Chapter 5. Basic O/R Mapping

5.1. Mapping declaration

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, not table declarations.
Note that, even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document, including XDoclet, Middlegen and AndroMDA.
Lets kick off with 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 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.)

5.1.1. Doctype

All XML mappings should declare the doctype shown. The actual DTD may 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 your DTD declaration against the contents of your claspath.

5.1.1.1. EntityResolver

As mentioned previously, Hibernate will first attempt to resolve DTDs in its classpath. The manner in which 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 namespace is recognized whenever the resolver encounteres a systemId starting with http://hibernate.sourceforge.net/; the resolver attempts to resolve these entities via the classlaoder which loaded the Hibernate classes.
  • a user namespace is recognized whenever the resolver encounteres a systemId using a classpath:// 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.
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 Section 5.2.3, “Custom value types” typedef.

5.1.2. hibernate-mapping

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 specified, tablenames will be qualified by the given schema and catalog names. If missing, tablenames will be unqualified. The default-cascade attribute specifies what cascade style should be assumed for properties and collections which do not specify a cascade attribute. The auto-import attribute lets us use unqualified class names in the query language, by default.
<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"
 />
schema (optional): The name of a database schema.
catalog (optional): The name of a database catalog.
default-cascade (optional - defaults to none): A default cascade style.
default-access (optional - defaults to property): The strategy Hibernate should use for accessing all properties. Can be a custom implementation of PropertyAccessor.
default-lazy (optional - defaults to true): The default value for unspecifed lazy attributes of class and collection mappings.
auto-import (optional - defaults to true): Specifies whether we can use unqualified class names (of classes in this mapping) in the query language.
package (optional): Specifies a package prefix to assume for unqualified class names in the mapping document.
If you have two persistent classes with the same (unqualified) name, you should set auto-import="false". Hibernate will throw an exception if you attempt to assign two classes to the same "imported" name.
Note that 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, e.g. Cat.hbm.xml, Dog.hbm.xml, or if using inheritance, Animal.hbm.xml.

5.1.3. class

You may declare a persistent class using the class element:
<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"
        entity-name="EntityName"
        check="arbitrary sql check condition"
        rowid="rowid"
        subselect="SQL expression"
        abstract="true|false"
        node="element-name"
/>
name (optional): The fully qualified Java class name of the persistent class (or interface). If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.
table (optional - defaults to the unqualified class name): The name of its database table.
discriminator-value (optional - defaults to the class name): A value that distiguishes individual subclasses, used for polymorphic behaviour. Acceptable values include null and not null.
mutable (optional, defaults to true): Specifies that instances of the class are (not) mutable.
schema (optional): Override the schema name specified by the root <hibernate-mapping> element.
catalog (optional): Override the catalog name specified by the root <hibernate-mapping> element.
proxy (optional): Specifies an interface to use for lazy initializing proxies. You may specify the name of the class itself.
dynamic-update (optional, defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed.
dynamic-insert (optional, defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.
select-before-update (optional, defaults to false): Specifies that Hibernate should never perform an SQL UPDATE unless it is certain that an object is actually modified. In certain cases (actually, only when a transient object has been associated with a new session using update()), this means that Hibernate will perform an extra SQL SELECT to determine if an UPDATE is actually required.
polymorphism (optional, defaults to implicit): Determines whether implicit or explicit query polymorphism is used.
where (optional) specify an arbitrary SQL WHERE condition to be used when retrieving objects of this class
persister (optional): Specifies a custom ClassPersister.
batch-size (optional, defaults to 1) specify a "batch size" for fetching instances of this class by identifier.
optimistic-lock (optional, defaults to version): Determines the optimistic locking strategy.
lazy (optional): Lazy fetching may be completely disabled by setting lazy="false".
entity-name (optional, defaults to the class name): Hibernate3 allows a class to be mapped multiple times (to different tables, potentially), and allows entity mappings that are represented by Maps or XML at the Java level. In these cases, you should provide an explicit arbitrary name for the entity. See Section 4.4, “Dynamic models” and Chapter 18, XML Mapping for more information.
check (optional): A SQL expression used to generate a multi-row check constraint for automatic schema generation.
rowid (optional): Hibernate can use so called ROWIDs on databases which support. E.g. on Oracle, Hibernate can use the rowid extra column for fast updates if you set this option to rowid. A ROWID is an implementation detail and represents the physical location of a stored tuple.
subselect (optional): Maps an immutable and read-only entity to a database subselect. Useful if you want to have a view instead of a base table, but don't. See below for more information.
abstract (optional): Used to mark abstract superclasses in <union-subclass> hierarchies.
It is perfectly acceptable for the named persistent class to be an interface. You would then declare implementing classes of that interface using the <subclass> element. You may persist any static inner class. You should specify the class name using the standard form ie. eg.Foo$Bar.
Immutable classes, mutable="false", may not 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 which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked. See "Proxies for Lazy Initialization" below.
Implicit polymorphism means that instances of the class will be returned by a query that names any superclass or implemented interface or the 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 and that 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 may, for example, specify your own subclass of org.hibernate.persister.EntityPersister or you might even provide a completely new implementation of the interface org.hibernate.persister.ClassPersister that implements persistence via, for example, stored procedure calls, serialization to flat files or LDAP. See org.hibernate.test.CustomPersister for a simple example (of "persistence" to a Hashtable).
Note that the dynamic-update and dynamic-insert settings are not inherited by subclasses and so may also be specified on the <subclass> or <joined-subclass> elements. These settings may increase performance in some cases, but might actually decrease performance in others. Use judiciously.
Use of select-before-update will usually decrease performance. It is very 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 columns
  • all check all columns
  • dirty check the changed columns, allowing some concurrent updates
  • none do not use optimistic locking
We very strongly recommend that you use version/timestamp columns for optimistic locking with Hibernate. This is the optimal strategy with respect to performance and is the only strategy that correctly handles modifications made to detached instances (ie. when Session.merge() is used).
There is no difference between a view and a base table for a Hibernate mapping, as expected this is transparent at the database level (note that some DBMS don't support views properly, especially with updates). Sometimes you want to use a view, but can't create one in the database (ie. 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 as both as an attribute and a nested mapping element.

