- 5.1. 映射定义（Mapping declaration）
- 5.1.1. Doctype
- 5.1.2. Hibernate-mapping
- 5.1.3. Class
- 5.1.4. id
- 5.1.5. Enhanced identifier generators
- 5.1.6. Identifier generator optimization
- 5.1.7. composite-id
- 5.1.8. Discriminator
- 5.1.9. Version (optional)
- 5.1.10. Timestamp (optional)
- 5.1.11. Property
- 5.1.12. Many-to-one
- 5.1.13. One-to-one
- 5.1.14. Natural-id
- 5.1.15. Component and dynamic-component
- 5.1.16. Properties
- 5.1.17. Subclass
- 5.1.18. Joined-subclass
- 5.1.19. Union-subclass
- 5.1.20. Join
- 5.1.21. Key
- 5.1.22. Column and formula elements
- 5.1.23. Import
- 5.1.24. Any
- 5.2. Hibernate types
- 5.3. 多次映射同一个类
- 5.4. SQL中引号包围的标识符
- 5.5. 其他元数据(Metadata)
- 5.6. Generated properties
- 5.7. Auxiliary database objects
Object/relational mappings are usually defined in an XML document. The mapping document is designed to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations and not table declarations.
Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.
Here is an example mapping:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat"
table="cats"
discriminator-value="C">
<id name="id">
<generator class="native"/>
</id>
<discriminator column="subclass"
type="character"/>
<property name="weight"/>
<property name="birthdate"
type="date"
not-null="true"
update="false"/>
<property name="color"
type="eg.types.ColorUserType"
not-null="true"
update="false"/>
<property name="sex"
not-null="true"
update="false"/>
<property name="litterId"
column="litterId"
update="false"/>
<many-to-one name="mother"
column="mother_id"
update="false"/>
<set name="kittens"
inverse="true"
order-by="litter_id">
<key column="mother_id"/>
<one-to-many class="Cat"/>
</set>
<subclass name="DomesticCat"
discriminator-value="D">
<property name="name"
type="string"/>
</subclass>
</class>
<class name="Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>We will now discuss the content of the mapping document. We will only describe, however, the document elements and attributes that are used by Hibernate at runtime. The mapping document also contains some extra optional attributes and elements that affect the database schemas exported by the schema export tool (for example, the not-null attribute).
All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above, in the directory hibernate-x.x.x/src/org/hibernate , or in hibernate3.jar. Hibernate will always look for the DTD in its classpath first. If you experience lookups of the DTD using an Internet connection, check the DTD declaration against the contents of your classpath.
Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a custom org.xml.sax.EntityResolver implementation with the SAXReader it uses to read in the xml files. This custom EntityResolver recognizes two different systemId namespaces:
a
hibernate namespaceis recognized whenever the resolver encounters a systemId starting withhttp://hibernate.sourceforge.net/. The resolver attempts to resolve these entities via the classloader which loaded the Hibernate classes.a
user namespaceis recognized whenever the resolver encounters a systemId using aclasspath://URL protocol. The resolver will attempt to resolve these entities via (1) the current thread context classloader and (2) the classloader which loaded the Hibernate classes.
The following is an example of utilizing user namespacing:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [
<!ENTITY types SYSTEM "classpath://your/domain/types.xml">
]>
<hibernate-mapping package="your.domain">
<class name="MyEntity">
<id name="id" type="my-custom-id-type">
...
</id>
<class>
&types;
</hibernate-mapping>Where types.xml is a resource in the your.domain package and contains a custom typedef.
This element has several optional attributes. The schema and catalog attributes specify that tables referred to in this mapping belong to the named schema and/or catalog. If they are specified, tablenames will be qualified by the given schema and catalog names. If they are missing, tablenames will be unqualified. The default-cascade attribute specifies what cascade style should be assumed for properties and collections that do not specify a cascade attribute. By default, the auto-import attribute allows you to use unqualified class names in the query language.
<hibernate-mapping
schema="schemaName" 
catalog="catalogName" 
default-cascade="cascade_style" 
default-access="field|property|ClassName" 
default-lazy="true|false" 
auto-import="true|false" 
package="package.name" 
/> |
|
|
|
|
|
|
|
|
|
|
|
|
|
If you have two persistent classes with the same unqualified name, you should set auto-import="false". An exception will result if you attempt to assign two classes to the same "imported" name.
The hibernate-mapping element allows you to nest several persistent <class> mappings, as shown above. It is, however, good practice (and expected by some tools) to map only a single persistent class, or a single class hierarchy, in one mapping file and name it after the persistent superclass. For example, Cat.hbm.xml, Dog.hbm.xml, or if using inheritance, Animal.hbm.xml.
You can declare a persistent class using the class element. For example:
<class
name="ClassName"
table="tableName"
discriminator-value="discriminator_value"
mutable="true|false"
schema="owner"
catalog="catalog"
proxy="ProxyInterface"
dynamic-update="true|false" 
dynamic-insert="true|false" 
select-before-update="true|false" 
polymorphism="implicit|explicit" 
where="arbitrary sql where condition" 
persister="PersisterClass" 
batch-size="N" 
optimistic-lock="none|version|dirty|all" 
lazy="true|false" (16)
entity-name="EntityName" (17)
check="arbitrary sql check condition" (18)
rowid="rowid" (19)
subselect="SQL expression" (20)
abstract="true|false" (21)
node="element-name"
/> |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| (16) |
|
| (17) |
|
| (18) |
|
| (19) |
|
| (20) |
|
| (21) |
|
It is acceptable for the named persistent class to be an interface. You can declare implementing classes of that interface using the <subclass> element. You can persist any static inner class. Specify the class name using the standard form i.e. e.g.Foo$Bar.
