Tuesday, March 29, 2011

Dependency Injection - An Introductory Tutorial Part 1 | Javalobby

CDI is the Java standard for dependency injection (DI) and interception (AOP). It is evident from the popularity of DI and AOP that Java needs to address DI and AOP so that it can build other standards and JSRs on top of it. DI and AOP are the foundation of many Java frameworks, and CDI will be the foundation of many future specifications and JSRs.

This article discusses dependency injection in a tutorial format. It covers some of the features of CDI such as type safe annotations configuration, alternatives and more. This tutorial is split into two parts, the first part covers the basis of dependency injection, @Inject, @Produces and @Qualifiers. The next part in this series covers advanced topics like creating pluggable components with Instance and processing annotations for configuration.

CDI is a foundational aspect of Java EE 6. It is or will be shortly supported by Caucho's Resin, IBM's WebSphere, Oracle's Glassfish, Red Hat's JBoss and many more application servers. CDI is similar to core Spring and Guice frameworks. Like JPA did for ORM, CDI simplifies and sanitizes the API for DI and AOP. If you have worked with Spring or Guice, you will find CDI easy to use and easy to learn. If you are new to Dependency Injection (DI), then CDI is an easy on ramp for picking up DI quickly. CDI is simpler to use and learn.

CDI can be used standalone and can be embedded into any application.

Source code for this tutorial, and instructions for use.

It is no accident that this tutorial follows this Spring 2.5 DI tutorial (using Spring "new" DI annotations) written three years ago. It will be interesting to compare and contrast the examples in this tutorial with the one written three years ago for Spring DI annotations.

Design goals of this tutorial

This tutorial series is meant to be a description and explanation of DI in CDI without the clutter of EJB 3.1 or JSF. There are already plenty of tutorials that cover EJB 3.1 and JSF with CDI as a supporting actor.

CDI has merit on its own outside of the EJB and JSF space. This tutorial only covers CDI. Repeat there is no JSF 2 or EJB 3.1 in this tutorial. There are plenty of articles and tutorials that cover using CDI as part of a larger JEE 6 application. This tutorial is not that. This tutorial series is CDI and only CDI.

This tutorial only has full, complete code examples with source code you can download and try out on your own. There are no code snippets where you can't figure out where in the code you are suppose to be.

We start out slow, step by step and basic. Then once you understand the fundamentals, we pick up the pace quite a bit.

All code examples have actually been run. We don't type in ad hoc code. If it did not run, it is not in our tutorial. We are not winging it.

There are clear headings for code listings so you can use this tutorial as a cookbook when you want to use some feature of CDI DI in the future. This is a code centric tutorial. Again, the code listings are in the TOC on the wiki page so you can find just the code listing you are looking for quickly like an index for a cookbook.

Decorators, Extentions, Interceptors, Scopes are out of scope for this first tutorial. Expect them in future tutorials.

If this tutorial is well recieved and we get enough feedback through, the JavaLobby articles, our google group and comments section of the wiki then we will add a comprehensive tutorial on CDI AOP (Decorators and Interceptors) and one on Extentions. The more positive and/or constructive feedback we get the more encouraged we will be to add more.

Dependency Injection

Dependency Injection (DI) refers to the process of supplying an external dependency to a software component. DI can help make your code architecturally pure.

It aids in design by interface as well as test-driven development by providing a consistent way to inject dependencies. For example, a data access object (DAO) may depend on a database connection.

Instead of looking up the database connection with JNDI, you could inject it.

One way to think about a DI framework like CDI is to think of JNDI turned inside out. Instead of an object looking up other objects that it needs to get its job done (dependencies), a DI container injects those dependent objects. This is the so-called Hollywood Principle, "Don't call us‚" (lookup objects), "we'll call you" (inject objects).

If you have worked with CRC cards you can think of a dependency as a collaborator. A collaborator is an object that another object needs to perform its role, like a DAO (data access object) needs a JDBC connection object for example.

Dependency Injection-`AutomatedTellerMachine` without CDI or Spring or Guice

Let's say that you have an automated teller machine (ATM, also known as an automated banking machine in other countries) and it needs the ability to talk to a bank. It uses what it calls a transport object to do this. In this example, a transport object handles the low-level communication to the bank.

This example could be represented by these two interfaces as follows:

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Dependency Injection - An Introductory Tutorial Part 1 | Javalobby
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