5.1.4. id

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>
name (optional): The name of the identifier property.
type (optional): A name that indicates the Hibernate type.
column (optional - defaults to the property name): The name of the primary key column.
unsaved-value (optional - defaults to a "sensible" value): An identifier property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
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 to allow access to legacy data with composite keys. We strongly discourage its use for anything else.

5.1.4.1. Generator

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 may choose to provide their own specialized implementations. However, Hibernate provides a range of built-in implementations. There are shortcut names for the built-in generators:
increment
generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. Do not use in a cluster.
identity
supports identity columns in DB2, MySQL, MS SQL Server, Sybase and HypersonicSQL. The returned identifier is of type long, short or int.
sequence
uses a sequence in DB2, PostgreSQL, Oracle, SAP DB, McKoi or a generator in Interbase. The returned identifier is of type long, short or int
hilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a table and column (by default hibernate_unique_key and next_hi respectively) as a source of hi values. The hi/lo algorithm generates identifiers that are unique only for a particular database.
seqhilo
uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence.
uuid
uses a 128-bit UUID algorithm to generate identifiers of type string, unique within a network (the IP address is used). The UUID is encoded as a string of hexadecimal digits of length 32.
guid
uses a database-generated GUID string on MS SQL Server and MySQL.
native
picks identity, sequence or hilo depending upon the capabilities of the underlying database.
assigned
lets the application to assign an identifier to the object before save() is called. This is the default strategy if no <generator> element is specified.
select
retrieves a primary key assigned by a database trigger by selecting the row by some unique key and retrieving the primary key value.
foreign
uses the identifier of another associated object. Usually used in conjunction with a <one-to-one> primary key association.
sequence-identity
a specialized sequence generation strategy which utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to actually return the generated identifier value as part of the insert statement execution. This strategy is only known to be supported on Oracle 10g drivers targetted for JDK 1.4. Note comments on these insert statements are disabled due to a bug in the Oracle drivers.

5.1.4.2. Hi/lo algorithm

The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm, a favorite approach to identifier generation. The first implementation requires a "special" database table to hold the next available "hi" value. The second uses an Oracle-style sequence (where supported).
<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 can't use hilo when supplying your own Connection to Hibernate. When Hibernate is using an application server datasource to obtain connections enlisted with JTA, you must properly configure the hibernate.transaction.manager_lookup_class.

5.1.4.3. UUID algorithm

The UUID contains: IP address, startup time of the JVM (accurate to a quarter second), system time and a counter value (unique within the JVM). It's not possible to obtain a MAC address or memory address from Java code, so this is the best we can do without using JNI.

