OneToOne (Java EE 5)

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;
    	...
    }
 

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

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:

""

Copyright 2007 Sun Microsystems, Inc. All rights reserved. Use is subject to license terms.