Java 9 Modularity

Chapter 4. Services

In this chapter, you will learn how to use services, an important feature for creating modular codebases. After learning the basics of providing and consuming services, we will apply them to EasyText, making it more extensible.

Factory Pattern

In the previous chapter, you saw that encapsulation alone doesn’t get us very far when we want to create truly decoupled modules. If we still write

MyInterface i = new MyImpl();

every time we need to use an implementation class, it means the implementation class must be exported. Consequently, strong coupling still remains between the consumer and provider of the implementation: the consumer requires the provider module directly to use its exported implementation class. Changes in the implementation directly affect all consumers. As you will soon see, services are an excellent solution to this problem. But before diving into services, let’s see if we can fix this problem by using an existing pattern, building on our knowledge of the module system so far.

The factory pattern is a well-known creational design pattern that seems to address the very problem we are dealing with. Its goal is to decouple a consumer of objects from the instantiation of specific classes. Many variations on the factory pattern have emerged since it was first described in the iconic Gang of Four Design Patterns book by Gamma et al. (Addison-Wesley). Let’s try to implement a simple variation of this pattern and see how far it gets us with decoupling ...