Java - JPA @Basic和@Embedded注释 [英] Java - JPA @Basic and @Embedded annotations

问题描述

我正在从本教程。

我对理解以下注释有一些困惑：

I have some confusions in understanding the following annotations:

• @Basic


• @Embedded

• @Basic
• @Embedded

可嵌入类型的字段默认为持久化，就像使用@Embedded注释一样。

Fields of an embeddable type default to persistent, as if annotated with @Embedded.

如果embeddable类型的字段默认为持久性，那么为什么我们需要 @Embedded 注释

If the fields of embeddable types default to persistent, then why would we need the @Embedded annotation

推荐答案

@Embeddable 注释允许指定存储实例的类作为拥有实体的固有部分。此注释具有无属性。

The @Embeddable annotation allows to specify a class whose instances are stored as intrinsic part of the owning entity. This annotation has no attributes.

@Embeddable
public class EmploymentPeriod {
     java.util.Date startDate;
     java.util.Date endDate;
     ...
}

@Embedded 注释用于指定实体的持久字段或属性，该实体的值是可嵌入类的实例。默认情况下， @Embeddable 类中指定的列定义适用于拥有实体的表，但您可以使用 > @AttributeOverride ：

The @Embedded annotation is used to specify a persistent field or property of an entity whose value is an instance of an embeddable class. By default, column definitions specified in the @Embeddable class apply to the table of the owning entity but you can override them using@AttributeOverride:

@Embedded
@AttributeOverrides({
    @AttributeOverride(name="startDate", column=@Column(name="EMP_START")),
    @AttributeOverride(name="endDate", column=@Column(name="EMP_END"))
})
public EmploymentPeriod getEmploymentPeriod() { ... }

---

关于可选的 @Basic 注释，您可以使用它将获取类型配置为 LAZY 并使用可选属性将映射配置为禁止空值（对于非基本类型）。

---

Regarding the optional @Basic annotation, you may use it to configure the fetch type to LAZY and to configure the mapping to forbid null values (for non primitive types) with the optional attribute.

@Basic(fetch=LAZY)
protected String getName() { return name; }

您也可以将它放在字段或属性上以明确标记为持久性（用于文档目的） ）。

You can also place it on a field or property to explicitly mark it as persistent (for documentation purpose).

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