Immutable classes, mutable="false", cannot be updated or deleted by the application. This allows Hibernate to make some minor performance optimizations.
The optional proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies that implement the named interface. The persistent object will load when a method of the proxy is invoked. See "Initializing collections and proxies" below.
Implicit polymorphism means that instances of the class will be returned by a query that names any superclass or implemented interface or class, and that instances of any subclass of the class will be returned by a query that names the class itself. Explicit polymorphism means that class instances will be returned only by queries that explicitly name that class. Queries that name the class will return only instances of subclasses mapped inside this <class> declaration as a <subclass> or <joined-subclass>. For most purposes, the default polymorphism="implicit" is appropriate. Explicit polymorphism is useful when two different classes are mapped to the same table This allows a "lightweight" class that contains a subset of the table columns.
The persister attribute lets you customize the persistence strategy used for the class. You can, for example, specify your own subclass of org.hibernate.persister.EntityPersister, or you can even provide a completely new implementation of the interface org.hibernate.persister.ClassPersister that implements, for example, persistence via stored procedure calls, serialization to flat files or LDAP. See org.hibernate.test.CustomPersister for a simple example of "persistence" to a Hashtable.
The dynamic-update and dynamic-insert settings are not inherited by subclasses, so they can also be specified on the <subclass> or <joined-subclass> elements. Although these settings can increase performance in some cases, they can actually decrease performance in others.
Use of select-before-update will usually decrease performance. It is useful to prevent a database update trigger being called unnecessarily if you reattach a graph of detached instances to a Session.
如果你打开了dynamic-update，你可以选择几种乐观锁定的策略：
version: check the version/timestamp columnsall: check all columnsdirty: check the changed columns, allowing some concurrent updatesnone: do not use optimistic locking
It is strongly recommended that you use version/timestamp columns for optimistic locking with Hibernate. This strategy optimizes performance and correctly handles modifications made to detached instances (i.e. when Session.merge() is used).
There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level, although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but you cannot create one in the database (i.e. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression:
<class name="Summary">
<subselect>
select item.name, max(bid.amount), count(*)
from item
join bid on bid.item_id = item.id
group by item.name
</subselect>
<synchronize table="item"/>
<synchronize table="bid"/>
<id name="name"/>
...
</class>Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that queries against the derived entity do not return stale data. The <subselect> is available both as an attribute and a nested mapping element.
被映射的类必须定义对应数据库表主键字段。大多数类有一个JavaBeans风格的属性， 为每一个实例包含唯一的标识。<id> 元素定义了该属性到数据库表主键字段的映射。
<id
name="propertyName"
type="typename"
column="column_name"
unsaved-value="null|any|none|undefined|id_value"
access="field|property|ClassName">
node="element-name|@attribute-name|element/@attribute|."
<generator class="generatorClass"/>
</id> |
|
|
|
|
|
|
|
|
|
如果 name属性不存在，会认为这个类没有标识属性。
unsaved-value 属性在Hibernate3中几乎不再需要。
There is an alternative <composite-id> declaration that allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.
可选的<generator>子元素是一个Java类的名字， 用来为该持久化类的实例生成唯一的标识。如果这个生成器实例需要某些配置值或者初始化参数， 用<param>元素来传递。
<id name="id" type="long" column="cat_id">
<generator class="org.hibernate.id.TableHiLoGenerator">
<param name="table">uid_table</param>
<param name="column">next_hi_value_column</param>
</generator>
</id>All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple interface. Some applications can choose to provide their own specialized implementations, however, Hibernate provides a range of built-in implementations. The shortcut names for the built-in generators are as follows:
increment用于为
long,short或者int类型生成 唯一标识。只有在没有其他进程往同一张表中插入数据时才能使用。 在集群下不要使用。identity对DB2,MySQL, MS SQL Server, Sybase和HypersonicSQL的内置标识字段提供支持。 返回的标识符是
long,short或者int类型的。sequence在DB2,PostgreSQL, Oracle, SAP DB, McKoi中使用序列（sequence)， 而在Interbase中使用生成器(generator)。返回的标识符是
long,short或者int类型的。hilo使用一个高/低位算法高效的生成
long,short或者int类型的标识符。给定一个表和字段（默认分别是hibernate_unique_key和next_hi）作为高位值的来源。 高/低位算法生成的标识符只在一个特定的数据库中是唯一的。seqhilo使用一个高/低位算法来高效的生成
long,short或者int类型的标识符，给定一个数据库序列（sequence)的名字。uuiduses a 128-bit UUID algorithm to generate identifiers of type string that are unique within a network (the IP address is used). The UUID is encoded as a string of 32 hexadecimal digits in length.
guid在MS SQL Server 和 MySQL 中使用数据库生成的GUID字符串。
nativeselects
identity,sequenceorhilodepending upon the capabilities of the underlying database.assignedlets the application assign an identifier to the object before
save()is called. This is the default strategy if no<generator>element is specified.selectretrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving the primary key value.
foreignuses the identifier of another associated object. It is usually used in conjunction with a
<one-to-one>primary key association.sequence-identitya specialized sequence generation strategy that utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on these insert statements are disabled due to a bug in the Oracle drivers.
The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm. The first implementation requires a "special" database table to hold the next available "hi" value. Where supported, the second uses an Oracle-style sequence.
