Object/relational mappings are 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 define XML mappings be hand, a number of tools exist to generate the mapping document, including XDoclet, Middlegen and AndroMDA. Lets kick off with an example 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.) 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/net/sf/hibernate or in hibernate.jar. Hibernate will always look for the DTD in its classpath first. This element has three optional attributes. The schema attribute specifies that tables referred to by this mapping belong to the named schema. If specified, tablenames will be qualified by the given schema name. 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. ]]> schema (optional): The name of a database schema. default-cascade (optional - defaults to none): A default cascade style. 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. You may declare a persistent class using the class element: ]]> name: The fully qualified Java class name of the persistent class (or interface). table: 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. 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): Setting lazy="true" is a shortcut equalivalent to specifying the name of the class itself as the proxy interface. 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 be 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 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 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 outside of the session (ie. when Session.update() is used). Keep in mind that a version or timestamp property should never be null, no matter what unsaved-value strategy, or an instance will be detected as transient. 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. ]]> 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 null): An identifier property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from transient 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 important! If the identfier property of your class does not default to null, then you should specify the actual default. There is an alternative <composite-id> declaration to allow access to legacy data with composite keys. We strongly discourage its use for anything else. The required <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. uid_table next_hi_value_column ]]> 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. Do not use this generator with connections enlisted with JTA or with a user-supplied connection. seqhilo uses a hi/lo algorithm to efficiently generate identifiers of type long, short or int, given a named database sequence. uuid.hex 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. uuid.string uses the same UUID algorithm. The UUID is encoded a string of length 16 consisting of (any) ASCII characters. Do not use with PostgreSQL. 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. foreign uses the identifier of another associated object. Usually used in conjunction with a <one-to-one> primary key association. 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). hi_value next_value 100 ]]> hi_value 100 ]]> Unfortunately, you can't use hilo when supplying your own Connection to Hibernate, or when Hibernate is using an application server datasource to obtain connections enlisted with JTA. Hibernate must be able to fetch the "hi" value in a new transaction. A standard approach in an EJB environment is to implement the hi/lo algorithm using a stateless session bean. The UUIDs contain: 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. Don't try to use uuid.string in PostgreSQL. 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. uid_sequence ]]> ]]> For cross-platform development, the native strategy will choose from the identity, sequence and hilo strategies, dependant upon the capabilities of the underlying database. 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. Be very careful when using this feature to assign keys with business meaning (almost always a terrible design decision). Due to its inherent nature, entities that use this generator cannot be saved via the Session's saveOrUpdate() method. Instead you have to explicitly specify to Hibernate if the object should be saved or updated by calling either the save() or update() method of the Session. ...... ]]> 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. ]]> 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 will describe a much more convenient approach where the composite identifier is implemented as a seperate class in . The attributes described below apply only to this alternative approach: name (optional): A property of component type that holds the composite identifier (see next section). class (optional - defaults to the property type determined by reflection): The component class used as a composite identifier (see next section). unsaved-value (optional - defaults to none): Indicates that transient instances should be considered newly instantiated, if set to any. 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. ]]> 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. 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 <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). ]]> 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 transient instances that were saved or loaded in a previous session. (undefined specifies that the identifier property value should be used.) Version numbers may be of type long, integer, short, timestamp or calendar. 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. ]]> 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 transient instances that were saved or loaded in a previous session. (undefined specifies that the identifier property value should be used.) Note that <timestamp> is equivalent to <version type="timestamp">. The <property> element declares a persistent, JavaBean style property of the class. ]]> 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. 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. typename could be: The name of a Hibernate basic type (eg. integer, string, character, date, timestamp, float, binary, serializable, object, blob). 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). The name of a subclass of PersistentEnum (eg. eg.Color). The name of a serializable Java class. 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 ordinary association to another persistent class is declared using a many-to-one element. The relational model is a many-to-one association. (Its really just an object reference.) ]]> name: The name of the property. column (optional): The name of the column. 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. outer-join (optional - defaults to auto): enables outer-join fetching for this association when hibernate.use_outer_join is set. 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. The cascade attribute permits the following values: all, save-update, delete, none. Setting a value other than none will propagate certain operations to the associated (child) object. See "Lifecycle Objects" below. The outer-join attribute accepts three different values: auto (default) Fetch the association using an outerjoin if the associated class has no proxy true Always fetch the association using an outerjoin false Never fetch the association using an outerjoin A typical many-to-one declaration looks as simple as ]]> 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.) ]]> Then the mapping for OrderItem might use: ]]> This is certainly not encouraged, however. A one-to-one association to another persistent class is declared using a one-to-one element. ]]> 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 is also used by the schema export tool). outer-join (optional - defaults to auto): Enable outer-join fetching for this association when hibernate.use_outer_join is set. 