Using JPA with App Engine

Java Persistence API (JPA) is a standard interface for accessing databases in Java, providing an automatic mapping between Java classes and database tables. There is an open-source plugin available for using JPA with Datastore, and this page provides information on how to get started with it.

Warning: We think most developers will have a better experience using the low-level Datastore API, or one of the open-source APIs developed specifically for Datastore, such as Objectify. JPA was designed for use with traditional relational databases, and so has no way to explicitly represent some of the aspects of Datastore that make it different from relational databases, such as entity groups and ancestor queries. This can lead to subtle issues that are difficult to understand and fix.

Version 1.x of the plugin is included in the App Engine Java SDK, which implements JPA version 1.0. The implementation is based on DataNucleus Access Platform version 1.1.

Note: The instructions on this page apply to JPA version 1, which uses version 1.x of the DataNucleus plugin for App Engine. Version 2.x of the DataNucleus plugin is also available, which allows you to use JPA 2.0. The 2.x plugin provides a number of new APIs and features; however, the upgrade is not fully backwards-compatible with the 1.x version. If you rebuild an application using JPA 2.0, you need to update and retest your code. For more information on the new version, see Using JPA 2.0 with App Engine.

Setting Up JPA

To use JPA to access the datastore, an App Engine app needs the following:

• The JPA and datastore JARs must be in the app's war/WEB-INF/lib/ directory.
• A configuration file named persistence.xml must be in the app's war/WEB-INF/classes/META-INF/ directory, with configuration that tells JPA to use the App Engine datastore.
• The project's build process must perform a post-compilation "enhancement" step on the compiled data classes to associate them with the JPA implementation.

Copying the JARs

The JPA and datastore JARs are included with the App Engine Java SDK. You can find them in the appengine-java-sdk/lib/user/orm/ directory.

Copy the JARs to your application's war/WEB-INF/lib/ directory.

Make sure the appengine-api.jar is also in the war/WEB-INF/lib/ directory. (You may have already copied this when creating your project.) The App Engine DataNucleus plugin uses this JAR to access the datastore.

Creating the persistence.xml File

The JPA interface needs a configuration file named persistence.xml in the application's war/WEB-INF/classes/META-INF/ directory. You can create this file in this location directly, or have your build process copy this file from a source directory.

Create the file with the following contents:

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
        http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

    <persistence-unit name="transactions-optional">
        <provider>org.datanucleus.store.appengine.jpa.DatastorePersistenceProvider</provider>
        <properties>
            <property name="datanucleus.NontransactionalRead" value="true"/>
            <property name="datanucleus.NontransactionalWrite" value="true"/>
            <property name="datanucleus.ConnectionURL" value="appengine"/>
        </properties>
    </persistence-unit>

</persistence>

Datastore Read Policy and Call Deadline

As described on the Datastore Queries page, you can set the read policy (strong consistency vs. eventual consistency) and the datastore call deadline for a EntityManagerFactory in the persistence.xml file. These settings go in the <persistence-unit> element. All calls made with a given EntityManager instance use the configuration selected when the manager was created by the EntityManagerFactory. You can also override these options for an individual Query (described below).

To set the read policy, include a property named datanucleus.appengine.datastoreReadConsistency. Its possible values are EVENTUAL (for reads with eventual consistency) and STRONG (for reads with strong consistency). If not specified, the default is STRONG.

            <property name="datanucleus.appengine.datastoreReadConsistency" value="EVENTUAL" />

You can set separate datastore call deadlines for reads and for writes. For reads, use the JPA standard property javax.persistence.query.timeout. For writes, use datanucleus.datastoreWriteTimeout. The value is an amount of time, in milliseconds.

            <property name="javax.persistence.query.timeout" value="5000" />
            <property name="datanucleus.datastoreWriteTimeout" value="10000" />

If you want to use cross-group (XG) transactions, add the following property:

            <property name="datanucleus.appengine.datastoreEnableXGTransactions" value="true" />

You can have multiple <persistence-unit> elements in the same persistence.xml file, using different name attributes, to use EntityManager instances with different configurations in the same app. For example, the following persistence.xml file establishes two sets of configuration, one named "transactions-optional" and another named "eventual-reads-short-deadlines":

<?xml version="1.0" encoding="UTF-8" ?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/persistence
        http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

    <persistence-unit name="transactions-optional">
        <provider>org.datanucleus.store.appengine.jpa.DatastorePersistenceProvider</provider>
        <properties>
            <property name="datanucleus.NontransactionalRead" value="true"/>
            <property name="datanucleus.NontransactionalWrite" value="true"/>
            <property name="datanucleus.ConnectionURL" value="appengine"/>
        </properties>
    </persistence-unit>

