CDI caters to a very fundamental design idea, as that of dependency injection. When writing any application, you would typically define multiple classes and some relationship between them. Thus, to work with one object you would need to hand it its dependencies during its creation. When using CDI, these dependencies are provided to the object, rather than the owning object fetching them for itself. Dependency Injection (DI) leads to the owning object becoming independent of the dependency creation and lookup code, which makes it loosely coupled, and the code is testable. Both qualities, while difficult to pull off, are worth striving for.

Now, before we get into the nuts and bolts of CDI and how it helps to structure the code, let's explore what dependency injection is and what problems it helps solve with an example. Consider...

This programming model unifies the various approaches taken by existing dependency injection frameworks such as Spring, Guice, and Seam. It has been designed considering the scope of objects as well. CDI was introduced in Java EE 6, and now with Java EE 8, we have the latest CDI 2.0 version, which offers Java SE support as well. While Java EE 5 did make resource injection possible, it did not have the support for general purpose dependency injection for objects. With CDI, we have beans that are container managed, and these managed beans are Plain Old Java Objects (POJOs) whose life cycle is controlled by the Java EE container. The idea of the bean has been presented in Java for ages in various forms, such as JSF bean, EJB bean, and so on. CDI takes this idea and provides a unified view to managed beans across the entire Java platform, which is relied on by...

Before we start wielding JPA and CDI together, let’s get the basics out of the way for JPA. The Java Persistence API allows for the modelling of the domain objects for accessing, persisting, and managing data between POJOs and a relational database. JPA is the standard for working with object-relational mapping (ORM) solutions. Popular ORM solutions include Hibernate, EclipseLink (the reference implementation for all JPA versions), Apache OpenJPA, and DataNucleus. JPA puts the focus back on the domain model for retrieving and persisting data without having to deal with resource management and vendor specific SQL.

Most developers would be accustomed to hearing about Hibernate in articles and projects; it also shows up as a skill sought by employers. While Hibernate and the like can be used directly, using JPA helps us avoid falling in the vendor...

As you may have noticed, getting started with JPA and CDI isn't that complex. With an understanding of both, it's possible to build large applications which utilize these technologies for various business cases. We looked at how dependency injection promotes writing loosely coupled and testable code and how CDI, a ubiquitous API for dependency injection and context management in Java EE, helps us do that.

CDI 2 brings ordered and asynchronous events to further enrich this programming model. As of the 2.0 release, CDI is available for Java SE as well. JPA has been updated for utilizing Java 8 Streams, repeatable annotations, and date/time APIs along with better CDI integration support. The JPA entities are defined as classes using @Entity, @MappedSuperclass, and @Embeddable annotations. To track state changes, we get to use entity listeners that can be used with...