<id name="id" type="long" column="cat_id">
<generator class="hilo">
<param name="table">hi_value</param>
<param name="column">next_value</param>
<param name="max_lo">100</param>
</generator>
</id><id name="id" type="long" column="cat_id">
<generator class="seqhilo">
<param name="sequence">hi_value</param>
<param name="max_lo">100</param>
</generator>
</id>Unfortunately, you cannot use hilo when supplying your own Connection to Hibernate. When Hibernate uses an application server datasource to obtain connections enlisted with JTA, you must configure the hibernate.transaction.manager_lookup_class.
The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so this is the best option without using JNI.
For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can use identity key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can use sequence style key generation. Both of these strategies require two SQL queries to insert a new object. For example:
<id name="id" type="long" column="person_id">
<generator class="sequence">
<param name="sequence">person_id_sequence</param>
</generator>
</id><id name="id" type="long" column="person_id" unsaved-value="0">
<generator class="identity"/>
</id>For cross-platform development, the native strategy will, depending on the capabilities of the underlying database, choose from the identity, sequence and hilo strategies.
If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can use the assigned generator. This special generator uses the identifier value already assigned to the object's identifier property. The generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do not specify a <generator> element.
The assigned generator makes Hibernate use unsaved-value="undefined". This forces Hibernate to go to the database to determine if an instance is transient or detached, unless there is a version or timestamp property, or you define Interceptor.isUnsaved().
Hibernate does not generate DDL with triggers. It is for legacy schemas only.
<id name="id" type="long" column="person_id">
<generator class="select">
<param name="key">socialSecurityNumber</param>
</generator>
</id>In the above example, there is a unique valued property named socialSecurityNumber. It is defined by the class, as a natural key and a surrogate key named person_id, whose value is generated by a trigger.
Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifier generation. The first aspect is database portability; the second is optimization Optimization means that you do not have to query the database for every request for a new identifier value. These two new generators are intended to take the place of some of the named generators described above, starting in 3.3.x. However, they are included in the current releases and can be referenced by FQN.
The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator which is intended, firstly, as a replacement for the sequence generator and, secondly, as a better portability generator than native. This is because native generally chooses between identity and sequence which have largely different semantics that can cause subtle issues in applications eyeing portability. org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in a different manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on the capabilities of the dialect being used. The difference between this and native is that table-based and sequence-based storage have the same exact semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-based generators. This generator has a number of configuration parameters:
sequence_name(optional, defaults tohibernate_sequence): the name of the sequence or table to be used.initial_value(optional, defaults to1): the initial value to be retrieved from the sequence/table. In sequence creation terms, this is analogous to the clause typically named "STARTS WITH".increment_size(optional - defaults to1): the value by which subsequent calls to the sequence/table should differ. In sequence creation terms, this is analogous to the clause typically named "INCREMENT BY".force_table_use(optional - defaults tofalse): should we force the use of a table as the backing structure even though the dialect might support sequence?value_column(optional - defaults tonext_val): only relevant for table structures, it is the name of the column on the table which is used to hold the value.optimizer(optional - defaults tonone): See 第 5.1.6 节 “Identifier generator optimization”
The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which is intended, firstly, as a replacement for the table generator, even though it actually functions much more like org.hibernate.id.MultipleHiLoPerTableGenerator, and secondly, as a re-implementation of org.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the notion of pluggable optimizers. Essentially this generator defines a table capable of holding a number of different increment values simultaneously by using multiple distinctly keyed rows. This generator has a number of configuration parameters:
table_name(optional - defaults tohibernate_sequences): the name of the table to be used.value_column_name(optional - defaults tonext_val): the name of the column on the table that is used to hold the value.segment_column_name(optional - defaults tosequence_name): the name of the column on the table that is used to hold the "segment key". This is the value which identifies which increment value to use.segment_value(optional - defaults todefault): The "segment key" value for the segment from which we want to pull increment values for this generator.segment_value_length(optional - defaults to255): Used for schema generation; the column size to create this segment key column.initial_value(optional - defaults to1): The initial value to be retrieved from the table.increment_size(optional - defaults to1): The value by which subsequent calls to the table should differ.optimizer(optional - defaults to): See 第 5.1.6 节 “Identifier generator optimization”
For identifier generators that store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value. Instead, you can group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators (第 5.1.5 节 “Enhanced identifier generators” support this operation.
none(generally this is the default if no optimizer was specified): this will not perform any optimizations and hit the database for each and every request.hilo: applies a hi/lo algorithm around the database retrieved values. The values from the database for this optimizer are expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "group number". Theincrement_sizeis multiplied by that value in memory to define a group "hi value".pooled: as with the case ofhilo, this optimizer attempts to minimize the number of hits to the database. Here, however, we simply store the starting value for the "next group" into the database structure rather than a sequential value in combination with an in-memory grouping algorithm. Here,increment_sizerefers to the values coming from the database.
<composite-id
name="propertyName"
class="ClassName"
mapped="true|false"
access="field|property|ClassName">
node="element-name|."
<key-property name="propertyName" type="typename" column="column_name"/>
<key-many-to-one name="propertyName class="ClassName" column="column_name"/>
......
</composite-id>A table with a composite key can be mapped with multiple properties of the class as identifier properties. The <composite-id> element accepts <key-property> property mappings and <key-many-to-one> mappings as child elements.
<composite-id>
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>The persistent class must override equals() and hashCode() to implement composite identifier equality. It must also implement Serializable.
Unfortunately, this approach means that a persistent object is its own identifier. There is no convenient "handle" other than the object itself. You must instantiate an instance of the persistent class itself and populate its identifier properties before you can load() the persistent state associated with a composite key. We call this approach an embedded composite identifier, and discourage it for serious applications.