    <persistence-unit name="eventual-reads-short-deadlines">
        <provider>org.datanucleus.store.appengine.jpa.DatastorePersistenceProvider</provider>
        <properties>
            <property name="datanucleus.NontransactionalRead" value="true"/>
            <property name="datanucleus.NontransactionalWrite" value="true"/>
            <property name="datanucleus.ConnectionURL" value="appengine"/>

            <property name="datanucleus.appengine.datastoreReadConsistency" value="EVENTUAL" />
            <property name="javax.persistence.query.timeout" value="5000" />
            <property name="datanucleus.datastoreWriteTimeout" value="10000" />
        </properties>
    </persistence-unit>
</persistence>

See Getting an EntityManager Instance below for information on creating an EntityManager with a named configuration set.

You can override the read policy and call deadline for an individual Query object. To override the read policy for a Query, call its setHint() method as follows:

        Query q = em.createQuery("select from " + Book.class.getName());
        q.setHint("datanucleus.appengine.datastoreReadConsistency", "EVENTUAL");

As above, the possible values are "EVENTUAL" and "STRONG".

To override the read timeout, call setHint() as follows:

        q.setHint("javax.persistence.query.timeout", 3000);

There is no way to override the configuration for these options when you fetch entities by key.

Enhancing Data Classes

The DataNucleus implementation of JPA uses a post-compilation "enhancement" step in the build process to associate data classes with the JPA implementation.

You can perform the enhancement step on compiled classes from the command line with the following command:

java -cp classpath org.datanucleus.enhancer.DataNucleusEnhancer
class-files

The classpath must contain the JARs datanucleus-core-*.jar, datanucleus-jpa-*, datanucleus-enhancer-*.jar, asm-*.jar, and geronimo-jpa-*.jar (where * is the appropriate version number of each JAR) from the appengine-java-sdk/lib/tools/ directory, as well as all of your data classes.

For more information on the DataNucleus bytecode enhancer, see the DataNucleus documentation.

Getting an EntityManager Instance

An app interacts with JPA using an instance of the EntityManager class. You get this instance by instantiating and calling a method on an instance of the EntityManagerFactory class. The factory uses the JPA configuration (identified by the name "transactions-optional") to create EntityManager instances.

Because an EntityManagerFactory instance takes time to initialize, it's a good idea to reuse a single instance as much as possible. An easy way to do this is to create a singleton wrapper class with a static instance, as follows:

EMF.java

import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public final class EMF {
    private static final EntityManagerFactory emfInstance =
        Persistence.createEntityManagerFactory("transactions-optional");

    private EMF() {}

    public static EntityManagerFactory get() {
        return emfInstance;
    }
}

Tip: "transactions-optional" refers to the name of the configuration set in the persistence.xml file. If your app uses multiple configuration sets, you'll have to extend this code to call Persistence.createEntityManagerFactory() as desired. Your code should cache a singleton instance of each EntityManagerFactory.

The app uses the factory instance to create one EntityManager instance for each request that accesses the datastore.

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;

import EMF;

// ...
    EntityManager em = EMF.get().createEntityManager();

You use the EntityManager to store, update, and delete data objects, and to perform datastore queries.

When you are done with the EntityManager instance, you must call its close() method. It is an error to use the EntityManager instance after calling its close() method.

    try {
        // ... do stuff with em ...
    } finally {
        em.close();
    }

Class and Field Annotations

Each object saved by JPA becomes an entity in the App Engine datastore. The entity's kind is derived from the simple name of the class (without the package name). Each persistent field of the class represents a property of the entity, with the name of the property equal to the name of the field (with case preserved).

To declare a Java class as capable of being stored and retrieved from the datastore with JPA, give the class a @Entity annotation. For example:

import javax.persistence.Entity;

@Entity
public class Employee {
    // ...
}

Fields of the data class that are to be stored in the datastore must either be of a type that is persisted by default or expliclty declared as persistent. You can find a chart detailing JPA default persistence behavior on the DataNucleus website. To explicitly declare a field as persistent, you give it an @Basic annotation:

import java.util.Date;
import javax.persistence.Enumerated;

import com.google.appengine.api.datastore.ShortBlob;

// ...
    @Basic
    private ShortBlob data;

The type of a field can be any of the following:

• one of the core types supported by the datastore
• a Collection (such as a java.util.List<...>) of values of a core datastore type
• an instance or Collection of instances of a @Entity class
• an embedded class, stored as properties on the entity

