While the JPA specification says that you should not use rolled back objects, such objects are perfectly valid in OpenJPA. You can control whether the objects' managed state is rolled back to its pre-transaction values with the openjpa.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.

When loading data into a field, OpenJPA 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, OpenJPA uses the value type to hold the loaded data. OpenJPA 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.

public class Company {

    // OpenJPA 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, OpenJPA will detect that it's 
        // actually a TreeMap and use natural ordering for loaded data
        departments = new TreeMap();
    }

    // rest of class definition...
}

OpenJPA'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, OpenJPA will use the TimeZone that was used to initialize the field.

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 org.apache.openjpa.persistence.*;

...

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

import org.apache.openjpa.persistence.*;

@Entity
public class Company {

    @ManyToMany
    @LRS private Collection<Employee> employees;

    ...
}

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

OpenJPA 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, OpenJPA 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.

The OpenJPA org.apache.openjpa.persistence.Externalizer annotation 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, OpenJPA 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 OpenJPA org.apache.openjpa.persistence.Factory annotation 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, OpenJPA 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 OpenJPA, you can force a persistent field by annotating it with org.apache.openjpa.persistence.Persistent annotation.

You can externalize a field to virtually any value that is supported by OpenJPA'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 entities, or anything in between. If you do choose to externalize to a collection or map, OpenJPA recognizes a family of metadata extensions for specying type information for the externalized form of your fields - see Section 3.2.6, “ Type ”. If the external form of your field is an entity object or contains entities, OpenJPA 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.

import org.apache.openjpa.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
    @Persistent
    @Externalizer("Magazine.authorsFromCustomType")
    @Factory("Magazine.authorsToCustomType")
    @ElementType(Author.class)
    private CustomType customType;

    public static Collection authorsFromCustomType(CustomType customType) {
        ... logic to pack custom type into a list of authors ...
    }

    public static CustomType authorsToCustomType (Collection authors) {
        ... logic to create custom type from a collection of authors ...
    }

    ...
}

You can query externalized fields using parameters. Pass in a value of the field type when executing the query. OpenJPA 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, OpenJPA also allows you to use parameters or literals of the field's externalized type in queries, as demonstrated in the example below.

// 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();

public class Magazine {

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

    ...
}