第二种方法我们称为mapped(映射式)组合标识符 (mapped composite identifier),<composite-id>元素中列出的标识属性不但在持久化类出现，还形成一个独立的标识符类。
<composite-id class="MedicareId" mapped="true">
<key-property name="medicareNumber"/>
<key-property name="dependent"/>
</composite-id>In this example, both the composite identifier class, MedicareId, and the entity class itself have properties named medicareNumber and dependent. The identifier class must override equals() and hashCode() and implement Serializable. The main disadvantage of this approach is code duplication.
下面列出的属性是用来指定一个映射式组合标识符的：
mapped(optional - defaults tofalse): indicates that a mapped composite identifier is used, and that the contained property mappings refer to both the entity class and the composite identifier class.class(optional - but required for a mapped composite identifier): the class used as a composite identifier.
We will describe a third, even more convenient approach, where the composite identifier is implemented as a component class in 第 8.4 节 “组件作为联合标识符(Components as composite identifiers)”. The attributes described below apply only to this alternative approach:
name(optional - required for this approach): a property of component type that holds the composite identifier. Please see chapter 9 for more information.access(optional - defaults toproperty): the strategy Hibernate uses for accessing the property value.class(optional - defaults to the property type determined by reflection): the component class used as a composite identifier. Please see the next section for more information.
The third approach, an identifier component, is recommended for almost all applications.
The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy mapping strategy. It declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. A restricted set of types can be used: string, character, integer, byte, short, boolean, yes_no, true_false.
<discriminator
column="discriminator_column"
type="discriminator_type"
force="true|false"
insert="true|false"
formula="arbitrary sql expression"
/> |
|
|
|
|
|
|
|
|
|
鉴别器字段的实际值是根据<class>和<subclass>元素中 的discriminator-value属性得来的。
The force attribute is only useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This will not usually be the case.
The formula attribute allows you to declare an arbitrary SQL expression that will be used to evaluate the type of a row. For example:
<discriminator
formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
type="integer"/>The <version> element is optional and indicates that the table contains versioned data. This is particularly useful if you plan to use long transactions. See below for more information:
<version
column="version_column"
name="propertyName"
type="typename"
access="field|property|ClassName"
unsaved-value="null|negative|undefined"
generated="never|always"
insert="true|false"
node="element-name|@attribute-name|element/@attribute|."
/> |
|
|
|
|
|
|
|
|
|
|
|
|
|
Version numbers can be of Hibernate type long, integer, short, timestamp or calendar.
A version or timestamp property should never be null for a detached instance. Hibernate will detect any instance with a null version or timestamp as transient, irrespective of what other unsaved-value strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate. It is especially useful for people using assigned identifiers or composite keys.
The optional <timestamp> element indicates that the table contains timestamped data. This provides an alternative to versioning. Timestamps are a less safe implementation of optimistic locking. However, sometimes the application might use the timestamps in other ways.
<timestamp
column="timestamp_column"
name="propertyName"
access="field|property|ClassName"
unsaved-value="null|undefined"
source="vm|db"
generated="never|always"
node="element-name|@attribute-name|element/@attribute|."
/> |
|
|
|
|
|
|
|
|
|
|
|
Note
<Timestamp> is equivalent to <version type="timestamp">. And <timestamp source="db"> is equivalent to <version type="dbtimestamp">
The <property> element declares a persistent JavaBean style property of the class.
<property
name="propertyName"
column="column_name"
type="typename"
update="true|false"
insert="true|false"
formula="arbitrary SQL expression"
access="field|property|ClassName"
lazy="true|false"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
generated="never|insert|always"
node="element-name|@attribute-name|element/@attribute|."
index="index_name"
unique_key="unique_key_id"
length="L"
precision="P"
scale="S"
/> |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
typename可以是如下几种：
The name of a Hibernate basic type:
integer, string, character, date, timestamp, float, binary, serializable, object, blobetc.The name of a Java class with a default basic type:
int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clobetc.一个可以序列化的Java类的名字。
The class name of a custom type:
com.illflow.type.MyCustomTypeetc.
If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4. In certain cases you will need the type attribute. For example, to distinguish between Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.
The access attribute allows you to control how Hibernate accesses the property at runtime. By default, Hibernate will call the property get/set pair. If you specify access="field", Hibernate will bypass the get/set pair and access the field directly using reflection. You can specify your own strategy for property access by naming a class that implements the interface org.hibernate.property.PropertyAccessor.
A powerful feature is derived properties. These properties are by definition read-only. The property value is computed at load time. You declare the computation as an SQL expression. This then translates to a SELECT clause subquery in the SQL query that loads an instance:
<property name="totalPrice"
formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p
WHERE li.productId = p.productId
AND li.customerId = customerId
AND li.orderNumber = orderNumber )"/>You can reference the entity table by not declaring an alias on a particular column. This would be customerId in the given example. You can also use the nested <formula> mapping element if you do not want to use the attribute.
An ordinary association to another persistent class is declared using a many-to-one element. The relational model is a many-to-one association; a foreign key in one table is referencing the primary key column(s) of the target table.
<many-to-one
name="propertyName"
column="column_name"
class="ClassName"
cascade="cascade_style"
fetch="join|select"
update="true|false"
insert="true|false"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
unique="true|false"
not-null="true|false"
optimistic-lock="true|false"
lazy="proxy|no-proxy|false"
not-found="ignore|exception"
entity-name="EntityName"
formula="arbitrary SQL expression"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
index="index_name"
unique_key="unique_key_id"
foreign-key="foreign_key_name"
/> |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Setting a value of the cascade attribute to any meaningful value other than none will propagate certain operations to the associated object. The meaningful values are divided into three categories. First, basic operations, which include: persist, merge, delete, save-update, evict, replicate, lock and refresh; second, special values: delete-orphan; and third,all comma-separated combinations of operation names: cascade="persist,merge,evict" or cascade="all,delete-orphan". See 第 10.11 节 “传播性持久化(transitive persistence)” for a full explanation. Note that single valued, many-to-one and one-to-one, associations do not support orphan delete.
