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: ]]> 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/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. 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. ]]> 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. You may declare a persistent class using the class element: ]]> name: The fully qualified Java class name of the persistent class (or interface). 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): Hibernate3 supports class-less persistence: use the <dynamic-class> instead of a <class> mapping and an entity-name attribute instead of a class name. This allows you to implement your domain model using maps of maps or any other dynamic approach. See for more information. catalog (optional): The name of a database catalog used for this class and its table. 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 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 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: select id, name as ident, address as loc, 'human' as species from humans union select id, ident, planet as loc, species from aliens ...]]> 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. 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 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 important! If the identfier property of your class does not default to the normal Java default value (null or zero), 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 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. 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. 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 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. 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. person_id_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. This generator is used when the primary key is a natural key instead of a surrogate key. 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 timestampe property, or you define Interceptor.isUnsaved(). For legacy schemas only (Hibernate does not generate DDL with triggers). socialSecurityNumber ]]> 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. ...... ]]> 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 undefined): Indicates that transient instances should be considered newly instantiated, if set to any, or detached, if set to none. It is best to leave the default value in all cases. 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. (Tells Hibernate to not include the column in SQL INSERTs.) 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: ]]> 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 detached 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 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! 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 detached 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. 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, define if a version check should be made if this property is dirty. 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 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 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: ]]> 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. 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. ]]> 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. join (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. 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, define if a version check should be made if this property is dirty. Setting a value of the cascade attribute to not none will propagate certain operations to the associated object (usually in a parent/child fashion). See "Lifecycle Objects" discussed later. 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. If the referenced unique key comprises multiple properties of the associated entity, you should map the referenced properties inside a named <properties> element. 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). 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. 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. 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, define if a version check should be made if this property is dirty. 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 . 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. ..... ]]> 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 . 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. ..... ]]> 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: ]]> For information about inheritance mappings, see . 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. ..... ]]> 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 . Using the <join> element, it is possible to map properties of one class to several tables. ... ]]> 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 select for a join defined on a subclass, then rather than joining, a sequential select will be issued only if a row turns out to represent an instance of the subclass. 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 . For example, the address information for a person can be mapped to a separate table (while preserving value type semantics for all properties): ... ...]]> 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. 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. ]]> 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.) 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. 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 (not whats inside a collection), components and certain immutable objects. Unlike entities, values (in particular collections and components) are persisted and deleted by reachability. Since value objects (and primitives) are persisted and deleted along with their containing entity they may not be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections. 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. A Java property of type java.lang.String also has value semantics. Given this definition, we can say that all types (classes) provided by the JDK have value type semantics in Java, while user-defined types may be mapped with entity or value type semantics. This decision is up to the application developer. A good hint for an entity class in a domain model are shared references to a single instance of that class, while composition or aggregation usually translates to a value type. We'll revisit both concepts throughout the documentation. The challenge is to map the Java type system (and the developers definition of entities and value types) to the SQL/database type system. The bridge between both systems is provided by Hibernate: for entities we use <class>, <subclass< and so on. For value types we use <property>, <component>, etc, usually with a type attribute. The value of this attribute is the name of a Hibernate mapping type. Hibernate provides many mappings (for standard JDK value types) out of the box. You can write your own mapping types and implement your custom conversion strategies as well, as you'll see later. All built-in Hibernate types except collections support null semantics. The built-in basic mapping 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. 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. 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. You may also supply parameters to a UserType in the mapping file. To do this, your UserType has to implement the org.hibernate.usertype.ParameterizedType interface. To supply parameters to your UserType, you can use the <type> element in your mapping files. 1 ]]> The UserType can now retrieve the value for the parameter named default from the Properties object passed to it. If you use a certain UserType very often, it may be useful to define a shorter name for it. You can do this using the <typedef> element in your mapping files. Typedefs contain a UserType and a defined name for it, and may also contain a list of default paramter values if the type defined implements ParameterizedType. After defining the name, it can be used everywhere the original classname can be used. 2 ]]> It is also possible to override the parameters supplied in a typedef on a case-by-case basis by using type parameters on the property mapping. Even though Hibernate's rich range of built-in types and support for components means you will 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. TODO: document parameterizable usertypes 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. ]]> ..... ..... ]]> 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 check should be made if this property is dirty. 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, union-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. Note: Previously this feature made the ordering of the mapping documents important. Since Hibernate3, the ordering of mapping files does not matter when using the extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses before subclasses. ]]> Many Hibernate users prefer to embed mapping information directly in sourcecode using XDoclet @hibernate.tags. We will not cover this approach in this document, since strictly it is considered part of XDoclet. However, we include the following example of the Cat class with XDoclet mappings. See the Hibernate web site for more examples of XDoclet and Hibernate. JDK 5.0 introduced XDoclet-style annotations at the language level, type-safe and checked at compile time. This mechnism is more powerful than XDoclet annotations and better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification (JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans. Hibernate3 implements the EntityManager of JSR-220 (the persistence API), support for mapping metadata is available via the Hibernate Annotations package, as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported. This is an example of a POJO class annotated as an EJB entity bean: Note that support for JDK 5.0 Annotations (and JSR-220) is still work in progress and not completed.