DataNucleus - Tutorial for JPA for MongoDB

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Background

An application can be JPA-enabled via many routes depending on the development process of the project in question. For example the project could use Eclipse as the IDE for developing classes. In that case the project would typically use the Dali Eclipse plugin coupled with the DataNucleus Eclipse plugin. Alternatively the project could use Ant, Maven2 or some other build tool. In this case this tutorial should be used as a guiding way for using DataNucleus in the application. The JPA process is quite straightforward.

  1. Prerequisite : Download DataNucleus AccessPlatform
  2. Step 1 : Define their persistence definition using Meta-Data.
  3. Step 2 : Define the "persistence-unit"
  4. Step 3 : Compile your classes, and instrument them (using the DataNucleus enhancer).
  5. Step 4 : Write your code to persist your objects within the DAO layer.
  6. Step 5 : Run your application.

The tutorial guides you through this. You can obtain the code referenced in this tutorial from SourceForge (one of the files entitled "datanucleus-samples-jpa-tutorial-*").


Step 0 : Download DataNucleus AccessPlatform

You can download DataNucleus in many ways, but the simplest is to download the distribution zip appropriate to your datastore (in this case MongoDB). You can do this from SourceForge DataNucleus download page. When you open the zip you will find DataNucleus jars in the lib directory, and dependency jars in the deps directory.


Step 1 : Take your model classes and mark which are persistable

For our tutorial, say we have the following classes representing a store of products for sale.

package org.datanucleus.samples.jpa.tutorial;

public class Inventory
{
    String name = null;
    Set<Product> products = new HashSet();

    public Inventory(String name)
    {
        this.name = name;
    }

    public Set<Product> getProducts() {return products;}
}
package org.datanucleus.samples.jpa.tutorial;

public class Product
{
    long id;
    String name = null;
    String description = null;
    double price = 0.0;

    public Product(String name, String desc, double price)
    {
        this.name = name;
        this.description = desc;
        this.price = price;
    }
}
package org.datanucleus.samples.jpa.tutorial;

public class Book extends Product
{
    String author=null;
    String isbn=null;
    String publisher=null;

    public Book(String name, String desc, double price, String author, 
                String isbn, String publisher)
    {
        super(name,desc,price);
        this.author = author;
        this.isbn = isbn;
        this.publisher = publisher;
    }
}

So we have a relationship (Inventory having a set of Products), and inheritance (Product-Book). Now we need to be able to persist objects of all of these types, so we need to define persistence for them. There are many things that you can define when deciding how to persist objects of a type but the essential parts are

  • Mark the class as an Entity so it is visible to the persistence mechanism
  • Identify which field(s) represent the identity of the object.

So this is what we do now. Note that we could define persistence using XML metadata, annotations. In this tutorial we will use annotations.

package org.datanucleus.samples.jpa.tutorial;

@Entity
public class Inventory
{
    @Id
    String name = null;

    @OneToMany(cascade={CascadeType.PERSIST, CascadeType.MERGE, CascadeType.DETACH})
    Set<Product> products = new HashSet();
    ...
}
package org.datanucleus.samples.jpa.tutorial;

@Entity
@Inheritance(strategy=InheritanceType.JOINED)
public class Product
{
    @Id
    @GeneratedValue(strategy=GenerationType.TABLE)
    long id;

    ...
}
package org.datanucleus.samples.jpa.tutorial;

@Entity
public class Book extends Product
{
    ...
}

Note that we mark each class that can be persisted with @Entity and their primary key field(s) with @Id In addition we defined a valueStrategy for Product field id so that it will have its values generated automatically. In this tutorial we are using application identity which means that all objects of these classes will have their identity defined by the primary key field(s). You can read more in application identity when designing your systems persistence.