Here is an example of a typical many-to-one declaration:
<many-to-one name="product" class="Product" column="PRODUCT_ID"/>
The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is a complicated and confusing relational model. For example, if the Product class had a unique serial number that is not the primary key. The unique attribute controls Hibernate's DDL generation with the SchemaExport tool.
<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>
那么关于OrderItem 的映射可能是：
<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>
This is not encouraged, however.
如果被引用的唯一主键由关联实体的多个属性组成，你应该在名称为<properties>的元素 里面映射所有关联的属性。
If the referenced unique key is the property of a component, you can specify a property path:
<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>
持久化对象之间一对一的关联关系是通过one-to-one元素定义的。
<one-to-one
name="propertyName"
class="ClassName"
cascade="cascade_style"
constrained="true|false"
fetch="join|select"
property-ref="propertyNameFromAssociatedClass"
access="field|property|ClassName"
formula="any SQL expression"
lazy="proxy|no-proxy|false"
entity-name="EntityName"
node="element-name|@attribute-name|element/@attribute|."
embed-xml="true|false"
foreign-key="foreign_key_name"
/> |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
There are two varieties of one-to-one associations:
主键关联
惟一外键关联
Primary key associations do not need an extra table column. If two rows are related by the association, then the two table rows share the same primary key value. To relate two objects by a primary key association, ensure that they are assigned the same identifier value.
For a primary key association, add the following mappings to Employee and Person respectively:
<one-to-one name="person" class="Person"/>
<one-to-one name="employee" class="Employee" constrained="true"/>
Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You use a special Hibernate identifier generation strategy called foreign:
<class name="person" table="PERSON">
<id name="id" column="PERSON_ID">
<generator class="foreign">
<param name="property">employee</param>
</generator>
</id>
...
<one-to-one name="employee"
class="Employee"
constrained="true"/>
</class>A newly saved instance of Person is assigned the same primary key value as the Employee instance referred with the employee property of that Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person, can be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>
This association can be made bidirectional by adding the following to the Person mapping:
<one-to-one name="employee" class="Employee" property-ref="person"/>
<natural-id mutable="true|false"/>
<property ... />
<many-to-one ... />
......
</natural-id>Although we recommend the use of surrogate keys as primary keys, you should try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. It is also immutable. Map the properties of the natural key inside the <natural-id> element. Hibernate will generate the necessary unique key and nullability constraints and, as a result, your mapping will be more self-documenting.
It is recommended that you implement equals() and hashCode() to compare the natural key properties of the entity.
This mapping is not intended for use with entities that have natural primary keys.
mutable(optional - defaults tofalse): by default, natural identifier properties are assumed to be immutable (constant).
The <component> element maps properties of a child object to columns of the table of a parent class. Components can, in turn, declare their own properties, components or collections. See the "Component" examples below:
<component
name="propertyName"
class="className"
insert="true|false"
update="true|false"
access="field|property|ClassName"
lazy="true|false"
optimistic-lock="true|false"
unique="true|false"
node="element-name|."
>
<property ...../>
<many-to-one .... />
........
</component> |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
其<property>子标签为子类的一些属性与表字段之间建立映射。
<component>元素允许加入一个<parent>子元素，在组件类内部就可以有一个指向其容器的实体的反向引用。
The <dynamic-component> element allows a Map to be mapped as a component, where the property names refer to keys of the map. See 第 8.5 节 “动态组件 （Dynamic components）” for more information.
The <properties> element allows the definition of a named, logical grouping of the properties of a class. The most important use of the construct is that it allows a combination of properties to be the target of a property-ref. It is also a convenient way to define a multi-column unique constraint. For example:
<properties
name="logicalName"
insert="true|false"
update="true|false"
optimistic-lock="true|false"
unique="true|false"
>
<property ...../>
<many-to-one .... />
........
</properties> |
|
|
|
|
|
|
|
|
|
例如，如果我们有如下的<properties>映射:
<class name="Person">
<id name="personNumber"/>
...
<properties name="name"
unique="true" update="false">
<property name="firstName"/>
<property name="initial"/>
<property name="lastName"/>
</properties>
</class>You might have some legacy data association that refers to this unique key of the Person table, instead of to the primary key:
<many-to-one name="person"
class="Person" property-ref="name">
<column name="firstName"/>
<column name="initial"/>
<column name="lastName"/>
</many-to-one>The use of this outside the context of mapping legacy data is not recommended.
Polymorphic persistence requires the declaration of each subclass of the root persistent class. For the table-per-class-hierarchy mapping strategy, the <subclass> declaration is used. For example:
<subclass
name="ClassName"
discriminator-value="discriminator_value"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
entity-name="EntityName"
node="element-name"
extends="SuperclassName">
<property .... />
.....
</subclass> |
|
|
|
|
|
|
|
Each subclass declares its own persistent properties and subclasses. <version> and <id> properties are assumed to be inherited from the root class. Each subclass in a hierarchy must define a unique discriminator-value. If this is not specified, the fully qualified Java class name is used.
For information about inheritance mappings see 第 9 章 Inheritance mapping.
Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass. To do this you use the <joined-subclass> element. For example:
<joined-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<key .... >
<property .... />
.....
