- 5.1. 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. Mapping a class more than once
- 5.4. SQL quoted identifiers
- 5.5. Metadata alternatives
- 5.6. Generated properties
- 5.7. Column read and write expressions
- 5.8. 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 version "3.5.6-Final">
<!ENTITY today "September 15, 2010">
<!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
schem
a="schemaName"
catal
og="catalogName"
defau
lt-cascade="cascade_style"
defau
lt-access="field|property|ClassName"
defau
lt-lazy="true|false"
auto-
import="true|false"
packa
ge="package.name"
/>
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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"
dynami
c-update="true|false"
dynami
c-insert="true|false"
select
-before-update="true|false"
polymo
rphism="implicit|explicit"
where=
"arbitrary sql where condition"
persis
ter="PersisterClass"
batch-
size="N"
optimi
stic-lock="none|version|dirty|all"
lazy="(16)true|false"
entity(17)-name="EntityName"
check=(18)"arbitrary sql check condition"
rowid=(19)"rowid"
subsel(20)ect="SQL expression"
abstra(21)ct="true|false"
node="element-name"
/>
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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.
If you enable dynamic-update, you will have a choice of optimistic
locking strategies:
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.
Mapped classes must declare the primary key column of the database
table. Most classes will also have a JavaBeans-style property holding the unique identifier
of an instance. The <id> element defines the mapping from that
property to the primary key column.
<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>
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If the name attribute is missing, it is assumed that the class has no
identifier property.
The unsaved-value attribute is almost never needed in Hibernate3.
There is an alternative <composite-id> declaration that allows access to
legacy data with composite keys. Its use is strongly discouraged for anything else.
The optional <generator> child element names a Java class used
to generate unique identifiers for instances of the persistent class. If any parameters
are required to configure or initialize the generator instance, they are passed using the
<param> element.
<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:
incrementgenerates identifiers of type
long,shortorintthat are unique only when no other process is inserting data into the same table. Do not use in a cluster.identitysupports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type
long,shortorint.sequenceuses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type
long,shortorinthilouses a hi/lo algorithm to efficiently generate identifiers of type
long,shortorint, given a table and column (by defaulthibernate_unique_keyandnext_hirespectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.seqhilouses a hi/lo algorithm to efficiently generate identifiers of type
long,shortorint, given a named database sequence.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.
guiduses a database-generated GUID string on MS SQL Server and MySQL.
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 Section 5.1.6, “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 Section 5.1.6, “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 (Section 5.1.5, “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.
A second approach is what we call a mapped composite identifier,
where the identifier properties named inside the <composite-id>
element are duplicated on both the persistent class and a separate identifier class.
<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.
The following attributes are used to specify a mapped composite identifier:
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 Section 8.4, “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"
/>
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Actual values of the discriminator column are specified by the
discriminator-value attribute of the <class> and
<subclass> elements.
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|."
/>
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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|."
/>
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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"
/>
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typename could be:
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.The name of a serializable Java class.
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"
/>
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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 Section 10.11, “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"/>
Then the mapping for OrderItem might use:
<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>
This is not encouraged, however.
If the referenced unique key comprises multiple properties of the associated entity, you should
map the referenced properties inside a named <properties> element.
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"/>
A one-to-one association to another persistent class is declared using a
one-to-one element.
<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"
/>
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There are two varieties of one-to-one associations:
primary key associations
unique foreign key associations
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>
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The child <property> tags map properties of the
child class to table columns.
The <component> element allows a <parent>
subelement that maps a property of the component class as a reference back to the
containing entity.
The <dynamic-component> element allows a Map
to be mapped as a component, where the property names refer to keys of the map. See
Section 8.5, “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>
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For example, if we have the following <properties> mapping:
<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>
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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 Chapter 9, 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>
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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 Chapter 9, 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>
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No discriminator column or key column is required for this mapping strategy.