5.1.4.4. Identity columns and sequences

For databases which support identity columns (DB2, MySQL, Sybase, MS SQL), you may use identity key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you may use sequence style key generation. Both these strategies require two SQL queries to insert a new object.
<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 choose from the identity, sequence and hilo strategies, dependant upon the capabilities of the underlying database.

5.1.4.5. Assigned identifiers

If you want the application to assign identifiers (as opposed to having Hibernate generate them), you may use the assigned generator. This special generator will use the identifier value already assigned to the object's identifier property. This generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do no specify a <generator> element.
Choosing the assigned generator makes Hibernate use unsaved-value="undefined", forcing 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().

5.1.4.6. Primary keys assigned by triggers

For legacy schemas only (Hibernate does not generate DDL with triggers).
<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 defined by the class, as a natural key, and a surrogate key named person_id whose value is generated by a trigger.

5.1.5. composite-id

<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>
For a table with a composite key, you may map 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>
Your persistent class must override equals() and hashCode() to implement composite identifier equality. It must also implements Serializable.
Unfortunately, this approach to composite identifiers 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 disadvantage of this approach is quite obvious - code duplication.
The following attributes are used to specify a mapped composite identifier:
  • mapped (optional, defaults to false): 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 (see chapter 9).
  • access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
  • class (optional - defaults to the property type determined by reflection): The component class used as a composite identifier (see next section).
This third approach, an identifier component is the one we recommend for almost all applications.

5.1.6. discriminator

The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy mapping strategy and 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 may 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"
/>
column (optional - defaults to class) the name of the discriminator column.
type (optional - defaults to string) a name that indicates the Hibernate type
force (optional - defaults to false) "force" Hibernate to specify allowed discriminator values even when retrieving all instances of the root class.
insert (optional - defaults to true) set this to false if your discriminator column is also part of a mapped composite identifier. (Tells Hibernate to not include the column in SQL INSERT s.)
formula (optional) an arbitrary SQL expression that is executed when a type has to be evaluated. Allows content-based discrimination.
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.
Using the formula attribute you can declare an arbitrary SQL expression that will be used to evaluate the type of a row:
<discriminator
    formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
    type="integer"/>

5.1.7. version (optional)

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).
<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|."
/>
column (optional - defaults to the property name): The name of the column holding the version number.
name: The name of a property of the persistent class.
type (optional - defaults to integer): The type of the version number.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
unsaved-value (optional - defaults to undefined): A version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. (undefined specifies that the identifier property value should be used.)
generated (optional - defaults to never): Specifies that this version property value is actually generated by the database. See the discussion of Section 5.6, “Generated Properties” generated properties.
insert (optional - defaults to true): Specifies whether the version column should be included in SQL insert statements. May be set to false if and only if the database column is defined with a default value of 0.
Version numbers may be of Hibernate type long, integer, short, timestamp or calendar.
A version or timestamp property should never be null for a detached instance, so Hibernate will detact any instance with a null version or timestamp as transient, no matter what other unsaved-value strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid any problems with transitive reattachment in Hibernate, especially useful for people using assigned identifiers or composite keys!

5.1.8. timestamp (optional)

The optional <timestamp> element indicates that the table contains timestamped data. This is intended as an alternative to versioning. Timestamps are by nature 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|."
/>
column (optional - defaults to the property name): The name of a column holding the timestamp.
name: The name of a JavaBeans style property of Java type Date or Timestamp of the persistent class.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
unsaved-value (optional - defaults to null): A version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session. (undefined specifies that the identifier property value should be used.)
source (optional - defaults to vm): From where should Hibernate retrieve the timestamp value? From the database, or from the current JVM? Database-based timestamps incur an overhead because Hibernate must hit the database in order to determine the "next value", but will be safer for use in clustered environments. Note also, that not all Dialect s are known to support retrieving of the database's current timestamp, while others might be unsafe for usage in locking due to lack of precision (Oracle 8 for example).
generated (optional - defaults to never): Specifies that this timestamp property value is actually generated by the database. See the discussion of Section 5.6, “Generated Properties” generated properties.
Note that <timestamp> is equivalent to <version type="timestamp">. And <timestamp source="db"> is equivalent to <version type="dbtimestamp">

