Hibernate3 annotation 扩展
hibernate3 中用annotation实现数据库和实体的映射关系，那么在保存实体的时候，如果po中有个字段为null，但在数据库中设置了默认值，最终保存的记录中此字段并没有被设置默认值，而是null，解决此种问题，需要在po中添加下列映射代码：
public class Forest { ... }
programlisting
@Entity
@Inheritance(
strategy=InheritanceType.JOINED
)
public class Vegetable { ... }
@Entity
@OnDelete(action=OnDeleteAction.CASCADE)
public class Carrot extends Vegetable { ... }
Identifier
@org.hibernate.annotations.GenericGenerator
allows you to define an Hibernate specific id
generator.
@org.hibernate.annotations.GenericGenerator
允许你定义一个Hibernate特定的id生成器。
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@Id @GeneratedValue(generator="system-uuid")
@GenericGenerator(name="system-uuid", strategy = "uuid")
public String getId() {
@Id @GeneratedValue(generator="hibseq")
@GenericGenerator(name="hibseq", strategy = "seqhilo",
parameters = {
@Parameter(name="max_lo", value = "5"),
@Parameter(name="sequence", value="heybabyhey")
}
)
public Integer getId() {
strategy is the short name of an Hibernate3
generator strategy or the fully qualified class name of an
IdentifierGenerator implementation. You can add
some parameters through the parameters
attribute
strategy可以是Hibernate3生成器策略的简称，或者是一个IdentifierGenerator实现的（带包路径的）全限定类名。
你可以通过parameters属性增加一些参数。
Property
Access type
The access type is guessed from the position of
@Id or @EmbeddedId in the entity
hierarchy. Sub-entities, embedded objects and mapped superclass
inherit the access type from the root entity.
访问类型是根据@Id或@EmbeddedId在实体继承层次中所处的位置推演而得的。子实体（Sub-entities），内嵌对象和被映射的父类均从根实体（root entity）继承访问类型。
In Hibernate, you can override the access type to:
在Hibernate中，你可以把访问类型覆盖成：
use a custom access type strategy
fine tune the access type at the class level or at the
property level
使用定制的访问类型策略
优化类级别或属性级别的访问类型
An @AccessType annotation has been introduced to support this
behavior. You can define the access type on
为支持这种行为，Hibernate引入了@AccessType注解。你可以对以下元素定义访问类型：
an entity
a superclass
an embeddable object
a property
实体
父类
可内嵌的对象
属性
The access type is overriden for the annotated element, if
overriden on a class, all the properties of the given class inherit
the access type. For root entities, the access type is considered to
be the default one for the whole hierarchy (overridable at class or
property level).
被注解元素的访问类型会被覆盖，若覆盖是在类级别上，则所有的属性继承访问类型。
对于根实体，其访问类型会被认为是整个继承层次中的缺省设置（可在类或属性一级覆盖）。
If the access type is marked as "property", the getters are
scanned for annotations, if the access type is marked as "field", the
fields are scanned for annotations. Otherwise the elements marked with
@Id or @embeddedId are scanned.
若访问类型被标以"property"，则Hibernate会扫描getter方法的注解，若访问类型被标以"field"，则扫描字段的注解。否则，扫描标为@Id或@embeddedId的元素。
You can override an access type for a property, but the element
to annotate will not be influenced: for example an entity having
access type field, can annotate a field with
@AccessType("property"), the access type will then
be property for this attribute, the the annotations still have to be
carried on the field.
你可以覆盖某个属性（property）的访问类型，但是受注解的元素将不受影响：例如一个具有field访问类型的实体，（我们）可以将某个字段标注为 @AccessType("property")，则该字段的访问类型随之将成为property，但是其他字段上依然需要携带注解。
If a superclass or an embeddable object is not annotated, the
root entity access type is used (even if an access type has been
define on an intermediate superclass or embeddable object). The
russian doll principle does not apply.
若父类或可内嵌的对象没有被注解，则使用根实体的访问类型（即使已经在非直系父类或可内嵌对象上定义了访问类型）。此时俄罗斯套娃（Russian doll）原理就不再适用。（译注：俄罗斯套娃（матрёшка或 матрешка）是俄罗斯特产木制玩具，一般由多个一样图案的空心木娃娃一个套一个组成，最多可达十多个，通常为圆柱形，底部平坦可以直立。）
programlisting
@Entity
public class Person implements Serializable {
@Id @GeneratedValue //access type field
Integer id;
@Embedded
@AttributeOverrides({
@AttributeOverride(name = "iso2", column = @Column(name = "bornIso2")),
@AttributeOverride(name = "name", column = @Column(name = "bornCountryName"))
})
Country bornIn;
}
@Embeddable
@AccessType("property") //override access type for all properties in Country
public class Country implements Serializable {
private String iso2;
private String name;
public String getIso2() {
return iso2;
}
public void setIso2(String iso2) {
this.iso2 = iso2;
}
@Column(name = "countryName")
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Formula
Sometimes, you want the Database to do some computation for you
rather than in the JVM, you might also create some kind of virtual
column. You can use a SQL fragment (aka formula) instead of mapping a
property into a column. This kind of property is read only (its value
is calculated by your formula fragment).
Sometimes, you want the Database to do some computation for you
rather than in the JVM, you might also create some kind of virtual
column. You can use a SQL fragment (aka formula) instead of mapping a
property into a column. This kind of property is read only (its value
is calculated by your formula fragment).
有时候，你想让数据库，而非JVM，来替你完成一些计算，也可能想创建某种虚拟字段（译注：即数据库视图）。你可以使用一段SQL（亦称为公式），而不是将属性映射到（物理）字段。 这种属性是只读的(属性值由公求得)。
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@Formula("obj_length * obj_height * obj_width")
public long getObjectVolume()
The SQL fragment can be as complex as you want avec even include
subselects.
SQL片段可以是任意复杂的，甚至可包含子查询。
Type
@org.hibernate.annotations.Type overrides the
default hibernate type used: this is generally not necessary since the
type is correctly inferred by Hibernate. Please refer to the Hibernate
reference guide for more informations on the Hibernate types.
@org.hibernate.annotations.Type
覆盖了Hibernate所用的默认类型：这通常不是必须的，因为类型可以由Hibernate正确推得。
关于Hibernate类型的详细信息,请参考Hibernate使用手册。
@org.hibernate.annotations.TypeDef and
@org.hibernate.annotations.TypeDefs allows you to
declare type definitions. These annotations are placed at the class or
package level. Note that these definitions will be global for the
session factory (even at the class level) and that type definition has
to be defined before any usage.
@org.hibernate.annotations.TypeDef 和
@org.hibernate.annotations.TypeDefs允许你来声明类型定义。
这些注解被置于类或包一级。注意，对session factory来说，
这些定义将是全局的（即使定义于类一级），并且类型定义必须先于任何使用。
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@TypeDefs(
{
@TypeDef(
name="caster",
typeClass = CasterStringType.class,
parameters = {
@Parameter(name="cast", value="lower")
}
)
}
)
package org.hibernate.test.annotations.entity;
...
public class Forest {
@Type(type="caster")
public String getSmallText() {
...
}