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Castor JDO Mapping

Documentation Author(s):
Bruce Snyder
Werner Guttmann

The Mapping File
    The <mapping> element
    The <class> element
    The <map-to> element
    The <field> element
    The <sql> element


Release 1.0 M3:
-Added collection type 'iterator'.
-Added collection type 'enumerate'.
-Added additional syntax for specifying the identity of a class.


The Castor mapping file also provides a mechanism for binding a Java object model to a relational database model. This is usually referred to as object-to-relational mapping (O/R mapping). O/R mapping bridges the gap between an object model and a relational model.

The mapping file provides a description of the Java object model to Castor JDO. Via Castor XML, Castor JDO uses the information in the mapping file to map Java objects to relational database tables. The following is a high-level example of a mapping file:

      <class ... >
          <map-to ... />
          <field ... >
              <sql ... />

Each Java object is represented by a <class> element composed of attributes, a <map-to> element and <field> elements. The <map-to> element contains a reference to the relational table to which the Java object maps. Each <field> element represents either a public class variable or the variable's accessor/mutator methods (depending upon the mapping info). Each <field> element is composed of attributes and one <sql> element. The <sql> element represents the field in the relational table to which the <field> element maps.

It is possible to use the mapping file and Castor's default behavior in conjunction. When Castor handles an object but is unable to locate information about it in the mapping file, it will rely upon its default behavior. Castor makes use of the Java programming language Reflection API to introspect the Java objects to determine the methods to use. This is the reason some attributes are not required in the mapping file.

The Mapping File

The <mapping> element

  <!ELEMENT mapping ( description?, include*, class*, key-generator* )>

The <mapping> element is the root element of a mapping file. It contains:

-an optional description
-zero or more <include> which facilitates reusing mapping files
-zero or more <class> descriptions: one for each class we intend to give mapping information
-zero or more <key-generator>: not used for XML mapping

A mapping file look like this:

  <?xml version="1.0"?>
  <!DOCTYPE mapping PUBLIC "-//EXOLAB/Castor Object Mapping DTD Version 1.0//EN"

     <description>Description of the mapping&lt;/description>
     <include href="other_mapping_file.xml"/>

     <class name="A">

     <class name="B">

The <class> element

  <!ELEMENT class ( description?, cache-type?, map-to?, field+ )>
  <!ATTLIST class
            name            ID       #REQUIRED
            extends         IDREF    #IMPLIED
            depends         IDREF    #IMPLIED
            auto-complete   ( true |false ) "false"
            identity        CDATA    #IMPLIED
            access          ( read-only | shared | exclusive | db-locked )  "shared"
            key-generator   IDREF    #IMPLIED >

The <class> element contains all the information used to map a Java object to a relational database. The content of <class> is mainly used to describe the fields that will be mapped.

Description of the attributes:

-name: The fully qualified package name of the Java object to map to.
-extends: Should be used only if this Java object extends another Java object for which mapping information is provided. It should not be used if the extended Java object is not referenced in the mapping file.
-depends: For more information on this field, please see Dependent and related relationships.
-identity: For more information on this field, please see Design -> Persistence.
-access: For more information on this field, please see Locking Modes.
-key-generator: For more information on this field, please see KeyGen.

Description of the elements:

-<description>: An optional description.
-<cache-type>: For more information on this field please see Bounded Dirty Checking and Caching.
-<map-to>: Used to tell Castor the name of the relational table to which to map.
-<field>: Zero or more <field> elements are used to describe properties of each Java object.

The <map-to> element

  <!ELEMENT map-to EMPTY>
  <!ATTLIST map-to
            table      NMTOKEN  #IMPLIED
            xml        NMTOKEN  #IMPLIED
            ns-uri     NMTOKEN  #IMPLIED
            ns-prefix  NMTOKEN  #IMPLIED
            ldap-dn    NMTOKEN  #IMPLIED
            ldap-oc    NMTOKEN  #IMPLIED>

<map-to> is used to specify the name of the item that should be associated with the given Java object. The <map-to> element is only used for the root Java object.

