关联的目标实体，因为通常可以根据引用的对象类型推断出来。

示例 1：映射外键列的一对一关联

在 Customer 类中：

    @OneToOne(optional=false)
    @JoinColumn(
    	name="CUSTREC_ID", unique=true, nullable=false, updatable=false)
    public CustomerRecord getCustomerRecord() { return customerRecord; }

在 CustomerRecord 类中：

    @OneToOne(optional=false, mappedBy="customerRecord")
    public Customer getCustomer() { return customer; }

示例 2：假定源和目标共享同一个主键值的一对一关联。 

在 Employee 类中：

    @Entity
    public class Employee {
    	@Id Integer id;
    
    	@OneToOne @PrimaryKeyJoinColumn
    	EmployeeInfo info;
    	...
    }

对于 EmployeeInfo 类：

    @Entity
    public class EmployeeInfo {
    	@Id Integer id;
    	...
    }
 

since

en Java Persistence 1.0

英文文档：

This annotation defines a single-valued association to another entity that has one-to-one multiplicity. It is not normally necessary to specify the associated target entity explicitly since it can usually be inferred from the type of the object being referenced.

    Example 1: One-to-one association that maps a foreign key column

    On Customer class:

    @OneToOne(optional=false)
    @JoinColumn(
        name="CUSTREC_ID", unique=true, nullable=false, updatable=false)
    public CustomerRecord getCustomerRecord() { return customerRecord; }

    On CustomerRecord class:

    @OneToOne(optional=false, mappedBy="customerRecord")
    public Customer getCustomer() { return customer; }

    Example 2: One-to-one association that assumes both the source and target share the same primary key values. 

    On Employee class:

    @Entity
    public class Employee {
        @Id Integer id;
    
        @OneToOne @PrimaryKeyJoinColumn
        EmployeeInfo info;
        ...
    }

    On EmployeeInfo class:

    @Entity
    public class EmployeeInfo {
        @Id Integer id;
        ...
    }
 

Since:

Java Persistence 1.0

---

Optional Element Summary
CascadeType[]	cascade           (Optional) The operations that must be cascaded to the target of the association.
FetchType	fetch           (Optional) Whether the association should be lazily loaded or must be eagerly fetched.
String	mappedBy           (Optional) The field that owns the relationship.
boolean	optional           (Optional) Whether the association is optional.
Class	targetEntity           (Optional) The entity class that is the target of the association.

 

abstract public Class<T> targetEntity()
（可选）作为关联目标的实体类。

默认为存储关联的字段或属性的类型。

英文文档：

targetEntity

public abstract Class targetEntity

(Optional) The entity class that is the target of the association.

Defaults to the type of the field or property that stores the association.

Default:

void.class

---

abstract public CascadeType[] cascade()
（可选）必须串联到关联目标的操作。

默认情况下，不串联任何操作。

英文文档：

cascade

public abstract CascadeType[] cascade

(Optional) The operations that must be cascaded to the target of the association.

By default no operations are cascaded.

Default:

{}

---

abstract public FetchType fetch()
（可选）关联应该延迟加载还是必须立即获取。EAGER 战略是必须立即获取关联实体的持久性提供者运行时的要求。LAZY 战略是持久性提供者运行时的提示。

英文文档：

fetch

public abstract FetchType fetch

(Optional) Whether the association should be lazily loaded or must be eagerly fetched. The EAGER strategy is a requirement on the persistence provider runtime that the associated entity must be eagerly fetched. The LAZY strategy is a hint to the persistence provider runtime.

Default:

EAGER

---

abstract public boolean optional()
（可选）关联是否是可选的。如果设置为 false，则必须总是存在非 null 关系。

英文文档：

optional

public abstract boolean optional

(Optional) Whether the association is optional. If set to false then a non-null relationship must always exist.

Default:

true

---

abstract public String mappedBy()
（可选）拥有关系的字段。此元素仅在关联的反向端（非拥有端）指定。

英文文档：

mappedBy

public abstract String mappedBy

(Optional) The field that owns the relationship. This element is only specified on the inverse (non-owning) side of the association.

Default:

""

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