</joined-subclass> |
|
|
|
|
|
|
|
A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier using the <key> element. The mapping at the start of the chapter would then be re-written as:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="eg">
<class name="Cat" table="CATS">
<id name="id" column="uid" type="long">
<generator class="hilo"/>
</id>
<property name="birthdate" type="date"/>
<property name="color" not-null="true"/>
<property name="sex" not-null="true"/>
<property name="weight"/>
<many-to-one name="mate"/>
<set name="kittens">
<key column="MOTHER"/>
<one-to-many class="Cat"/>
</set>
<joined-subclass name="DomesticCat" table="DOMESTIC_CATS">
<key column="CAT"/>
<property name="name" type="string"/>
</joined-subclass>
</class>
<class name="eg.Dog">
<!-- mapping for Dog could go here -->
</class>
</hibernate-mapping>For information about inheritance mappings see 第 9 章 Inheritance mapping.
A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state. In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can map each class with a separate <class> declaration. However, if you wish use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use the <union-subclass> mapping. For example:
<union-subclass
name="ClassName"
table="tablename"
proxy="ProxyInterface"
lazy="true|false"
dynamic-update="true|false"
dynamic-insert="true|false"
schema="schema"
catalog="catalog"
extends="SuperclassName"
abstract="true|false"
persister="ClassName"
subselect="SQL expression"
entity-name="EntityName"
node="element-name">
<property .... />
.....
</union-subclass> |
|
|
|
|
|
|
|
这种映射策略不需要指定辨别标志(discriminator)字段。
For information about inheritance mappings see 第 9 章 Inheritance mapping.
Using the <join> element, it is possible to map properties of one class to several tables that have a one-to-one relationship. For example:
<join
table="tablename"
schema="owner"
catalog="catalog"
fetch="join|select"
inverse="true|false"
optional="true|false">
<key ... />
<property ... />
...
</join> |
|
|
|
|
|
|
|
|
|
|
|
For example, address information for a person can be mapped to a separate table while preserving value type semantics for all properties:
<class name="Person"
table="PERSON">
<id name="id" column="PERSON_ID">...</id>
<join table="ADDRESS">
<key column="ADDRESS_ID"/>
<property name="address"/>
<property name="zip"/>
<property name="country"/>
</join>
...This feature is often only useful for legacy data models. We recommend fewer tables than classes and a fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, as explained later.
The <key> element has featured a few times within this guide. It appears anywhere the parent mapping element defines a join to a new table that references the primary key of the original table. It also defines the foreign key in the joined table:
<key
column="columnname"
on-delete="noaction|cascade"
property-ref="propertyName"
not-null="true|false"
update="true|false"
unique="true|false"
/> |
|
|
|
|
|
|
|
|
|
|
|
For systems where delete performance is important, we recommend that all keys should be defined on-delete="cascade". Hibernate uses a database-level ON CASCADE DELETE constraint, instead of many individual DELETE statements. Be aware that this feature bypasses Hibernate's usual optimistic locking strategy for versioned data.
The not-null and update attributes are useful when mapping a unidirectional one-to-many association. If you map a unidirectional one-to-many association to a non-nullable foreign key, you must declare the key column using <key not-null="true">.
Mapping elements which accept a column attribute will alternatively accept a <column> subelement. Likewise, <formula> is an alternative to the formula attribute. For example:
<column
name="column_name"
length="N"
precision="N"
scale="N"
not-null="true|false"
unique="true|false"
unique-key="multicolumn_unique_key_name"
index="index_name"
sql-type="sql_type_name"
check="SQL expression"
default="SQL expression"/><formula>SQL expression</formula>
column and formula attributes can even be combined within the same property or association mapping to express, for example, exotic join conditions.
<many-to-one name="homeAddress" class="Address"
insert="false" update="false">
<column name="person_id" not-null="true" length="10"/>
<formula>'MAILING'</formula>
</many-to-one>If your application has two persistent classes with the same name, and you do not want to specify the fully qualified package name in Hibernate queries, classes can be "imported" explicitly, rather than relying upon auto-import="true". You can also import classes and interfaces that are not explicitly mapped:
<import class="java.lang.Object" rename="Universe"/>
<import
class="ClassName"
rename="ShortName"
/> |
|
|
|
There is one more type of property mapping. The <any> mapping element defines a polymorphic association to classes from multiple tables. This type of mapping requires more than one column. The first column contains the type of the associated entity. The remaining columns contain the identifier. It is impossible to specify a foreign key constraint for this kind of association. This is not the usual way of mapping polymorphic associations and you should use this only in special cases. For example, for audit logs, user session data, etc.
The meta-type attribute allows the application to specify a custom type that maps database column values to persistent classes that have identifier properties of the type specified by id-type. You must specify the mapping from values of the meta-type to class names.
<any name="being" id-type="long" meta-type="string">
<meta-value value="TBL_ANIMAL" class="Animal"/>
<meta-value value="TBL_HUMAN" class="Human"/>
<meta-value value="TBL_ALIEN" class="Alien"/>
<column name="table_name"/>
<column name="id"/>
</any><any
name="propertyName"
id-type="idtypename"
meta-type="metatypename"
cascade="cascade_style"
access="field|property|ClassName"
optimistic-lock="true|false"
>
<meta-value ... />
<meta-value ... />
.....
<column .... />
<column .... />
.....
</any> |
|
|
|
|
|
|
|
|
|
|
|
In relation to the persistence service, Java language-level objects are classified into two groups:
An entity exists independently of any other objects holding references to the entity. Contrast this with the usual Java model, where an unreferenced object is garbage collected. Entities must be explicitly saved and deleted. Saves and deletions, however, can be cascaded from a parent entity to its children. This is different from the ODMG model of object persistence by reachability and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They can also be versioned.