For information about inheritance mappings see Chapter 9, 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>
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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"
/>
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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"
read="SQL expression"
write="SQL expression"/>
<formula>SQL expression</formula>
Most of the attributes on column provide a means of tailoring the
DDL during automatic schema generation. The read and write
attributes allow you to specify custom SQL that Hibernate will use to access the column's value.
For more on this, see the discussion of
column read and write expressions.
The column and formula elements 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"
/>
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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>
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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_falseType mappings from Java primitives or wrapper classes to appropriate (vendor-specific) SQL column types.
boolean, yes_noandtrue_falseare all alternative encodings for a Javabooleanorjava.lang.Boolean.stringA type mapping from
java.lang.StringtoVARCHAR(or OracleVARCHAR2).date, time, timestampType mappings from
java.util.Dateand its subclasses to SQL typesDATE,TIMEandTIMESTAMP(or equivalent).calendar, calendar_dateType mappings from
java.util.Calendarto SQL typesTIMESTAMPandDATE(or equivalent).big_decimal, big_integerType mappings from
java.math.BigDecimalandjava.math.BigIntegertoNUMERIC(or OracleNUMBER).locale, timezone, currencyType mappings from
java.util.Locale,java.util.TimeZoneandjava.util.CurrencytoVARCHAR(or OracleVARCHAR2). Instances ofLocaleandCurrencyare mapped to their ISO codes. Instances ofTimeZoneare mapped to theirID.classA type mapping from
java.lang.ClasstoVARCHAR(or OracleVARCHAR2). AClassis mapped to its fully qualified name.binaryMaps byte arrays to an appropriate SQL binary type.
textMaps long Java strings to a SQL
CLOBorTEXTtype.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.
The basic value types have corresponding Type constants defined on
org.hibernate.Hibernate. For example, Hibernate.STRING
represents the string type.
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>
Notice the use of <column> tags to map a property to multiple
columns.
The CompositeUserType, EnhancedUserType,
UserCollectionType, and UserVersionType
interfaces provide support for more specialized uses.
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>
The UserType can now retrieve the value for the parameter named
default from the Properties object passed to it.
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"/>
It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping.
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.
This is an example of a POJO class annotated as an EJB entity bean:
@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.
Hibernate allows you to customize the SQL it uses to read and write the values of columns mapped to simple properties. For example, if your database provides a set of data encryption functions, you can invoke them for individual columns like this:
<!-- XML : generated by JHighlight v1.0 (http://jhighlight.dev.java.net) --> <span class="xml_tag_symbols"><</span><span class="xml_tag_name">property</span><span class="xml_plain"> </span><span class="xml_attribute_name">name</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"creditCardNumber"</span><span class="xml_tag_symbols">></span><span class="xml_plain"></span><br /> <span class="xml_plain"> </span><span class="xml_tag_symbols"><</span><span class="xml_tag_name">column</span><span class="xml_plain"> </span><br /> <span class="xml_plain"> </span><span class="xml_attribute_name">name</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"credit_card_num"</span><span class="xml_plain"></span><br /> <span class="xml_plain"> </span><span class="xml_attribute_name">read</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"decrypt(credit_card_num)"</span><span class="xml_plain"></span><br /> <span class="xml_plain"> </span><span class="xml_attribute_name">write</span><span class="xml_tag_symbols">=</span><span class="xml_attribute_value">"encrypt(?)"</span><span class="xml_tag_symbols">/></span><span class="xml_plain"></span><br /> <span class="xml_tag_symbols"></</span><span class="xml_tag_name">property</span><span class="xml_tag_symbols">></span><span class="xml_plain"></span><br />
Hibernate applies the custom expressions automatically whenever the property is
referenced in a query. This functionality is similar to a derived-property
formula with two differences:
The property is backed by one or more columns that are exported as part of automatic schema generation.
The property is read-write, not read-only.
The write expression, if specified, must contain exactly one '?' placeholder
for the value.
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>