The simplest way of using application identity is where you have a single PK field, and in this case you use the JDO 2 SingleFieldIdentity . mechanism. This provides a PrimaryKey for cases where you have a single PK field and you don't need to specify the objectid-class . Let's take an example

public class MyClass
{
    long id;
    String name;
    String description;
    ...
}

<class name="MyClass" identity-type="application">
    <field name="id" primary-key="true"/>
    <field name="name"/>
    <field name="description"/>
</class>

So we didnt specify the JDO "objectid-class". You will, of course, have to give the field a value before persisting the object, either by setting it yourself, or by using a value-strategy on that field.

If you need to create an identity of this form for use in querying via pm.getObjectById() then you can create the identities in the following way

For a "long" type :
javax.jdo.identity.LongIdentity id = new javax.jdo.identity.LongIdentity(myClass, 101);

For a "String" type :
javax.jdo.identity.StringIdentity id = new javax.jdo.identity.StringIdentity(myClass, "ABCD");

We have shown an example above for type "long", but you can also use this for the following

short, Short       - javax.jdo.identity.ShortIdentity
int, Integer       - javax.jdo.identity.IntIdentity
long, Long         - javax.jdo.identity.LongIdentity
String             - javax.jdo.identity.StringIdentity
char, Character    - javax.jdo.identity.CharIdentity
byte, Byte         - javax.jdo.identity.ByteIdentity
java.util.Date     - javax.jdo.identity.ObjectIdentity
java.util.Currency - javax.jdo.identity.ObjectIdentity
java.util.Locale   - javax.jdo.identity.ObjectIdentity

If you wish to use application identity and don't want to use the "SingleFieldIdentity" builtin PK classes then you must define a Primary Key class of your own. You can't use classes like java.lang.String, or java.lang.Long directly. You must follow these rules when defining your primary key class.

• the Primary Key class must be public
• the Primary Key class must implement Serializable
• the Primary Key class must have a public no-arg constructor, which might be the default constructor
• the field types of all non-static fields in the Primary Key class must be serializable, and are recommended to be primitive, String, Date, or Number types
• all serializable non-static fields in the Primary Key class must be public
• the names of the non-static fields in the Primary Key class must include the names of the primary key fields in the JDO class, and the types of the common fields must be identical
• the equals() and hashCode() methods of the Primary Key class must use the value(s) of all the fields corresponding to the primary key fields in the JDO class
• if the Primary Key class is an inner class, it must be static
• the Primary Key class must override the toString() method defined in Object, and return a String that can be used as the parameter of a constructor
• the Primary Key class must provide a String constructor that returns an instance that compares equal to an instance that returned that String by the toString() method.
• the Primary Key class must be only used within a single inheritence tree.

Here's an example of a composite (multiple field) primary key class

public class ComposedIdKey implements Serializable
{
    public String field1;
    public String field2;

    /**
     *  Default constructor.
     */
    public ComposedIdKey ()
    {
    }

    /**
     * String constructor.
     */
    public ComposedIdKey(String value) 
    {
        StringTokenizer token = new StringTokenizer (value, "::");
        //className
        token.nextToken ();
        //field1
        this.field1 = token.nextToken ();
        //field2
        this.field2 = token.nextToken ();
    }

    /**
     * Implementation of equals method.
     */
    public boolean equals(Object obj)
    {
        if (obj == this)
        {
            return true;
        }
        if (!(obj instanceof ComposedIdKey))
        {
            return false;
        }
        ComposedIdKey c = (ComposedIdKey)obj;

        return field1.equals(c.field1) && field2.equals(c.field2);
    }

    /**
     *  Implementation of hashCode method that supports the
     *  equals-hashCode contract.
     */
    public int hashCode ()
    {
        return this.field1.hashCode() ^ this.field2.hashCode();
    }

    /**
     *  Implementation of toString that outputs this object id's PK values.
     */
    public String toString ()
    {
        return this.getClass().getName() + "::"  + this.field1 + "::" + this.field2;
    }
}