A data class must have a public or protected default constructor and one field dedicated to storing the primary key of the corresponding datastore entity. You can choose between four different kinds of key fields, each using a different value type and annotations. (See Creating Data: Keys for more information.) The simplest key field is a long integer value that is automatically populated by JPA with a value unique across all other instances of the class when the object is saved to the datastore for the first time. Long integer keys use a @Id annotation, and a @GeneratedValue(strategy = GenerationType.IDENTITY) annotation:

import com.google.appengine.api.datastore.Key;

import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

// ...
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key key;

Here is an example data class:

import com.google.appengine.api.datastore.Key;

import java.util.Date;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Employee {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key key;

    private String firstName;

    private String lastName;

    private Date hireDate;

    // Accessors for the fields. JPA doesn't use these, but your application
    does.

    public Key getKey() {
        return key;
    }

    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Date getHireDate() {
        return hireDate;
    }
    public void setHireDate(Date hireDate) {
        this.hireDate = hireDate;
    }
}

Inheritance

JPA supports creating data classes that use inheritance. Before we talk about how JPA inheritance works on App Engine, we recommend you read the DataNucleus documentation on this subject and then come back. Done? OK. JPA inheritance on App Engine works as described in the DataNucleus documentation with some additional restrictions. We'll discuss these restrictions and then give some concrete examples.

The "JOINED" inheritance strategy allows you to split the data for a single data object across multiple "tables," but since the App Engine datastore does not support joins, operating on a data object with this inheritance strategy requires a remote procedure call for each level of inheritance. This is potentially very inefficient, so the "JOINED" inheritance strategy is not supported on data classes.

Second, the "SINGLE_TABLE" inheritance strategy allows you to store the data for a data object in a single "table" associated with the persistent class at the root of your inheritance hierarchy. Although there are no inherent inefficiencies in this strategy, it is not currently supported. We may revisit this in future releases.

Now the good news: The "TABLE_PER_CLASS" and "MAPPED_SUPERCLASS" strategies work as described in the DataNucleus documentation. Let's look at an example:

Worker.java

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MappedSuperclass;

@Entity
@MappedSuperclass
public abstract class Worker {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key key;

    private String department;
}

Employee.java

// ... imports ...

@Entity
public class Employee extends Worker {
    private int salary;
}

Intern.java

import java.util.Date;
// ... imports ...

@Entity
public class Intern extends Worker {
    private Date internshipEndDate;
}

In this example we've added an @MappedSuperclass annotation to the Worker class declaration. This tells JPA to store all persistent fields of the Worker in the datastore entities of its subclasses. The datastore entity created as the result of calling persist() with an Employee instance will have two properties named "department" and "salary". The datastore entity created as the result of calling persist() with an Intern instance will have two properties named "department" and "inernshipEndDate". There will not be any entities of kind "Worker" in the datastore.

Now let's make things a little more interesting. Suppose, in addition to having Employee and Intern, we also want a specialization of Employee that describes employees who have left the company:

FormerEmployee.java

import java.util.Date;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
// ... imports ...

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class FormerEmployee extends Employee {
    private Date lastDay;
}

In this example we've added an @Inheritance annotation to the FormerEmployee class declaration with its strategy attribute set to InheritanceType.TABLE_PER_CLASS. This tells JPA to store all persistent fields of the FormerEmployee and its superclasses in datastore entities corresponding to FormerEmployee instances. The datastore entity created as the result of calling persist() with an FormerEmployee instance will have three properties named "department", "salary", and "lastDay". There will never be an entity of kind "Employee" that corresponds to a FormerEmployee, but if you call persist() with a object whose runtime type is Employee you will create an entity of kind "Employee.

Mixing relationships with inheritance works so long as the declared types of your relationship fields match the runtime types of the objects you are assigning to those fields. Refer to the section on Polymorphic Relationships for more information. This section contains JDO examples, but the concepts and the restrictions are the same for JPA.

Unsupported Features of JPA 1.0

The following features of the JPA interface are not supported by the App Engine implementation:

• Owned many-to-many relationships, and unowned relationships. You can implement unowned relationships using explicit Key values, though type checking is not enforced in the API.
• "Join" queries. You cannot use a field of a child entity in a filter when performing a query on the parent kind. Note that you can test the parent's relationship field directly in a query using a key.
• Aggregation queries (group by, having, sum, avg, max, min)
• Polymorphic queries. You cannot perform a query of a class to get instances of a subclass. Each class is represented by a separate entity kind in the datastore.