Description of the attributes:

-table: The name of the relational database table to which the Java object is associated.

The <field> element

  <!ELEMENT field ( description?, sql?, xml?, ldap? )>
  <!ATTLIST field
            name           NMTOKEN  #REQUIRED
            type           NMTOKEN  #IMPLIED
            required       ( true | false )  "false"
            direct         ( true | false )  "false"
            lazy           ( true | false )  "false"
            transient      ( true | false )  "false"
            identity       ( true | false )  "false"
            get-method     NMTOKEN  #IMPLIED
            set-method     NMTOKEN  #IMPLIED
            create-method  NMTOKEN  #IMPLIED
            collection     ( array | enumerate | collection | set | 
                             arraylist | vector | map | hashtable | sortedset | iterator )  #IMPLIED
            comparator     NMTOKEN  #IMPLIED>

The <field> element is used to describe a property of a Java object. It provides:

-the identity ('name') of the property
-the type of the property (inferred from 'type' and 'collection')
-the access method of the property (inferred from 'direct', 'get-method', 'set-method')

From this information, Castor is able to access a given property in the Java object.

In order to determine the signature that Castor expects, there are two easy rules to apply.

1. Determine <type>.

  • If there is no 'collection' attribute, the object type is the value of the 'type' attribute. The value of the type attribute can be a fully qualified Java object like 'java.lang.String' or one of the allowed aliases:

    short name Primitive type? Java Class

    Castor will try to cast the data in the mapping file to the proper Java type.

  • If there is a collection attribute, the items in the following table can be used:

    name type default implementation added in
    enumeratejava.util.Enumeration-1.0 M3
    sortedsetjava.util.SortedSetjava.util.TreeSet1.0 M2
    iteratorjava.util.Iterator n/a 1.0 M3

    The type of the object inside the collection is the 'type' attribute. The 'default implementation' is the type used if the object holding the collection is found to be null and needs to be instantiated.

    For hashtable and map, Castor will add an object using the put(object, object) method - the object is both the key and the value. This will be improved in the future.

  • It is necessary to use a collection when the content model of the element expects more than one element of the specified type. This is how the 'to-many' portion of a relationship is described.

    It is not possible to use a collection of type 'iterator' or 'enumerate' with lazy loading enabled.

    2. Determine the signature of the method

  • If 'direct' is set to true, Castor expects to find a public Java object variable with the given signature:

      public <type> <name>;

  • If 'direct' is set to false or omitted, Castor will access the property though accessor methods. Castor determines the signature of the accessors and mutators as follows: If the 'get-method' or 'set-method' attributes are supplied, it will try to find a function with the following signature:

      public <type> <get-method>();


      public void <set-method>(<type> value);

    If 'get-method' or 'set-method' attributes are not provided, Castor will try to find the following functions:

      public <type> is<capitalized-name>();


      public <type> get<capitalized-name>();

    the former for boolean types, the latter for all other types (or if the 'is<capitalized-name>()' method is not defined for a boolean type), and a standard set method of

      public void set<capitalized-name>(<type> value);

    If there are more than one set<capitalized-name> method it first tries to find the one that exactly matches <type>. If no such method is available and <type> is a java primitive type it tries to find a method with the corresponing java object type.

    <capitalized-name> means that Castor uses the <name> attribute by changing its first letter to uppercase without modifying the other letters.

    The content of the <field> element will contain the information about how to map this field to the relational table.

  • Description of the attributes:

    -name: If 'direct' access is used, 'name' should be the name of a public variable in the object we are mapping (the field must be public, not static and not transient). If no direct access and no 'get-/set-method' is specified, this name will be used to infer the name of the accessor and mutator methods.
    -type: The Java type of the field. This is used to access the field. Castor will use this information to cast the data type(e.g. string into integer). It is also used to define the signature of the accessor and mutator methods. If a collection is specified, this is used to specify the type of the object inside the collection. See description above for more details.
    -required: If true, the field is not optional.
    -transient: If true, the field will be ignored during persistence (and XML un-/marshalling). If you want this field to be ignored during any persistence-related operations only, please use the 'transient' attribute at the <sql> level.
    -identity: If true, the field is part of the class identity. Please use this as an alternative way of specifying the identity of an object.
    -direct: If true, Castor expects a public variable in the object and will modify it directly.
    -collection: If a parent object expects more than one occurrence of one of its fields, it is necessary to specify which collection type Castor will use to handle them. The type specified is used to define the type of the content inside the collection.
    -comparator: If the collection type equals 'sortedset', it is possible to specify a java.util.Comparator instance that will be used with the java.util.SortedSet (implementation) to specify a custom sort order. Please use this attribute to specify the class name of the Comparator instance to be used. Alternatively, it is possible to not specify a Comparator instance and have the Java objects stored in the SortedSet implement java.util.Comparable to specify the sort order.
    -get-method: An optional name of the accessor method Castor should use. If this attribute is not set, Castor will try to guess the name with the algorithm described above.
    -set-method: An optional name of the mutator method Castor should use. If this attribute is not set, Castor will try to guess the name with the algorithm described above.
    -create-method: Factory method for instantiation of the object.

    The <sql> element

      <!ELEMENT sql EMPTY>
      <!ATTLIST sql
                name        NMTOKENS  #IMPLIED
                type        NMTOKENS  #IMPLIED
                many-key    NMTOKENS  #IMPLIED
                many-table  NMTOKEN   #IMPLIED
                transient   ( true | false )    "false"
                read-only   ( true | false )    "false"
                dirty       ( check | ignore )  "check">
                cascading	( create | delete | update | all | none ) "none"

    The <sql> element is used to denote information about the database column to which a Java object is mapped. It should be declared for all <field> elements. Each <field> element contains one <sql> element. The <sql> element correlates directly to the <map-to> element for the containing <class> element. The <sql> elements contains the following attributes:

    -name: The name of the column in the database table.
    -type: The JDBC type of the column. It is inferred from the object when the type of this field is a persistent Java class that is defined elsewhere in the mapping. The complete list of automatic type conversions, and which values require manual mapping (e.g, java.util.Date) is listed in the SQL Type Conversion section of the Type Support document.
    -read-only: If true, the column in the relational database table will only be read, not updated or deleted.
    -transient (as of 0.9.9): If true, the field will be ignored during persistence only. If you want this field to be ignored during XML un-/marshalling as well, please use the 'transient' attribute at the <field> level.
    -dirty: If the value is 'ignore', the field will not be checked against the database for modification.
    -cascading: If the field is a relation, this attribute specifies which operations to cascade. Possible values are: 'all', none', 'create', 'update' or 'delete'; when not specifying 'none' or 'all', it is possibel to specify more than one value, using whitespace as a delimiter (e.g. 'create update'). For further information see HOW-TO on using cascading operation.

    There are two more attributes used only with 'to-many' relations.

    -many-key: Specifies the name of the column that holds the foreign key to this object. That column is in the database table that stores objects of the Java type of this field.
    -many-table: Specifies the name of the bridge table that contains the primary keys of the object on each side of the relationship. This is only used for many-to-many relationships.

    Copyright © 1999-2005 ExoLab Group, Intalio Inc., and Contributors. All rights reserved.
    Java, EJB, JDBC, JNDI, JTA, Sun, Sun Microsystems are trademarks or registered trademarks of Sun Microsystems, Inc. in the United States and in other countries. XML, XML Schema, XSLT and related standards are trademarks or registered trademarks of MIT, INRIA, Keio or others, and a product of the World Wide Web Consortium. All other product names mentioned herein are trademarks of their respective owners.