Book Image

Java 9 Dependency Injection

By : Nilang Patel, Krunal Patel
Book Image

Java 9 Dependency Injection

By: Nilang Patel, Krunal Patel

Overview of this book

Dependency Injection (DI) is a design pattern that allows us to remove the hard-coded dependencies and make our application loosely coupled, extendable, and maintainable. We can implement DI to move the dependency resolution from compile-time to runtime. This book will be your one stop guide to write loosely coupled code using the latest features of Java 9 with frameworks such as Spring 5 and Google Guice. We begin by explaining what DI is and teaching you about IoC containers. Then you’ll learn about object compositions and their role in DI. You’ll find out how to build a modular application and learn how to use DI to focus your efforts on the business logic unique to your application and let the framework handle the infrastructure work to put it all together. Moving on, you’ll gain knowledge of Java 9’s new features and modular framework and how DI works in Java 9. Next, we’ll explore Spring and Guice, the popular frameworks for DI. You’ll see how to define injection keys and configure them at the framework-specific level. After that, you’ll find out about the different types of scopes available in both popular frameworks. You’ll see how to manage dependency of cross-cutting concerns while writing applications through aspect-oriented programming. Towards the end, you’ll learn to integrate any third-party library in your DI-enabled application and explore common pitfalls and recommendations to build a solid application with the help of best practices, patterns, and anti-patterns in DI.
Table of Contents (14 chapters)
Title Page
Copyright and Credits
Packt Upsell
Contributors
Preface
Index

Best practices and anti-patterns


So far, we have talked about using IoC containers to achieve DI, but one of the most common mistakes is to use IoC containers without doing real DI. This may sound strange, but it is a fact. Such mistakes are possible in the absence of having a proper understanding of underlying concepts.

Ideally, DI implementation should only reference the IoC container during the time of the application's startup. If a developer wraps the IoC container itself and passes it into other component to reduce any dependency, this is not a good design. Let's understand this issue with an example.

What to inject – the container itself or just dependencies?

The situation of injecting container occurs when you try to wrap the container itself either in a singleton class or a public static method to provide the dependency to other components or modules, as in the following snippet:

public class AccountService {
  //Service method.
  public void getVariablePay() {
    System.out.println...