5.1.9. property

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"
/>
name: the name of the property, with an initial lowercase letter.
column (optional - defaults to the property name): the name of the mapped database table column. This may also be specified by nested <column> element(s).
type (optional): a name that indicates the Hibernate type.
update, insert (optional - defaults to true) : specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" property whose value is initialized from some other property that maps to the same colum(s) or by a trigger or other application.
formula (optional): an SQL expression that defines the value for a computed property. Computed properties do not have a column mapping of their own.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
lazy (optional - defaults to false): Specifies that this property should be fetched lazily when the instance variable is first accessed (requires build-time bytecode instrumentation).
unique (optional): Enable the DDL generation of a unique constraint for the columns. Also, allow this to be the target of a property-ref.
not-null (optional): Enable the DDL generation of a nullability constraint for the columns.
optimistic-lock (optional - defaults to true): Specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, determines if a version increment should occur when this property is dirty.
generated (optional - defaults to never): Specifies that this property value is actually generated by the database. See the discussion of Section 5.6, “Generated Properties” generated properties.
typename could be:
  1. The name of a Hibernate basic type (eg. integer, string, character, date, timestamp, float, binary, serializable, object, blob).
  2. The name of a Java class with a default basic type (eg. int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob).
  3. The name of a serializable Java class.
  4. The class name of a custom type (eg. com.illflow.type.MyCustomType).
If you do not specify a type, Hibernate will use reflection upon the named property to take a guess at the correct Hibernate type. Hibernate will try to interpret the name of the return class of the property getter using rules 2, 3, 4 in that order. However, this is not always enough. In certain cases you will still need the type attribute. (For example, to distinguish between Hibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.)
The access attribute lets you control how Hibernate will access 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 may specify your own strategy for property access by naming a class that implements the interface org.hibernate.property.PropertyAccessor.
An especially powerful feature are derived properties. These properties are by definition read-only, the property value is computed at load time. You declare the computation as a SQL expression, this 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 )"/>
Note that you can reference the entities own table by not declaring an alias on a particular column (customerId in the given example). Also note that you can use the nested <formula> mapping element if you don't like to use the attribute.

5.1.10. many-to-one

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"
/>
name: The name of the property.
column (optional): The name of the foreign key column. This may also be specified by nested <column> element(s).
class (optional - defaults to the property type determined by reflection): The name of the associated class.
cascade (optional): Specifies which operations should be cascaded from the parent object to the associated object.
fetch (optional - defaults to select): Chooses between outer-join fetching or sequential select fetching.
update, insert (optional - defaults to true) specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both to false allows a pure "derived" association whose value is initialized from some other property that maps to the same colum(s) or by a trigger or other application.
property-ref: (optional) The name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
unique (optional): Enable the DDL generation of a unique constraint for the foreign-key column. Also, allow this to be the target of a property-ref. This makes the association multiplicity effectively one to one.
not-null (optional): Enable the DDL generation of a nullability constraint for the foreign key columns.
optimistic-lock (optional - defaults to true): Specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, dertermines if a version increment should occur when this property is dirty.
lazy (optional - defaults to proxy): By default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed (requires build-time bytecode instrumentation). lazy="false" specifies that the association will always be eagerly fetched.
not-found (optional - defaults to exception): Specifies how foreign keys that reference missing rows will be handled: ignore will treat a missing row as a null association.
entity-name (optional): The entity name of the associated class.
formula (optional): an SQL expression that defines the value for a computed foreign key.
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 the names of Hibernate's basic operations, persist, merge, delete, save-update, evict, replicate, lock, refresh, as well as the special values delete-orphan and all and comma-separated combinations of operation names, for example, cascade="persist,merge,evict" or cascade="all,delete-orphan". See Section 10.11, “Transitive persistence” for a full explanation. Note that single valued associations (many-to-one and one-to-one associations) do not support orphan delete.
A typical many-to-one declaration looks as simple as this:
<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 an ugly relational model. For example, suppose 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 certainly 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 may specify a property path:
<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>

