The class element may contain
extension elements, field
elements, and fetch-group elements.
field elements represent fields declared
by the persistent class. These elements are optional; if a field
declared in the class is not named by some field
element, then its properties are defaulted as explained in the
attribute listings below. Thanks to JDO's comprehensive set of
defaults, most fields do not need to be listed explicitly.
field elements accept the following attributes:
name: The name of the field, as it is declared in the persistent class. This attribute is required.persistence-modifier: Specifies how JDO should manage the field. Legal values arepersistentfor persistent fields,transactionalfor fields that are non-persistent but can be rolled back along with the current transaction, andnone. The default value of this attribute is based on the type of the field:Fields declared
static, transient, orfinaldefault tonone.Fields of any primitive or primitive wrapper type default to
persistent.Fields of types
java.lang.String, java.lang.Number, java.math.BigDecimal, java.math.BigInteger, java.util.Locale, andjava.util.Datedefault topersistent.Fields of any user-defined persistence-capable type default to
persistent.Arrays of any of the types above default to
persistent.Fields of the following container types in the
java.utilpackage default topersistent:Collection, Set, List, Map, ArrayList, HashMap, HashSet, Hashtable, LinkedList, TreeMap, TreeSet, Vector.All other fields default to
none.
primary-key: Set this attribute totrueif the class uses application identity and this field is a primary key field. Defaults tofalse.null-value: Specifies the treatment ofnullvalues when the field is written to the datastore. Use a value ofnoneif the data store should hold anullvalue for the field. Usedefaultto write a datastore default value instead. Finally, useexceptionif you want the JDO implementation to throw an exception if the field isnullat flush time. Defaults tonone.default-fetch-group: Default fetch group fields are managed as a single unit for efficiency. They are typically loaded as a block from the datastore, and are often written as a block as well. This attribute defaults totruefor primitive, primitive wrapper,String, Date, BigDecimal, andBigIntegertypes. All other types default tofalse.dependent: Set this attribute totrueto create a dependent relation. In a dependent relation, the referenced object is deleted whenever the owning object is deleted, or whenever the relation is severed by nulling or resetting the owning field. For example, if theMagazine.coverArticlefield is marked dependent, then settingMagazine.coverArticleto a newArticleinstance will automatically delete the oldArticlestored in the field. Similarly, deleting aMagazineobject will automatically delete its current coverArticle.The
dependentattribute is only meaningful if the field holds a reference to another persistence-capable object.![[Note]](img/note.gif)
Note Kodo allows you to assign a dependent object that has been severed from its owning field or whose owner has been deleted to another relation within the same transaction. This will prevent the dependent object from being deleted.
embedded: This is a hint to the JDO implementation to store the field as part of the class instance in the datastore, rather than as a separate entity. For example, a relational implementation might store aCompany'sAddressproperties in the same database row as theCompanyrecord. Embedded defaults totruefor primitive, primitive wrapper,Date, BigDecimal, BigInteger, array, collection, and map types. All other types default tofalse. Embedded objects do not appear in theExtentfor their class, and cannot be retrieved directly by query.Any embedded relation to another persistence-capable object is automatically dependent as well.
All field elements may contain
extension child elements. field
elements that represent array, collection, or map fields may also
contain a single array,
collection, or map child element,
respectively. Each of these elements may contain additional
extension elements in turn.
The array element has two attributes:
dependent-element: Equivalent to thefieldelement'sdependentattribute, but applies to the values stored in each array index. In addition to the owning field being reset or nulled, dependent elements can be severed from their owner by removing the element from the array. This will cause the element value to be deleted.embedded-element: This attribute mirrors theembeddedattribute of thefieldelement, but applies to the values stored in each array index.
The collection element also has the
dependent-element and embedded-element
attributes. Additionally, it declares the
element-type attribute. Use this
attribute to tell the JDO implementation what class of objects the
collection contains. This is important for efficient storage of the
collection contents. The element-type defaults to
java.lang.Object unless you are using Java 5
parameterized types, in which case it defaults to the type parameter
in your field declaration.
The map element define six attributes. They are:
key-type: The class of objects used for map keys. Similar toelement-typeas described above. Thekey-typedefaults tojava.lang.Objectunless you are using Java 5 parameterized types, in which case it defaults to the key type parameter in your field declaration.dependent-key: Same as thedependent-elementattribute of arrays and collections, but applies to map keys.embedded-key: Same as theembedded-elementattribute of arrays and collections, but applies to map keys.value-type: The class of objects used for map values. Thevalue-typedefaults tojava.lang.Objectunless you are using Java 5 parameterized types, in which case it defaults to the value type parameter in your field declaration.dependent-value: Same as thedependent-elementelement of arrays and collections, but applies to map values.embedded-value: Same as theembedded-elementelement of arrays and collections, but applies to map values.
