Considering Java Modules for Better Project Structure—Need Advice

Hi,

I have been writing Java using packages for years now, and it has worked very well. However, ever since I started training for the OCP, I've read a lot about Java Modules, and now I'm starting to think I might be missing out on a valuable feature of Java.

Let's say we have a simple API with these packages:

model

repository

service

controller

Do you think it is a good idea to create separate modules for each package? Additionally, since I like using Gradle or Maven, do they work well with Java Module feature?

What do you guys think about this?

Mert Y. Ertugrul wrote:HI've read a lot about Java Modules, and now I'm starting to think I might be missing out on a valuable feature of Java.

I'm a big fan of modules. I'm happy you're giving them a shot.

Do you think it is a good idea to create separate modules for each package?

No, for two reasons.

First, modules don't have a one-to-one relationship with packages. A module may contain exactly one package, and while this is indeed often the case, there could be good reasons for a module to contain more than one package. Blindly creating a separate module per package is not a good idea. Some thought and planning is always required.

Secondly, you are using packages wrongly. A package is a collection of classes that depend strongly on each other. A class might want to access package private members of another class that is in the same package.

Instead, you seem to have divided your classes over packages based on how much the classes resemble each other, not how strongly the classes depend on each other. Why does a UserAccountController (for example) need to be in the same package as a ProductCategoryController (another example). Sure, they're both controllers, but neither of them has anything to do with the other.

Instead, you might want to have a package com.example.shop.user that contains the classes related to users and user accounts, and a package com.example.shop.product that contains the classes related to products and product categories. Web API controllers and the request and response types could be in com.example.shop.web.user and com.example.shop.web.product.


Modules then contain all the packages that an application might want to use together. It's unlikely that you'll want to use com.example.shop.user in an application, but not com.example.shop.product. On the other hand, you could put com.example.shop.user in a different module than com.example.shop.web.user, because the controllers are only used in a web application, while the model classes might also be used in a client application or administrative tool.

So we could have the following two modules:
module com.example.shop {  exports com.example.shop.product;  exports com.example.shop.user; }module com.example.shop.web {  requires com.example.shop;  exports com.example.shop.web.product;  exports com.example.shop.web.user; }

Additionally, since I like using Gradle or Maven, do they work well with Java Module feature?

Yes. There's no issue in using these tools together with JPMS modules.