9.2. JPA Extensions
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## 9.2. JPA Extensions

The following sections outline the runtime interfaces you can use to access Kodo-specific functionality from JPA. Each interface contains services and convenience methods missing from the JPA specification. Kodo strives to use the same naming conventions and API patterns as standard JPA methods in all extensions, so that Kodo JDO APIs feel as much as possible like standard JPA.

You may have noticed the examples throughout this document using the `OpenJPAPersistence.cast` methods to cast from standard JPA interfaces to Kodo extended interfaces. This is the recommended practice. Some application server vendors may proxy Kodo's JPA implementation, preventing a straight cast. `OpenJPAPersistence`'s `cast` methods work around these proxies.

    public static OpenJPAEntityManagerFactory cast (EntityManagerFactory emf);
    public static OpenJPAEntityManager cast (EntityManager em);
    public static OpenJPAQuery cast (Query q);

We provide additional information on the `OpenJPAPersistence` helper below.

### 9.2.1. OpenJPAEntityManagerFactory

The `org.apache.openjpa.persistence.OpenJPAEntityManagerFactory` interface extends the basic `javax.persistence.EntityManagerFactory` with Kodo-specific features. The `OpenJPAEntityManagerFactory` offers APIs to obtain managed and unmanaged `EntityManager`s from the same factory, to access the Kodo data and query caches, and to perform other Kodo-specific operations. See the interface Javadoc for details.

### 9.2.2. OpenJPAEntityManager

All Kodo `EntityManager`s implement the `org.apache.openjpa.persistence.OpenJPAEntityManager` interface. This interface extends the standard `javax.persistence.EntityManager`. Just as the standard `EntityManager` is the primary window into JPA services, the `OpenJPAEntityManager` is the primary window from JPA into Kodo-specific functionality. We strongly encourage you to investigate the API extensions this interface contains.

### 9.2.3. OpenJPAQuery

Kodo extends JPA's standard query functionality with the `org.apache.openjpa.persistence.OpenJPAQuery` interface. See its Javadoc for details on the convenience methods it provides.

### 9.2.4. Extent

An `Extent` is a logical view of all persistent instances of a given entity class, possibly including subclasses. Kodo adds the `org.apache.openjpa.persistence.Extent` class to the set of Java Persistence APIs. The following code illustrates iterating over all instances of the `Magazine` entity, without subclasses:

Example 9.3. Using a JPA Extent

    import org.apache.openjpa.persistence.*;

    ...

    OpenJPAEntityManager oem = OpenJPAPersistence.cast (em);
    Extent<Magazine> mags = oem.createExtent (Magazine.class, false);
    for (Magazine m : mags)
        processMagazine (m);

### 9.2.5. StoreCache

In addition to the `EntityManager` object cache mandated by the JPA specification, Kodo includes a flexible datastore-level cache. You can access this cache from your JPA code using the `org.apache.openjpa.persistence.StoreCache` facade. Section 10.1, “Data Cache” has detailed information on Kodo's data caching system, including the `StoreCache` facade.

### 9.2.6. QueryResultCache

Kodo can cache query results as well as persistent object data. The `org.apache.openjpa.persistence.QueryResultCache` is an JPA-flavored facade to Kodo's internal query cache. See Section 10.1.3, “Query Cache” for details on query caching in Kodo.

### 9.2.7. FetchPlan

Many of the aforementioned Kodo interfaces give you access to a `org.apache.openjpa.persistence.FetchPlan` instance. The `FetchPlan` allows you to exercise some control over how objects are fetched from the datastore, including large result set support, custom fetch groups, and lock levels.

Kodo goes one step further, extending `FetchPlan` with `org.apache.openjpa.persistence.jdbc.JDBCFetchPlan` to add additional JDBC-specific tuning methods. Unless you have customized Kodo to use a non-relational back-end (see Section 9.9, “Non-Relational Stores”), all `FetchPlan`s in Kodo implement `JDBCFetchPlan`, so feel free to cast to this interface.

Fetch plans pass on from parent components to child components. The `EntityManagerFactory` settings (via your configuration properties) for things like the fetch size, result set type, and custom fetch groups are passed on to the fetch plan of the `EntityManager`s it produces. The settings of each `EntityManager`, in turn, are passed on to each `Query` and `Extent` it returns. Note that the opposite, however, is not true. Modifying the fetch plan of a `Query` or `Extent` does not affect the `EntityManager`'s configuration. Likewise, modifying an `EntityManager`'s configuration does not affect the `EntityManagerFactory`.

Section 5.6, “Fetch Groups” includes examples using `FetchPlan`s.

### 9.2.8. OpenJPAPersistence

`org.apache.openjpa.persistence.OpenJPAPersistence` is a static helper class that adds Kodo-specific utility methods to `javax.persistence.Persistence`.

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