Reducing coupling of Java 9 modules

Mar 29 2017 by Tomasz Fijałkowski

In the Quick Introduction to Java 9 Modularization article, I quickly introduced the basic concept of Java 9 modularization. Now is the time to improve module design. In this article, we will focus on reducing module coupling.

Following the Dependency Inversion Principle, which states that:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
2. Abstractions should not depend on details. Details should depend on abstractions.

There should be a possibility to declare that a module depends on some API, not an implementation.

Fortunately, Java 9 introduces the concept of services - a module can register an implementation of an interface as a service. The other modules can consume this service. To take advantage of this solution, at least three modules are required: a client code, a module with an API, and a module with an implementation. The following example presents the consumption of a service in module-info.java (a client code):

module pl.tfij.java9modules.app {
    requires pl.tfij.java9modules.greetings.api;
    uses pl.tfij.java9modules.greetings.Greeting;
}

A module pl.tfij.java9modules.app depends on the pl.tfij.java9modules.greetings.api API, not its implementation. To access the implementation of the Greeting class, just use the ServiceLoader class:

ServiceLoader<Greeting> services = ServiceLoader.load(Greeting.class);

However, to run such code, you have to provide the required implementation in another module. A module that provides a service (a module with implementation) declares a module descriptor like this:

module pl.tfij.java9modules.greetings.standard {
    requires pl.tfij.java9modules.greetings.api;
    provides pl.tfij.java9modules.greetings.Greeting
        with pl.tfij.java9modules.greetings.standard.StandardGreeting;
}

You can provide many modules and run code with many implementations. Just put them all on the module path.

To complete the example, a trivial module-info.java file from the module with the API is presented below:

module pl.tfij.java9modules.greetings.api {
    exports pl.tfij.java9modules.greetings;
}

In this way, the coupling of modules is reduced - high-level modules depend on abstractions, not on low-level modules. The full example of the code is on my GitHub: Java-9-modules—reducing-coupling-of-modules.