Step 2 : Define the 'persistence-unit'

Writing your own classes to be persisted is the start point, but you now need to define which objects of these classes are actually persisted. You do this via a file META-INF/persistence.xml at the root of the CLASSPATH. Like this

<?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_2_0.xsd" version="2.0">

    <!-- JPA tutorial "unit" -->
    <persistence-unit name="Tutorial">
        <class>org.datanucleus.samples.jpa.tutorial.Inventory</class>
        <class>org.datanucleus.samples.jpa.tutorial.Product</class>
        <class>org.datanucleus.samples.jpa.tutorial.Book</class>
        <exclude-unlisted-classes/>
        <properties>
            <property name="javax.persistence.jdbc.url" value="mongodb:/nucleus1"/>
            <property name="datanucleus.autoCreateSchema" value="true"/>
            <property name="datanucleus.validateTables" value="false"/>
            <property name="datanucleus.validateConstraints" value="false"/>
        </properties>
    </persistence-unit>
</persistence>

Step 3 : Enhance your classes

DataNucleus relies on the classes that you want to persist be enhanced to implement the interface PersistenceCapable. You could write your classes manually to do this but this would be laborious. Alternatively you can use a post-processing step to compilation that "enhances" your compiled classes, adding on the necessary extra methods to make them PersistenceCapable. There are several ways to do this, most notably at post-compile, or at runtime. We use the post-compile step in this tutorial. DataNucleus JPA provides its own byte-code enhancer for instrumenting/enhancing your classes (in datanucleus-core) and this is included in the DataNucleus AccessPlatform zip file prerequisite.

To understand on how to invoke the enhancer you need to visualise where the various source and jdo files are stored

src/java/org/datanucleus/samples/jpa/tutorial/Book.java
src/java/org/datanucleus/samples/jpa/tutorial/Inventory.java
src/java/org/datanucleus/samples/jpa/tutorial/Product.java

target/classes/org/datanucleus/samples/jpa/tutorial/Book.class
target/classes/org/datanucleus/samples/jpa/tutorial/Inventory.class
target/classes/org/datanucleus/samples/jpa/tutorial/Product.class

[when using Ant]
lib/persistence-api.jar
lib/jdo-api.jar
lib/datanucleus-core.jar
lib/datanucleus-api-jpa.jar

The first thing to do is compile your domain/model classes. You can do this in any way you wish, but the downloadable JAR provides an Ant task, and a Maven2 project to do this for you.

Using Ant :
ant compile

Using Maven :
mvn compile

To enhance classes using the DataNucleus Enhancer, you need to invoke a command something like this from the root of your project.

Using Ant :
ant enhance

Using Maven : (this is usually done automatically after the "compile" goal)
mvn datanucleus:enhance

Manually on Linux/Unix :
java -cp target/classes:lib/datanucleus-core.jar:
         lib/datanucleus-api-jpa.jar:lib/persistence-api.jar;lib/jdo-api.jar
     org.datanucleus.enhancer.DataNucleusEnhancer 
     -api JPA -pu Tutorial

Manually on Windows :
java -cp target\classes;lib\datanucleus-core.jar;
         lib\datanucleus-api-jpa.jar;lib\persistence-api.jar;lib\jdo-api.jar
     org.datanucleus.enhancer.DataNucleusEnhancer
     -api JPA -pu Tutorial

[Command shown on many lines to aid reading - should be on single line]

This command enhances all class files specified in the persistence-unit "Tutorial". If you accidentally omitted this step, at the point of running your application and trying to persist an object, you would get a ClassNotPersistenceCapableException thrown. The use of the enhancer is documented in more detail in the Enhancer Guide. The output of this step are a set of class files that represent persistable classes.


Step 4 : Write the code to persist objects of your classes

Writing your own classes to be persisted is the start point, but you now need to define which objects of these classes are actually persisted, and when. Interaction with the persistence framework of JPA is performed via an EntityManager. This provides methods for persisting of objects, removal of objects, querying for persisted objects, etc. This section gives examples of typical scenarios encountered in an application.

The initial step is to obtain access to an EntityManager, which you do as follows

EntityManagerFactory emf = Persistence.createEntityManagerFactory("Tutorial");
EntityManager em = emf.createEntityManager();

So we created an EntityManagerFactory for our "persistence-unit" called "Tutorial". Now that the application has an EntityManager it can persist objects. This is performed as follows

Transaction tx = em.getTransaction();
try
{
    tx.begin();

    Inventory inv = new Inventory("My Inventory");
    Product product = new Product("Sony Discman", "A standard discman from Sony", 49.99);
    inv.getProducts().add(product);
    em.persist(inv);

    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }

    em.close();
}

Please note that the finally step is important in that it tidies up connections to the datastore and the EntityManager.

Now we want to retrieve some objects from persistent storage, so we will use a "Query". In our case we want access to all Product objects that have a price below 150.00 and ordering them in ascending order.

Transaction tx = em.getTransaction();
try
{
    tx.begin();

    Query q = pm.createQuery("SELECT p FROM Product p WHERE p.price < 150.00");
    List results = q.getResultList();
    Iterator iter = results.iterator();
    while (iter.hasNext())
    {
        Product p = (Product)iter.next();

        ... (use the retrieved object)
    }

    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }

    em.close();
}

If you want to delete an object from persistence, you would perform an operation something like

Transaction tx = em.getTransaction();
try
{
    tx.begin();

    // Find and delete all objects whose last name is 'Jones'
    Query q = em.createQuery("DELETE FROM Person p WHERE p.lastName = 'Jones'");
    int numberInstancesDeleted = q.executeUpdate();

    tx.commit();
}
finally
{
    if (tx.isActive())
    {
        tx.rollback();
    }

    em.close();
}

Clearly you can perform a large range of operations on objects. We can't hope to show all of these here. Any good JPA book will provide many examples.

Step 5 : Run your application

To run your JPA-enabled application will require a few things to be available in the Java CLASSPATH, these being

  • The "persistence.xml" file (stored under META-INF/)
  • Any ORM MetaData files for your persistable classes
  • MongoDB Java driver jar needed for accessing your datastore
  • The JDO API JAR (defining the JDO interface) - DataNucleus JPA also requires this currently
  • The JPA API JAR (defining the JPA interface)
  • The DataNucleus Core, DataNucleus JPA API and DataNucleus MongoDB JARs

After that it is simply a question of starting your application and all should be taken care of. You can access the DataNucleus Log file by specifying the logging configuration properties, and any messages from DataNucleus will be output in the normal way. The DataNucleus log is a very powerful way of finding problems since it can list all SQL actually sent to the datastore as well as many other parts of the persistence process.

Using Ant (you need the included persistence.xml to specify your database)
ant run


Using Maven:
mvn exec:java


Manually on Linux/Unix :
java -cp lib/persistence-api.jar:lib/jdo-api.jar:lib/datanucleus-core.jar:lib/datanucleus-mongodb.jar:
         lib/datanucleus-api-jpa.jar:lib/mongo-java.jar:target/classes/:. 
     org.datanucleus.samples.jpa.tutorial.Main


Manually on Windows :
java -cp lib\persistence-api.jar;lib\jdo-api.jar;lib\datanucleus-core.jar;lib\datanucleus-mongodb.jar;
         lib\datanucleus-api-jpa.jar;lib\mongo-java.jar;target\classes\;.
     org.datanucleus.samples.jpa.tutorial.Main


Output :

DataNucleus Tutorial with JPA
=============================
Persisting products
Product and Book have been persisted

Executing Query for Products with price below 150.00
>  Book : JRR Tolkien - Lord of the Rings by Tolkien

Deleting all products from persistence

End of Tutorial

Part 2 : Next steps

In the above simple tutorial we showed how to employ JPA and persist objects to a MongoDB database. Obviously this just scratches the surface of what you can do, and to use JPA requires minimal work from the user. In this second part we show some further things that you are likely to want to do.

  1. Step 6 : Controlling the schema.
  2. Step 7 : Generate the database tables where your classes are to be persisted using SchemaTool.

Step 6 : Controlling the schema

In the above simple tutorial we didn't look at controlling the schema generated for these classes. Now let's pay more attention to this part by defining XML Metadata for the schema. We define this in XML to separate schema information from persistence information. So we define a file orm.xml

<?xml version="1.0" encoding="UTF-8" ?>
<entity-mappings>
    <description>DataNucleus JPA tutorial</description>
    <package>org.datanucleus.samples.jpa.tutorial</package>
    <entity class="org.datanucleus.samples.jpa.tutorial.Product" name="Product">
        <table name="JPA_PRODUCTS"/>
        <attributes>
            <id name="id">
                <generated-value strategy="TABLE"/>
            </id>
            <basic name="name">
                <column name="PRODUCT_NAME"/>
            </basic>
            <basic name="description">
                <column name="Desc"/>
            </basic>
        </attributes>
    </entity>

    <entity class="org.datanucleus.samples.jpa.tutorial.Book" name="Book">
        <table name="JPA_BOOKS"/>
        <attributes>
            <basic name="isbn">
                <column name="ISBN"/>
            </basic>
            <basic name="author">
                <column name="AUTHOR"/>
            </basic>
            <basic name="publisher">
                <column name="PUBLISHER"/>
            </basic>
        </attributes>
    </entity>

    <entity class="org.datanucleus.samples.jpa.tutorial.Inventory" name="Inventory">
        <table name="JPA_INVENTORY"/>
        <attributes>
            <id name="name">
                <column name="NAME" length="40"></column>
            </id>
            <one-to-many name="products">
            </one-to-many>
        </attributes>
    </entity>
</entity-mappings>

This file should be placed at the root of the CLASSPATH under META-INF.


Step 7 : Generate any schema required for your domain classes

This step is optional, depending on whether you have an existing database schema. If you haven't, at this point you can use the DataNucleus SchemaTool to generate the tables where these domain objects will be persisted. DataNucleus SchemaTool is a command line utility (it can be invoked from Maven/Ant in a similar way to how the Enhancer is invoked). The first thing that you need is to update the src/java/META-INF/persistence.xml file with your database details. Here we have a sample file (for HSQLDB) that contains

<?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_2_0.xsd" version="2.0">

    <!-- Tutorial "unit" -->
    <persistence-unit name="Tutorial">
        <class>org.datanucleus.samples.jpa.tutorial.Inventory</class>
        <class>org.datanucleus.samples.jpa.tutorial.Product</class>
        <class>org.datanucleus.samples.jpa.tutorial.Book</class>
        <exclude-unlisted-classes/>
        <properties>
            <property name="javax.persistence.jdbc.url" value="mongodb:/nucleus1"/>
            <property name="datanucleus.autoCreateSchema" value="true"/>
            <property name="datanucleus.validateTables" value="false"/>
            <property name="datanucleus.validateConstraints" value="false"/>
        </properties>
    </persistence-unit>

</persistence>

Now we need to run DataNucleus SchemaTool. For our case above you would do something like this

Using Ant :
ant createschema


Using Maven :
mvn datanucleus:schema-create


Manually on Linux/Unix :
java -cp target/classes:lib/jdo-api.jar:lib/persistence-api.jar:lib/datanucleus-core.jar:
         lib/datanucleus-mongodb.jar:lib/datanucleus-api-jpa.jar:lib/{mongo-java.jar}
     org.datanucleus.store.schema.SchemaTool
     -create -api JPA -pu Tutorial

Manually on Windows :
java -cp target\classes;lib\jdo-api.jar;lib\persistence-api.jar;lib\datanucleus-core.jar;
         lib\datanucleus-mongodb.jar;lib\datanucleus-api-jpa.jar;lib\{mongo-java.jar}
     org.datanucleus.store.schema.SchemaTool
     -create -api JPA -pu Tutorial

[Command shown on many lines to aid reading. Should be on single line]

This will generate the required tables, indexes, and foreign keys for the classes defined in the annotations and orm.xml Meta-Data file.


Any questions?

If you have any questions about this tutorial and how to develop applications for use with DataNucleus please read the online documentation since answers are to be found there. If you don't find what you're looking for go to our Forums.

The DataNucleus Team