While the JPA specification says that you should not use rolled back objects, such objects are perfectly valid in Kodo. You can control whether the objects' managed state is rolled back to its pre-transaction values with the kodo.RestoreState configuration property. none does not roll back state (the object becomes hollow, and will re-load its state the next time it is accessed), immutable restores immutable values (primitives, primitive wrappers, strings) and clears mutable values so that they are reloaded on next access, and all restores all managed values to their pre-transaction state.

JDO requires that all immutable fields be restored to their pre-transaction state when a transaction rollback occurs. Kodo also has the ability to restore the state of mutable fields including collections, maps, and arrays. To have the state of mutable fields restored on rollback, set the kodo.RestoreState or javax.jdo.option.RestoreValues configuration property to all.

When loading data into a field, Kodo examines the value you assign the field in your declaration code or in your no-args constructor. If the field value's type is more specific than the field's declared type, Kodo uses the value type to hold the loaded data. Kodo also uses the comparator you've initialized the field with, if any. Therefore, you can use custom comparators on your persistent field simply by setting up the comparator and using it in your field's initial value.

Kodo also attempts to preserve the Java type and order of all collections declared in your JDO metadata. By default, Kodo uses an ordering column to maintain sequencing for all fields declared to be a List or array type. See Section 7.1.3, “ JDO Forward Mapping Hints ” for how to prevent the creation of ordering columns during forward mapping.

Kodo does not automatically add an ordering column to List and array fields under JPA metadata. When you use JPA metadata, Kodo respects JPA's default of leaving collections and arrays without database sequencing.

public class Company
{
    // Kodo will detect the custom comparator in the initial field value
    // and use it whenever loading data from the database into this field
    private Collection employeesBySal = new TreeSet (new SalaryComparator ());
    private Map departments;

    public Company
    {
        // or we can initialize fields in our no-args constructor; even though
        // this field is declared type Map, Kodo will detect that it's actually
        // a TreeMap and use natural ordering for loaded data
        departments = new TreeMap ();
    }

    // rest of class definition...
}

Kodo's support for the java.util.Calendar type will store only the Date part of the field, not the TimeZone associated with the field. When loading the date into the Calendar field, Kodo will use the TimeZone that was used to initialize the field.

	Note
	Kodo will automatically track changes made via modification methods in fields of type Calendar, with one exception: when using Java version 1.3, the set() method cannot be overridden, so when altering the calendar using that method, the field must be explicitly marked as dirty. This limitation does not apply when running with Java version 1.4 and higer.

At runtime, the values of all mutable second class object fields in persistent and transactional objects are replaced with implementation-specific proxies. On modification, these proxies notify their owning instance that they have been changed, so that the appropriate updates can be made on the datastore.

import kodo.persistence.*;

...

Collection employees = company.getEmployees (); // employees is a lrs collection
Iterator itr = employees.iterator ();
while (itr.hasNext ())
    process ((Employee) itr.next ());
KodoPersistence.close (itr);

import kodo.jdo.*;

...

Collection employees = company.getEmployees (); // employees is a lrs collection
Iterator itr = employees.iterator ();
while (itr.hasNext ())
    process ((Employee) itr.next ());
KodoJDOHelper.close (itr);

import kodo.persistence.*;

@Entity
public class Company
{
    @ManyToMany 
    @LRS private Collection<Employee> employees;     

    ...
}

<class name="Company">
    <field name="employees">
        <collection element-type="Employee"/>
        <extension vendor-name="kodo" key="lrs" value="true"/>
    </field>
    ...
</class>

<property name="kodo.ProxyManager" value="TrackChanges=false"/>

kodo.ProxyManager: TrackChanges=false

Kodo offers the ability to write custom field mappings in order to have complete control over the mechanism with which fields are stored, queried, and loaded from the datastore. Often, however, a custom mapping is overkill. There is often a simple transformation from a Java field value to its database representation. Thus, Kodo provides the externalization service. Externalization allows you to specify methods that will externalize a field value to its database equivalent on store and then rebuild the value from its externalized form on load.

	Note
	Fields of embeddable classes used for @EmbeddedId values in JPA cannot have externalizers.

The JPA kodo.persistence.Externalizer annotation or JDO externalizer metadata extension sets the name of a method that will be invoked to convert the field into its external form for database storage. You can specify either the name of a non-static method, which will be invoked on the field value, or a static method, which will be invoked with the field value as a parameter. Each method can also take an optional StoreContext parameter for access to a persistence context. The return value of the method is the field's external form. By default, Kodo assumes that all named methods belong to the field value's class (or its superclasses). You can, however, specify static methods of other classes using the format <class-name>.<method-name>.

Given a field of type CustomType that externalizes to a string, the table below demonstrates several possible externalizer methods and their corresponding metadata extensions.

The JPA kodo.persistence.Factory annotation or JDO factory metadata extension contains the name of a method that will be invoked to instantiate the field from the external form stored in the database. Specify a static method name. The method will will be invoked with the externalized value and must return an instance of the field type. The method can also take an optional StoreContext parameter for access to a persistence context. If a factory is not specified, Kodo will use the constructor of the field type that takes a single argument of the external type, or will throw an exception if no constructor with that signature exists.

Given a field of type CustomType that externalizes to a string, the table below demonstrates several possible factory methods and their corresponding metadata extensions.

If your externalized field is not a standard persistent type, you must explicitly mark it persistent. In JPA, you can force a persistent field by annotating it with kodo.persistence.Persistent annotation. In JDO, set the field's persistence-modifier to true. If you want the field to be in the default fetch group, also set its default-fetch-group metadata attribute to true. See Chapter 5, Metadata for complete coverage of JDO metadata.