5.1.11. one-to-one

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"
/>
name: The name of the property.
class (optional - defaults to the property type determined by reflection): The name of the associated class.
cascade (optional) specifies which operations should be cascaded from the parent object to the associated object.
constrained (optional) specifies that a foreign key constraint on the primary key of the mapped table references the table of the associated class. This option affects the order in which save() and delete() are cascaded, and determines whether the association may be proxied (it is also used by the schema export tool).
fetch (optional - defaults to select): Chooses between outer-join fetching or sequential select fetching.
property-ref: (optional) The name of a property of the associated class that is joined to the primary key of this class. If not specified, the primary key of the associated class is used.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
formula (optional): Almost all one to one associations map to the primary key of the owning entity. In the rare case that this is not the case, you may specify a some other column, columns or expression to join on using an SQL formula. (See org.hibernate.test.onetooneformula for an example.)
lazy (optional - defaults to proxy): By default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed (requires build-time bytecode instrumentation). lazy="false" specifies that the association will always be eagerly fetched. Note that if constrained="false", proxying is impossible and Hibernate will eager fetch the association!
entity-name (optional): The entity name of the associated class.
There are two varieties of one-to-one association:
  • primary key associations
  • unique foreign key associations
Primary key associations don't need an extra table column; if two rows are related by the association then the two table rows share the same primary key value. So if you want two objects to be related by a primary key association, you must make sure 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"/>
Now we must ensure that the primary keys of related rows in the PERSON and EMPLOYEE tables are equal. We 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 then assigned the same primary key value as the Employee instance refered with the employee property of that Person.
Alternatively, a foreign key with a unique constraint, from Employee to Person, may be expressed as:
<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>
And this association may be made bidirectional by adding the following to the Person mapping:
<one-to-one name"employee" class="Employee" property-ref="person"/>

5.1.12. natural-id

<natural-id mutable="true|false"/>
        <property ... />
        <many-to-one ... />
        ......
</natural-id>
Even though we recommend the use of surrogate keys as primary keys, you should still try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. If it is also immutable, even better. Map the properties of the natural key inside the <natural-id> element. Hibernate will generate the necessary unique key and nullability constraints, and your mapping will be more self-documenting.
We strongly recommend that you implement equals() and hashCode() to compare the natural key properties of the entity.
This mapping is not intended for use with entities with natural primary keys.
  • mutable (optional, defaults to false): By default, natural identifier properties as assumed to be immutable (constant).

5.1.13. component, dynamic-component

The <component> element maps properties of a child object to columns of the table of a parent class. Components may, in turn, declare their own properties, components or collections. See "Components" 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>
name: The name of the property.
class (optional - defaults to the property type determined by reflection): The name of the component (child) class.
insert: Do the mapped columns appear in SQL INSERT s?
update: Do the mapped columns appear in SQL UPDATE s?
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
lazy (optional - defaults to false): Specifies that this component should be fetched lazily when the instance variable is first accessed (requires build-time bytecode instrumentation).
optimistic-lock (optional - defaults to true): Specifies that updates to this component do or do not require acquisition of the optimistic lock. In other words, determines if a version increment should occur when this property is dirty.
unique (optional - defaults to false): Specifies that a unique constraint exists upon all mapped columns of the component.
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”.

5.1.14. properties

The <properties> element allows the definition of a named, logical grouping of 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.
<properties 
        name="logicalName" 
        insert="true|false"
        update="true|false"
        optimistic-lock="true|false"
        unique="true|false"
>
        
        <property ...../>
        <many-to-one .... />
        ........
</properties>
name: The logical name of the grouping - not an actual property name.
insert: Do the mapped columns appear in SQL INSERT s?
update: Do the mapped columns appear in SQL UPDATE s?
optimistic-lock (optional - defaults to true): Specifies that updates to these properties do or do not require acquisition of the optimistic lock. In other words, determines if a version increment should occur when these properties are dirty.
unique (optional - defaults to false): Specifies that a unique constraint exists upon all mapped columns of the component.
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>
Then we might have some legacy data association which 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>
We don't recommend the use of this kind of thing outside the context of mapping legacy data.

5.1.15. subclass

Finally, 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.
<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>
name: The fully qualified class name of the subclass.
discriminator-value (optional - defaults to the class name): A value that distiguishes individual subclasses.
proxy (optional): Specifies a class or interface to use for lazy initializing proxies.
lazy (optional, defaults to true): Setting lazy="false" disables the use of lazy fetching.
Each subclass should declare its own persistent properties and subclasses. <version> and <id> properties are assumed to be inherited from the root class. Each subclass in a heirarchy must define a unique discriminator-value. If none is specified, the fully qualified Java class name is used.
For information about inheritance mappings, see Chapter 9, Inheritance Mapping.

5.1.16. joined-subclass

Alternatively, each subclass may be mapped to its own table (table-per-subclass mapping strategy). Inherited state is retrieved by joining with the table of the superclass. We use the <joined-subclass> element.
<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>
name: The fully qualified class name of the subclass.
table: The name of the subclass table.
proxy (optional): Specifies a class or interface to use for lazy initializing proxies.
lazy (optional, defaults to true): Setting lazy="false" disables the use of lazy fetching.
No discriminator column is 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 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.

