Exploring Apache DeltaSpike Data Module

Apache DeltaSpike is a collection of portable CDI extensions.

Introduction

DeltaSpike Data Module provides capabilities for implementing repository patterns and thereby simplifying the repository layer not unlike Spring Data. In this post we will take a JPA project and rewrite the repository layer to use DeltaSpike Data. Basically, we will strip off the boilerplate EntityManager queries and enable centralization of query logic and consequently reducing code duplication and improve testability.

Prerequisites

Project Structure

At the end of this guide our folder structure will look similar to the following:

.
|__src/
|  |__main/
|  |  |__java/
|  |  |  |__com/
|  |  |  |  |__juliuskrah/
|  |  |  |  |  |__cdi/
|  |  |  |  |  |  |__ApplicationResources.java
|  |  |  |  |  |  |__business/
|  |  |  |  |  |  |  |__CustomerService.java
|  |  |  |  |  |  |  |__dto/
|  |  |  |  |  |  |  |  |__CustomerBean.java
|  |  |  |  |  |  |  |__mapper/
|  |  |  |  |  |  |  |  |__CustomerMapper.java
|  |  |  |  |  |  |__entity/
|  |  |  |  |  |  |  |__Customer.java
|  |  |  |  |  |  |__repository/
|  |  |  |  |  |  |  |__CustomerRepository.java
|  |  |  |  |  |  |__web/
|  |  |  |  |  |  |  |__IndexController.java
|  |  |__resources/
|  |  |  |__db/
|  |  |  |  |__migration/
|  |  |  |  |  |__`V1__Create_customer_table.sql`
|  |  |  |__META-INF/
|  |  |  |  |__persistence.xml
|  |  |  |__modules/
|  |  |  |  |__com/
|  |  |  |  |  |__h2database/
|  |  |  |  |  |  |__h2/
|  |  |  |  |  |  |  |__main/
|  |  |  |  |  |  |  |  |__module.xml
|  |  |  |__project-defaults.yaml
|  |  |__webapp/
|  |  |  |__WEB-INF/
|  |  |  |  |__templates/
|  |  |  |  |  |__default.xhtml
|  |  |  |  |__beans.xml
|  |  |  |  |__web.xml
|  |  |  |__index.html
|  |  |  |__index.xhtml
|__pom.xml

Setting Up

Download the initial project (zip| tar.gz) and extract. From the extracted directory run the following command:

C:\> mvn clean wildfly-swarm:run

Wait for all the dependencies to download and swarm to start.

DeltaSpike

Add the DeltaSpike Data dependencies:

file: pom.xml

...
<dependency>
  <groupId>org.apache.deltaspike.modules</groupId>
  <artifactId>deltaspike-data-module-api</artifactId>
  <version>1.8.1</version>
  <scope>compile</scope>
</dependency>
<dependency>
  <groupId>org.apache.deltaspike.modules</groupId>
  <artifactId>deltaspike-data-module-impl</artifactId>
  <version>1.8.1</version>
  <scope>runtime</scope>
</dependency>
...

Replace the code in CustomerRepository:

file: src/main/java/com/juliuskrah/cdi/repository/CustomerRepository.java

@Repository
public interface CustomerRepository extends EntityRepository<Customer, UUID> {

  @Query("SELECT c FROM Customer c WHERE c.id = :id")
  Optional<Customer> findOne(@QueryParam("id") UUID id);
}

There are a few things to note here. The @Repository annotation tells the extension that this is a repository for the Customer entity. Any method defined on the repository will be processed by the framework.

The EntityRepository Interface is mainly intended to hold complex query logic, working with both a repository and an EntityManager in the service layer might unnecessarily clutter code. The top base type is the EntityRepository interface, providing common methods used with an EntityManager.

DeltaSpike Data module supports also annotating methods for more control on the generated query using @Query. If the JPQL query requires named parameters to be used, this can be done by annotating the arguments with the @QueryParam annotation.

For all these to work, DeltaSpike Data requires an EntityManager exposed via a CDI producer - which is common practice in Java EE applications:

@Produces
@PersistenceContext
EntityManager em;

Transactions

DeltaSpike Data module uses a ResourceLocalTransactionStrategy as default TransactionStrategy when demarcating transactions. Our example is configured to use jta-data-source, and the ResourceLocalTransactionStrategy does not quite fit our needs here.

Let’s talk a little about JPA transactions to understand our decision not to use ResourceLocalTransactionStrategy. A transaction is a set of operations that either fail or succeed as a unit. Transactions are a fundamental part of persistence. A database transaction consists of a set of SQL DML (Data Manipulation Language) operations that are committed or rolled back as a single unit. An object level transaction is one in which a set of changes made to a set of objects are committed to the database as a single unit.

JPA provides two mechanisms for transactions. When used in Java EE JPA provides integration with JTA (Java Transaction API). JPA also provides its own EntityTransaction implementation for Java SE and for use in a non-managed mode in Java EE. Transactions in JPA are always at the object level, this means that all changes made to all persistent objects in the persistence context are part of the transaction.

Resource local transactions are used in Java SE, or in application managed (non-managed) mode in Java EE. To use resource local transactions the transaction-type attribute in the persistence.xml is set to RESOURCE_LOCAL. Local JPA transactions are defined through the EntityTransaction class. It contains basic transaction API including begin, commit and rollback.

JTA transactions are used in Java EE, in managed mode (CMT). To use JTA transactions the transaction-type attribute in the persistence.xml is set to JTA. JTA transactions are defined through the JTA UserTransaction class.

JTA transactions can be used in two modes in Java EE. In Java EE managed mode such as an EntityManager injected through @PersistenceContext, this mode is also known as Container Managed Transaction (CMT).

The second mode allows the EntityManager to be application managed, (normally obtained from an injected EntityManagerFactory, or directly from JPA Persistence) also known as Bean Managed Transaction (BMT). This allows the persistence context to survive transaction boundaries, and follow the normal EntityManager life-cycle similar to resource local.

DeltaSpike has three implementations of TransactionStrategy:

  • ResourceLocalTransactionStrategy which uses EntityTransaction
  • BeanManagedUserTransactionStrategy which uses UserTransaction
  • ContainerManagedTransactionStrategy

We have to tell DeltaSpike to use ContainerManagedTransactionStrategy:

file: src/main/resources/META-INF/apache-deltaspike.properties

globalAlternatives.org.apache.deltaspike.jpa.spi.transaction.TransactionStrategy = org.apache.deltaspike.jpa.impl.transaction.ContainerManagedTransactionStrategy

Conclusion

We have been introduced to DeltaSpike Data Module, and we saw how easy it is to use for contructing queries. We also did a quick introduction to Transactions in JPA.

As usual you can find the full example to this guide in the github repository. Until the next post, keep doing cool things :+1:.

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