Note

If your custom field type is mutable and is not a standard collection, map, or date class, Kodo will not be able to detect changes to the field. You must mark the field dirty manully, or create a custom field proxy. See KodoEntityManager.dirty for how to mark a field dirty manually in JPA. See JDOHelper.makeDirty for how to mark a field dirty in JDO. See Section 5.5.4, “Proxies” for a discussion of proxies. Section 1.3.6, “Custom Proxies” covers Kodo's custom proxy samples.

You can externalize a field to virtually any value that is supported by Kodo's field mappings (embedded relations are the exception; you must declare your field to be a persistence-capable type in order to embed it). This means that a field can externalize to something as simple as a primitive, something as complex as a collection or map of persistence-capable objects, or anything in between. If you do choose to externalize to a collection or map, Kodo recognizes a family of metadata extensions for specying type information for the externalized form of your fields - see Section 6.4.2.7, “Type”. If the external form of your field is a persistence-capable object, or contains persistence-capable objects, Kodo will correctly include the objects in its persistence-by-reachability algorithms and its delete-dependent algorithms.

The example below demonstrates a few forms of externalization. See Section 1.3.4, “Using Externalization to Persist Second Class Objects” for a working example of externalizing many different field types.

import kodo.persistence.*;

@Entity
public class Magazine
{
    // use Class.getName and Class.forName to go to/from strings
    @Persistent
    @Externalizer("getName") 
    @Factory("forName")
    private Class cls;

    // use URL.getExternalForm for externalization. no factory; 
    // we can rely on the URL string constructor
    @Persistent
    @Externalizer("toExternalForm")
    private URL url;

    // use our custom methods; notice how we use the KeyType and ElementType 
    // annotations to specify the metadata for our externalized map
    @Persistent 
    @Externalizer("Magazine.mapFromCustomType")
    @Factory("Magazine.mapToCustomType")
    @KeyType(String.class) @ElementType(String.class)
    private CustomType customType;

    public static Map mapFromCustomType (CustomType customType)
    {
        ... logic to pack custom type into a map ...
    }

    public static CustomType mapToCustomType (Map map)
    {
        ... logic to create custom type from a map ...
    }

    ...
}

public class Magazine
{
    private Class      cls;
    private URL        url;
    private CustomType customType;

    public static Map mapFromCustomType (CustomType customType)
    {
        ... logic to pack custom type into a map ...
    }

    public static CustomType mapToCustomType (Map map)
    {
        ... logic to create custom type from a map ...
    }

    ...
}

<class name="Magazine">
    <field name="cls" persistence-modifier="persistent"
        default-fetch-group="true">
        <!-- use Class.getName and Class.forName to go to/from strings -->
        <extension vendor-name="kodo" key="externalizer" value="getName"/>
        <extension vendor-name="kodo" key="factory" value="forName"/>
    </field>
    <field name="url" persistence-modifier="persistent"
        default-fetch-group="true">
        <!-- use URL.getExternalForm for externalization. no    -->
        <!-- factory; we can rely on the URL string constructor -->
        <extension vendor-name="kodo" key="externalizer" 
            value="toExternalForm"/>
    </field>
    <field name="customType" persistence-modifier="persistent">
        <!-- use our custom methods; notice how we use the  -->
        <!-- key-type and value-type extensions to specify  -->
        <!-- the metadata for our externalized map          -->
        <extension vendor-name="kodo" key="externalizer" 
            value="Magazine.mapFromCustomType"/>
        <extension vendor-name="kodo" key="factory" 
            value="Magazine.mapToCustomType"/>
        <extension vendor-name="kodo" key="key-type" value="String"/>
        <extension vendor-name="kodo" key="value-type" value="String"/>
    </field>
    ... 
</class>

You can query externalized fields using parameters. Pass in a value of the field type when executing the query. Kodo will externalize the parameter using the externalizer method named in your metadata, and compare the externalized parameter with the value stored in the database. As a shortcut, Kodo also allows you to use parameters or literals of the field's externalized type in queries, as demonstrated in the example below.

	Note
	Currently, queries are limited to fields that externalize to a primitive, primitive wrapper, string, or date types, due to constraints on query syntax.

// you can query using parameters
Query q = em.createQuery ("select m from Magazine m where m.url = :u");
q.setParameter ("u", new URL ("http://www.solarmetric.com"));
List results = q.getResultList ();

// or as a shortcut, you can use the externalized form directly
q = em.createQuery ("select m from Magazine m where m.url = 'http://www.solarmetric.com'");
results = q.getResultList ();

// you can query using parameters
Query q = pm.newQuery (Magazine.class, "url == :u");
List results = (List) q.execute (new URL ("http://www.solarmetric.com"));

// or as a shortcut, you can use the externalized form directly
q = pm.newQuery (Magazine.class, "url == 'http://www.solarmetric.com'");
results = (List) q.execute ();

public class Magazine
{
    @ExternalValues({"SMALL=5", "MEDIUM=8", "LARGE=10"})
    @Type(int.class)
    private String sizeWidth;

    ...
}

public class Magazine
{
    private String sizeWidth;

    ...
}

<class name="Magazine">
	<field name="sizeWidth">
		<extension vendor-name="kodo" key="external-values"
			value="SMALL=5,MEDIUM=8,LARGE=10"/>
		<extension vendor-name="kodo" key="type" value="int"/>
	</field>
	... 
</class>