Java Persistence API (JPA) is a standard interface for storing objects containing data into a relational database. The standard defines interfaces for annotating Java objects, retrieving objects using queries, and interacting with a database using transactions. An application that uses the JPA interface can work with different databases without using any vendor-specific database code. JPA makes your application easy to port between different database vendors.
The App Engine Java SDK includes an implementation of JPA 1.0 for the App Engine datastore. The implementation is based on DataNucleus Access Platform. Since JPA presents a standard interface for interacting with relational databases and the App Engine datastore is not a relational database, there are features of JPA that the App Engine implementation simply cannot support. We have done our best to call attention to these features wherever possible.
See the Access Platform 1.1 documentation for more information about JPA. In particular, see "JPA Mapping" and "JPA API".
To use JPA to access the datastore, an App Engine app needs the following:
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war/WEB-INF/lib/ directory.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.If you are using the Google Plugin for Eclipse, the first and third items are taken care of for you. The new project wizard puts the JPA and datastore JARs in the correct location and the build process performs the "enhancement" step automatically. You must still manually create persistence.xml and put it in war/WEB-INF/classes/META-INF/. The plugin will be updated soon to do this automatically as well.
If you are using Apache Ant to build your project, you can use an Ant task included with the SDK to perform the enhancement step. You must copy the JARs and create the configuration file when you set up your project. See Using Apache Ant for more information about the Ant task.
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.
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>
The DataNucleus implementation of JPA uses a post-compilation "enhancement" step in the build process to associate data classes with the JPA implementation.
If you are using Apache Ant, the SDK includes an Ant task to perform this step. See Using Apache Ant for more information on using the Ant task.
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.
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;
}
}
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();
}
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:
java.util.List<...>) of values of a core datastore type@Entity classA 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 4 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 String getHireDate() {
return hireDate;
}
public void setHireDate(Date hireDate) {
this.hireDate = hireDate;
}
}
Creating data classes that make use of inheritance is a natural thing to do, and JPA supports this. 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 would require 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 Workerclass 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. Please 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.
The following features of the JPA interface are not supported by the App Engine implementation: