Sunday, April 13, 2014

Configuring Hibernate JPA in Spring Framework

tl;dr To configure JPA in Spring via Hibernate, you wire up one of the classes provided by Spring that creates a bean of type EntityManagerFactory which then makes EntityManager available for you. There are two types of Entity Managers, and the class you wire up depends on which Entity Manager factory you would be using. We have Application Managed and Container Managed Entity Managers.

This post gives an overview of how to configure Hibernate as a JPA implantation, for use in a Spring application. It is part of the Configuring Spring Series.


In Configuring Hibernate in Spring Framework, we saw that in other to talk to the database via Hibernate, your application code would do so via Spring's implementation of Hibernate's main interface: org.hibernate.Session which is made available via an implementation of  org.hibernate.SessionFactory. And the process of configuration is just a process of wiring up Spring's to provide this needed SessionFactory as a bean.

You have a similar procedure when it comes to configuring Hibernate JPA in Spring. 

Instead of working with a SessionFactory, you work with EntityManagerFactory. And instead of Hibernates' Session you have EntityManager  through which the actual database access is done.

The process of configuring Hibernate JPA in Spring hence, is all about wiring the Spring bean that would make available for you the EntityManagerFactory.

Before we get to the business of actual configuration, let's get some key concept's explained.


Types of EntityManagers


There are two types of EntityManagers. Application Managed and Container Managed the difference between these two lie not in their nature (they are technically the same as they both implement the EntityManager interface), but in how they are created. An Application Managed EntityManager are the type that are created when an application directly request one from the EntityManagerFactory while a Container Managed EntityManager are the type that are created by a Java EE container and then made available to your application code via dependency injection or via JNDI.

With Application Managed EnityManager, the application is concerned with managing the Entity Manager after creation; ie. opening and closing etc. With Container Managed, this task is delegated to the container that provides it.

Technically this difference is not much of a concern when working with Hibernate JPA via Spring as Spring plays the role of "the application" when an Application Managed EntityManager is needed and also "the Container", in case of Container Managed EntityManager. The only thing  to be aware of is the slight difference in the configuration process for these two types of Entity Managers.

Persistence.xml and Persistence Unit


Hibernate (via JPA or not) is an ORM tool; meaning it bridges the domain between Object Oriented Programming and the Relational domain of databases; making an OOP developer interact with relational database as if it was still in the OOP domain. Working with such a tool requires what is usually referred to as Entity Classes. Which are representation of database tables via Java classes and whose instances can be stored in a database. You can read Understanding Entity Relationship Mapping, Part 1: The basic concepts for some more fundamental discuss on Entity and relationship mapping between Entities.

A Persistence Unit in JPA is therefore a logical grouping of one or more of these Entity Classes together with any required extra configuration needed to work with them e.g. the data source or the ORM mapping file in case where XML based mapping is used.

The Persistence Unit is defined in the persistence.xml file, which must be located in the META-INF directory within the class path.

A typical persistence.xml file may look thus:



   com.MyApp.Enitity.Person
   com.MyApp.Enitity.Votes
        
            
            
            
            
        



In the persistence.xml above, two Entity classes are defined: com.MyApp.Enitity.Person and com.MyApp.Enitity.Votes, while additional configuration is provided within the tag

@PersistenceUnit and @PersistenceContext


The PersistenceContext is a cache. it is the representation in memory, of the database.

It has already being pointed out that when using Hibernate JPA, you work with the EntityManager. The EntityManager is actually a PersistenceContext; or in another words, the EntityManager is a cache. It is the representation of your database state in memory and it is this representation that your application code manipulates.

And like any other kind of cache, to make modifications to the cache lasting, it needs to be flushed; which is basically moving the modification into a persistent facility: a database, file system etc.

When working with Application Managed Entity Managers, the EntityManagerFactory that makes the EntityManager available for your code can be injected via the @PeristenceUnit annotation while in case of Container Managed Entity Managers, the EntityManager can be injected to your code via @PersistenceContext

Note the difference: With Application Managed Entity Managers, your application gets the EntityManagerFactory via @PeristenceUnit  from which you have to manually create (by calling the createEntityManager method) and manage an EntityManager. In the case of Container Managers Entity Managers, on the other hand, the EntityManager is created already and handed to your application code via @PersistenceContext. This is in line with the difference between the two Entity Manager as already explained.


With these concepts explained and out of the way, we can now look into how you do the actual configuration.

The configuration differs depending on whether you are configuring a Application Managed Entity Manager or a Container Managed Entity Manager.

Dependencies


I am using Maven as dependency manager, and as such the POM file to enable the configuration example to work would include these dependencies



    
        org.springframework
        spring-core
        4.0.3.RELEASE
    

    
        org.springframework
        spring-context
        4.0.3.RELEASE
    

    
        org.springframework
        spring-orm
        4.0.3.RELEASE
    

     
            org.hibernate
            hibernate-core
            4.3.4.Final
        

        
            org.hibernate
            hibernate-entitymanager
            4.3.4.Final
        

    
        mysql
        mysql-connector-java
        5.1.23
    


    
        cglib
        cglib 
        3.1
    


Make sure you have these dependencies in whatever dependency management tool you are using.

Configuring Application Managed Entity Manager


To get this flavor of EntityManager, you do so by configuring the corresponding Spring factory bean: The LocalEntityManagerFactoryBean, which is responsible for producing an Application Managed Entity Manager.

Configuring the LocalEnityManagerFactoryBean requires the presence of a persistence.xml file. So using the persistence.xml file already defined above, configuring the LocalEntityManagerFactoryBean would look thus:


  



The value of persistenceUntiName above, mySpringApp, refers to the persistence name defined in the persistence.xml


Configuring Container Managed Entity Manager


The bean configuration for Container Managed Entity Manager has more to it compared to the bean configuration for Application Managed Entity Managers. There are quite a couple of options that the configuration can specify. Would highlight some of this based on what function they perform.

We have the part of the configuration that specifies:
1. The Data Source
2. The JPA implementation that would be used. ie. the vendor via jpaVendorAdapter
3. The specific JPA properties via jpaProperties
4. The database access configuration in case Data Source is not used
5. And the configuration that specifies the path to scan for Entities (this applies when persistence.xml is not used)

So a typical configuration would look thus:


      
      
      
      
      
         
      

      
      
         
            create-drop
            org.hibernate.dialect.MySQL5Dialect
         
      
   

in case dataSource is not used, the required configuration can be provided inline: i.e.


       
       
       
       
       
       
       
       
         
       

     
      
         
            create-drop
            org.hibernate.dialect.MySQL5Dialect
         
      
   

These configuration requires that persistence.xml is in META-INF. It is possible to out rightly omit the persistence.xml file. This is possible starting from Spring 3.1, in which case you now need to specify the path Spring should search within to find Entity classes. This you do using the packagesToScan property. The configuration would then look like


      
      
         
      
      
      
         
            create-drop
            org.hibernate.dialect.MySQL5Dialect
         
      

   
    
        
            my.package.name
            my.anotherpackage.name
        
    


A quick tip to actually know the available properties to configure for the bean (or any other bean for that matter) is to check the Javadoc of the classes you want to configure as a bean. The setter methods gives away the available properties that can be configured.

Retrieving EntityManager For Use


Once the configuration is done, retrieving an EntityManager for use is as simple as you would retrieve any other bean. You could either retrieve the bean from the context manually or via annotations. You can read Configuring Beans in Spring Framework for some information on how this process works.

In case you would be retrieving the bean via annotations, then @PersistenceContext  is the annotation to use to enable this "auto wiring".

An annotated class can then look like this

@Repository
class PersonDao() {

@PersistenceContext
public EntityManager em

public void save(Person person) {
  em.persist(person);
}

....

}

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