5.1.17. union-subclass

A third option is to map only the concrete classes of an inheritance hierarchy to tables, (the table-per-concrete-class strategy) where each table defines all persistent state of the class, including inherited state. In Hibernate, it is not absolutely necessary to explicitly map such inheritance hierarchies. You can simply 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.
<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>
name: The fully qualified class name of the subclass.
table: The name of the subclass table.
proxy (optional): Specifies a class or interface to use for lazy initializing proxies.
lazy (optional, defaults to true): Setting lazy="false" disables the use of lazy fetching.
No discriminator column or key column is required for this mapping strategy.
For information about inheritance mappings, see Chapter 9, Inheritance Mapping.

5.1.18. join

Using the <join> element, it is possible to map properties of one class to several tables, when there's a 1-to-1 relationship between the tables.
<join
        table="tablename"
        schema="owner"
        catalog="catalog"
        fetch="join|select"
        inverse="true|false"
        optional="true|false">
        
        <key ... />
        
        <property ... />
        ...
</join>
table: The name of the joined table.
schema (optional): Override the schema name specified by the root <hibernate-mapping> element.
catalog (optional): Override the catalog name specified by the root <hibernate-mapping> element.
fetch (optional - defaults to join): If set to join, the default, Hibernate will use an inner join to retrieve a <join> defined by a class or its superclasses and an outer join for a <join> defined by a subclass. If set to select then Hibernate will use a sequential select for a <join> defined on a subclass, which will be issued only if a row turns out to represent an instance of the subclass. Inner joins will still be used to retrieve a <join> defined by the class and its superclasses.
inverse (optional - defaults to false): If enabled, Hibernate will not try to insert or update the properties defined by this join.
optional (optional - defaults to false): If enabled, Hibernate will insert a row only if the properties defined by this join are non-null and will always use an outer join to retrieve the properties.
For example, the 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.

5.1.19. key

We've seen the <key> element crop up a few times now. It appears anywhere the parent mapping element defines a join to a new table, and defines the foreign key in the joined table, that references the primary key of the original table.
<key
        column="columnname"
        on-delete="noaction|cascade"
        property-ref="propertyName"
        not-null="true|false"
        update="true|false"
        unique="true|false"
/>
column (optional): The name of the foreign key column. This may also be specified by nested <column> element(s).
on-delete (optional, defaults to noaction): Specifies whether the foreign key constraint has database-level cascade delete enabled.
property-ref (optional): Specifies that the foreign key refers to columns that are not the primary key of the orginal table. (Provided for legacy data.)
not-null (optional): Specifies that the foreign key columns are not nullable (this is implied whenever the foreign key is also part of the primary key).
update (optional): Specifies that the foreign key should never be updated (this is implied whenever the foreign key is also part of the primary key).
unique (optional): Specifies that the foreign key should have a unique constraint (this is implied whenever the foreign key is also the primary key).
We recommend that for systems where delete performance is important, all keys should be defined on-delete="cascade", and Hibernate will use 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 to a non-nullable foreign key, you must declare the key column using <key not-null="true">.

5.1.20. column and formula elements

Any mapping element which accepts a column attribute will alternatively accept a <column> subelement. Likewise, <formula> is an alternative to the formula attribute.
<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 may 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>

5.1.21. import

Suppose your application has two persistent classes with the same name, and you don't want to specify the fully qualified (package) name in Hibernate queries. Classes may be "imported" explicitly, rather than relying upon auto-import="true". You may even import classes and interfaces that are not explicitly mapped.
<import class="java.lang.Object" rename="Universe"/>
<import
        class="ClassName"
        rename="ShortName"
/>
class: The fully qualified class name of of any Java class.
rename (optional - defaults to the unqualified class name): A name that may be used in the query language.

5.1.22. any

There is one further type of property mapping. The <any> mapping element defines a polymorphic association to classes from multiple tables. This type of mapping always requires more than one column. The first column holds the type of the associated entity. The remaining columns hold the identifier. It is impossible to specify a foreign key constraint for this kind of association, so this is most certainly not meant as the usual way of mapping (polymorphic) associations. You should use this only in very special cases (eg. audit logs, user session data, etc).
The meta-type attribute lets the application specify a custom type that maps database column values to persistent classes which 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>
name: the property name.
id-type: the identifier type.
meta-type (optional - defaults to string): Any type that is allowed for a discriminator mapping.
cascade (optional- defaults to none): the cascade style.
access (optional - defaults to property): The strategy Hibernate should use for accessing the property value.
optimistic-lock (optional - defaults to true): Specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, define if a version increment should occur if this property is dirty.