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Is it worth using the Java Platform Module System in application code?
Is it worth using the Java Platform Module System introduced in Java 9 to structure application code?
Given that the Java Platform Module System introduced in Java 9 doesn't manage dependency versions, I understand I still need a dependency management tool such as Maven - but if I am already using such a tool, what additional value does the JPMS provide? In particular, if I am already using Maven modules to structure my app?
1 answer
If you are planning to share the application with individual users (rather than deploying it to a server), declaring your module dependencies explicitly allows jlink to create a stripped-down Java runtime in the application image (and thus in the user’s installation).
Aside from that, using modules is like using the private keyword, but at the package level rather than at the class level. You could make all of your fields and methods public… but any competent programmer knows that they should not do that. You only want to expose encapsulated functionality, not internal implementation.
In a module, packages work the same way. A module descriptor should only export the packages that are intended for consumption by other modules. Packages which are not exported are private to that module, and cannot be accessed by other modules. That is a good thing. That’s object-oriented design, just like private class members is.
The module system is explicitly not intended to replace Maven, Gradle, or Ivy. From the original specification:
In other words, this specification need not define yet another dependency-management mechanism. Maven, Ivy, and Gradle have all tackled this difficult problem. We should leave it to these and other build tools, and container applications, to discover and select a set of candidate modules for a given library or application.
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