Is IntelliJ IDEA project Module different concept from Java 9 Module?

Question

I never used modules in IntelliJ IDEA, but in Java 9 appeared modules ( which I also never used but wanna study now what is this)

So the question is: are there match

Answer 1

Much as I love Intellij, I have to answer yes: there is one big difference.

Java 9 Modules are a much needed step toward encapsulation and decoupling. But there is a form of (accidental? pathological?) coupling in Intellij modules by virtue of their membership in a single Intellij (and therefore VCS) project. Java 9 Modules can (and probably should, from the standpoint of encapsulation) be developed in separate VCS/IDE projects, from which they can expose only the APIs that make sense. A super (parent) POM is a mechanism providing non-pathological coupling across projects to reduce redundancy.

A module based on a well-defined domain and bounded context should be freely available for reuse: it's a big step toward the componentization we have been talking about for years. These domains are not random - they reflect an emerging analysis of the world. If it sounds like I'm advocating something like the anarchy and Balkanization faced by Node developers - I'm not: the well-analyzed domain is the key.

Either Intellij modules are "benefiting corruptly" from the kind of otherwise highly desirable coupling-under-the-hood that is one of the fantastic benefits of Intellij projects, or there is no value added by maintaining them in a single project. Working in a non-TBD environment with a branch for every JIRA ticket increases the cost of this kind of coupling (personal experience).

The justification in another answer for this coupling is the possibility of a "refactoring" that involves changes to multiple modules. But this is a code/design smell: either breaking changes are being made to a public API, which is a problem for all clients and can usually be avoided or mitigated with some imagination, deprecation, EOL-warnings (Strangler pattern), etc., or the modules exhibit pathological cohesion, probably due to incomplete analysis into bounded contexts.

Answer 2

It's a similar concept that appeared long before Java 9 modules. It's also not IDE specific. Build systems like Maven and Gradle also use this concept when working with the projects that consist of multiple sub-projects. In IntelliJ IDEA terminology the module is just a sub-project (In Eclipse the module is a Project and Workspace can have multiple projects).

Java 9 modules map to IntelliJ IDEA modules and provide additional features via the module descriptor specifying:

• the packages it explicitly makes available to other modules (all other packages in the module are implicitly unavailable to other modules)
• the services it offers
• the services it consumes
• to what other modules it allows reflection

IntelliJ IDEA already has a concept of modules for a project. Every IntelliJ IDEA module builds its own classpath. With the introduction of the new Java platform module system, IntelliJ IDEA modules had to extend their capability by supporting the Java platform's module-path if it is used instead of the classpath.

Related links:

• IntelliJ IDEA modules explained
• Migrating From Eclipse to IntelliJ IDEA
• Understanding Java 9 Modules
• Getting Started with Java 9 Module System
• Support for Java 9 Modules in IntelliJ IDEA