An entity's persistent state consists of references to other entities and instances of value types. Values are primitives: collections (not what is inside a collection), components and certain immutable objects. Unlike entities, values in particular collections and components, are persisted and deleted by reachability. Since value objects and primitives are persisted and deleted along with their containing entity, they cannot be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections.
Until now, we have been using the term "persistent class" to refer to entities. We will continue to do that. Not all user-defined classes with a persistent state, however, are entities. A component is a user-defined class with value semantics. A Java property of type java.lang.String also has value semantics. Given this definition, all types (classes) provided by the JDK have value type semantics in Java, while user-defined types can be mapped with entity or value type semantics. This decision is up to the application developer. An entity class in a domain model will normally have shared references to a single instance of that class, while composition or aggregation usually translates to a value type.
We will revisit both concepts throughout this reference guide.
The challenge is to map the Java type system, and the developers' definition of entities and value types, to the SQL/database type system. The bridge between both systems is provided by Hibernate. For entities, <class>, <subclass> and so on are used. For value types we use <property>, <component>etc., that usually have a type attribute. The value of this attribute is the name of a Hibernate mapping type. Hibernate provides a range of mappings for standard JDK value types out of the box. You can write your own mapping types and implement your own custom conversion strategies.
With the exception of collections, all built-in Hibernate types support null semantics.
The built-in basic mapping types can be roughly categorized into the following:
integer, long, short, float, double, character, byte, boolean, yes_no, true_false这些类型都对应Java的原始类型或者其封装类，来符合(特定厂商的)SQL 字段类型。
boolean, yes_no和true_false都是Java 中boolean或者java.lang.Boolean的另外说法。string从
java.lang.String到VARCHAR(或者 Oracle的VARCHAR2)的映射。date, time, timestamp从
java.util.Date和其子类到SQL类型DATE,TIME和TIMESTAMP(或等价类型)的映射。calendar, calendar_date从
java.util.Calendar到SQL 类型TIMESTAMP和DATE(或等价类型)的映射。big_decimal, big_integer从
java.math.BigDecimal和java.math.BigInteger到NUMERIC(或者 Oracle 的NUMBER类型)的映射。locale, timezone, currency从
java.util.Locale,java.util.TimeZone和java.util.Currency到VARCHAR(或者 Oracle 的VARCHAR2类型)的映射.Locale和Currency的实例被映射为它们的ISO代码。TimeZone的实例被影射为它的ID。class从
java.lang.Class到VARCHAR(或者 Oracle 的VARCHAR2类型)的映射。Class被映射为它的全限定名。binary把字节数组(byte arrays)映射为对应的 SQL二进制类型。
text把长Java字符串映射为SQL的
CLOB或者TEXT类型。serializableMaps serializable Java types to an appropriate SQL binary type. You can also indicate the Hibernate type
serializablewith the name of a serializable Java class or interface that does not default to a basic type.clob, blobType mappings for the JDBC classes
java.sql.Clobandjava.sql.Blob. These types can be inconvenient for some applications, since the blob or clob object cannot be reused outside of a transaction. Driver support is patchy and inconsistent.-
imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date, imm_serializable, imm_binary Type mappings for what are considered mutable Java types. This is where Hibernate makes certain optimizations appropriate only for immutable Java types, and the application treats the object as immutable. For example, you should not call
Date.setTime()for an instance mapped asimm_timestamp. To change the value of the property, and have that change made persistent, the application must assign a new, nonidentical, object to the property.
Unique identifiers of entities and collections can be of any basic type except binary, blob and clob. Composite identifiers are also allowed. See below for more information.
在org.hibernate.Hibernate中，定义了基础类型对应的Type常量。比如，Hibernate.STRING代表string 类型。
It is relatively easy for developers to create their own value types. For example, you might want to persist properties of type java.lang.BigInteger to VARCHAR columns. Hibernate does not provide a built-in type for this. Custom types are not limited to mapping a property, or collection element, to a single table column. So, for example, you might have a Java property getName()/setName() of type java.lang.String that is persisted to the columns FIRST_NAME, INITIAL, SURNAME.
To implement a custom type, implement either org.hibernate.UserType or org.hibernate.CompositeUserType and declare properties using the fully qualified classname of the type. View org.hibernate.test.DoubleStringType to see the kind of things that are possible.
<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
<column name="first_string"/>
<column name="second_string"/>
</property>注意使用<column>标签来把一个属性映射到多个字段的做法。
CompositeUserType, EnhancedUserType, UserCollectionType, 和 UserVersionType 接口为更特殊的使用方式提供支持。
You can even supply parameters to a UserType in the mapping file. To do this, your UserType must implement the org.hibernate.usertype.ParameterizedType interface. To supply parameters to your custom type, you can use the <type> element in your mapping files.
<property name="priority">
<type name="com.mycompany.usertypes.DefaultValueIntegerType">
<param name="default">0</param>
</type>
</property>现在，UserType 可以从传入的Properties对象中得到default 参数的值。
If you regularly use a certain UserType, it is useful to define a shorter name for it. You can do this using the <typedef> element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type is parameterized.
<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero">
<param name="default">0</param>
</typedef><property name="priority" type="default_zero"/>
也可以根据具体案例通过属性映射中的类型参数覆盖在typedef中提供的参数。
Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use a custom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your application. For example, a MonetaryAmount class is a good candidate for a CompositeUserType, even though it could be mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would be protected against changes to the way monetary values are represented.