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. 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. ]]> ]]> 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: employee ... ]]> A newly saved instance of Person is then assigned the same primar 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: ]]> And this association may be made bidirectional by adding the following to the Person mapping: ]]> 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. ........ ]]> 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 INSERTs? update: Do the mapped columns appear in SQL UPDATEs? access (optional - defaults to property): The strategy Hibernate should use for accessing the property value. 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. Finally, polymorphic persistence requires the declaration of each subclass of the root persistent class. For the (recommended) table-per-class-hierarchy mapping strategy, the <subclass> declaration is used. ..... ]]> 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): Setting lazy="true" is a shortcut equalivalent to specifying the name of the class itself as the proxy interface. 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. Alternatively, a subclass that is persisted to its own table (table-per-subclass mapping strategy) is declared using a <joined-subclass> element. ..... ]]> name: The fully qualified class name of the subclass. proxy (optional): Specifies a class or interface to use for lazy initializing proxies. lazy (optional): Setting lazy="true" is a shortcut equalivalent to specifying the name of the class itself as the proxy interface. 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: ]]> Collections are discussed later. 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. ]]> ]]> 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. To understand the behaviour of various Java language-level objects with respect to the persistence service, we need to classify them 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 (except that saves and deletions may be cascaded from a parent entity to its children). This is different from the ODMG model of object persistence by reachablity - and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They may also be versioned. An entity's persistent state consists of references to other entities and instances of value types. Values are primitives, collections, 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 may not be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections. All Hibernate types except collections support null semantics. Up until now, we've been using the term "persistent class" to refer to entities. We will continue to do that. Strictly speaking, however, not all user-defined classes with persistent state are entities. A component is a user defined class with value semantics. The basic types may be roughly categorized into integer, long, short, float, double, character, byte, boolean, yes_no, true_false Type mappings from Java primitives or wrapper classes to appropriate (vendor-specific) SQL column types. boolean, yes_no and true_false are all alternative encodings for a Java boolean or java.lang.Boolean. string A type mapping from java.lang.String to VARCHAR (or Oracle VARCHAR2). date, time, timestamp Type mappings from java.util.Date and its subclasses to SQL types DATE, TIME and TIMESTAMP (or equivalent). calendar, calendar_date Type mappings from java.util.Calendar to SQL types TIMESTAMP and DATE (or equivalent). big_decimal A type mapping from java.math.BigDecimal to NUMERIC (or Oracle NUMBER). locale, timezone, currency Type mappings from java.util.Locale, java.util.TimeZone and java.util.Currency to VARCHAR (or Oracle VARCHAR2). Instances of Locale and Currency are mapped to their ISO codes. Instances of TimeZone are mapped to their ID. class A type mapping from java.lang.Class to VARCHAR (or Oracle VARCHAR2). A Class is mapped to its fully qualified name. binary Maps byte arrays to an appropriate SQL binary type. text Maps long Java strings to a SQL CLOB or TEXT type. serializable Maps serializable Java types to an appropriate SQL binary type. You may also indicate the Hibernate type serializable with the name of a serializable Java class or interface that does not default to a basic type or implement PersistentEnum. clob, blob Type mappings for the JDBC classes java.sql.Clob and java.sql.Blob. These types may be inconvenient for some applications, since the blob or clob object may not be reused outside of a transaction. (Furthermore, driver support is patchy and inconsistent.) Unique identifiers of entities and collections may be of any basic type except binary, blob and clob. (Composite identifiers are also allowed, see below.) The basic value types have corresponding Type constants defined on org.hibernate.Hibernate. For example, Hibernate.STRING represents the string type. An enumerated type is a common Java idiom where a class has a constant (small) number of immutable instances. You may create a persistent enumerated type by implementing org.hibernate.PersistentEnum, defining the operations toInt() and fromInt(): The Hibernate type name is simply the name of the enumerated class, in this case eg.Color. 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. But 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. Check out org.hibernate.test.DoubleStringType to see the kind of things that are possible. ]]> Notice the use of <column> tags to map a property to multiple columns. Even though Hibernate's rich range of built-in types and support for components means you will very rarely need to use a custom type, it is nevertheless considered good form to use custom types for (non-entity) classes that occur frequently in your application. For example, a MonetoryAmount class is a good candidate for a CompositeUserType, even though it could easily be mapped as a component. One motivation for this is abstraction. With a custom type, your mapping documents would be future-proofed against possible changes in your way of representing monetory values. 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. If the meta-type returns instances of java.lang.Class, nothing else is required. On the other hand, if it is a basic type like string or character, you must specify the mapping from values to classes. ]]> ..... ..... ]]> name: the property name. id-type: the identifier type. meta-type (optional - defaults to class): a type that maps java.lang.Class to a single database column or, alternatively, a 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. The old object type that filled a similar role in Hibernate 1.2 is still supported, but is now semi-deprecated. You may 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 (usually double quotes, but brackets for SQL Server and backticks for MySQL). ... ]]> It is possible to define subclass and joined-subclass mappings in seperate mapping documents, directly beneath hibernate-mapping. This allows you to extend a class hierachy just by adding a new mapping file. You must specify an extends attribute in the subclass mapping, naming a previously mapped superclass. Use of this feature makes the ordering of the mapping documents important! ]]>