It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify an entity name to disambiguate between instances of the two mapped entities. By default, the entity name is the same as the class name. Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or when mapping associations to the named entity.
<class name="Contract" table="Contracts"
entity-name="CurrentContract">
...
<set name="history" inverse="true"
order-by="effectiveEndDate desc">
<key column="currentContractId"/>
<one-to-many entity-name="HistoricalContract"/>
</set>
</class>
<class name="Contract" table="ContractHistory"
entity-name="HistoricalContract">
...
<many-to-one name="currentContract"
column="currentContractId"
entity-name="CurrentContract"/>
</class>Associations are now specified using entity-name instead of class.
You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks in the mapping document. Hibernate will use the correct quotation style for the SQL Dialect. This is usually double quotes, but the SQL Server uses brackets and MySQL uses backticks.
<class name="LineItem" table="`Line Item`">
<id name="id" column="`Item Id`"/><generator class="assigned"/></id>
<property name="itemNumber" column="`Item #`"/>
...
</class>XML does not suit all users so there are some alternative ways to define O/R mapping metadata in Hibernate.
Many Hibernate users prefer to embed mapping information directly in sourcecode using XDoclet @hibernate.tags. We do not cover this approach in this reference guide since it is considered part of XDoclet. However, we include the following example of the Cat class with XDoclet mappings:
package eg;
import java.util.Set;
import java.util.Date;
/**
* @hibernate.class
* table="CATS"
*/
public class Cat {
private Long id; // identifier
private Date birthdate;
private Cat mother;
private Set kittens
private Color color;
private char sex;
private float weight;
/*
* @hibernate.id
* generator-class="native"
* column="CAT_ID"
*/
public Long getId() {
return id;
}
private void setId(Long id) {
this.id=id;
}
/**
* @hibernate.many-to-one
* column="PARENT_ID"
*/
public Cat getMother() {
return mother;
}
void setMother(Cat mother) {
this.mother = mother;
}
/**
* @hibernate.property
* column="BIRTH_DATE"
*/
public Date getBirthdate() {
return birthdate;
}
void setBirthdate(Date date) {
birthdate = date;
}
/**
* @hibernate.property
* column="WEIGHT"
*/
public float getWeight() {
return weight;
}
void setWeight(float weight) {
this.weight = weight;
}
/**
* @hibernate.property
* column="COLOR"
* not-null="true"
*/
public Color getColor() {
return color;
}
void setColor(Color color) {
this.color = color;
}
/**
* @hibernate.set
* inverse="true"
* order-by="BIRTH_DATE"
* @hibernate.collection-key
* column="PARENT_ID"
* @hibernate.collection-one-to-many
*/
public Set getKittens() {
return kittens;
}
void setKittens(Set kittens) {
this.kittens = kittens;
}
// addKitten not needed by Hibernate
public void addKitten(Cat kitten) {
kittens.add(kitten);
}
/**
* @hibernate.property
* column="SEX"
* not-null="true"
* update="false"
*/
public char getSex() {
return sex;
}
void setSex(char sex) {
this.sex=sex;
}
}See the Hibernate website for more examples of XDoclet and Hibernate.
JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe and checked at compile time. This mechanism is more powerful than XDoclet annotations and better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification (JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans. Hibernate3 implements the EntityManager of JSR-220 (the persistence API). Support for mapping metadata is available via the Hibernate Annotations package as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.
这是一个被注解为EJB entity bean 的POJO类的例子
@Entity(access = AccessType.FIELD)
public class Customer implements Serializable {
@Id;
Long id;
String firstName;
String lastName;
Date birthday;
@Transient
Integer age;
@Embedded
private Address homeAddress;
@OneToMany(cascade=CascadeType.ALL)
@JoinColumn(name="CUSTOMER_ID")
Set<Order> orders;
// Getter/setter and business methods
}Note
Support for JDK 5.0 Annotations (and JSR-220) is currently under development. Please refer to the Hibernate Annotations module for more details.
Generated properties are properties that have their values generated by the database. Typically, Hibernate applications needed to refresh objects that contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. When Hibernate issues an SQL INSERT or UPDATE for an entity that has defined generated properties, it immediately issues a select afterwards to retrieve the generated values.
Properties marked as generated must additionally be non-insertable and non-updateable. Only versions, timestamps, and simple properties, can be marked as generated.
never (the default): the given property value is not generated within the database.
insert: the given property value is generated on insert, but is not regenerated on subsequent updates. Properties like created-date fall into this category. Even though version and timestamp properties can be marked as generated, this option is not available.
always: the property value is generated both on insert and on update.
Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with Hibernate's schema evolution tools, they have the ability to fully define a user schema within the Hibernate mapping files. Although designed specifically for creating and dropping things like triggers or stored procedures, any SQL command that can be run via a java.sql.Statement.execute() method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for defining auxiliary database objects:
The first mode is to explicitly list the CREATE and DROP commands in the mapping file:
<hibernate-mapping>
...
<database-object>
<create>CREATE TRIGGER my_trigger ...</create>
<drop>DROP TRIGGER my_trigger</drop>
</database-object>
</hibernate-mapping>The second mode is to supply a custom class that constructs the CREATE and DROP commands. This custom class must implement the org.hibernate.mapping.AuxiliaryDatabaseObject interface.
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
</database-object>
</hibernate-mapping>Additionally, these database objects can be optionally scoped so that they only apply when certain dialects are used.
<hibernate-mapping>
...
<database-object>
<definition class="MyTriggerDefinition"/>
<dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
<dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
</database-object>
</hibernate-mapping>