97 Things Every Java Programmer Should Know

Chapter 74. Take Care of Your Module Declarations

Nicolai Parlog

If you’re creating Java modules, your module declarations (module-info.java files) are easily your most important source files. Each one represents an entire JAR and governs how it interacts with other JARs, so take good care of your declarations! Here are a few things to look out for.

Keep Module Declarations Clean

Module declarations are code and should be treated as such, so make sure your code style is applied. Beyond that, rather than placing directives randomly, structure your module declarations. Here’s the order the JDK uses:

1. Requires, including static and transitive

2. Exports

3. Exports to

4. Opens

5. Opens to

6. Uses

7. Provides

Whatever you decide, if you have a document defining your code style, record the decision there. If you have your IDE, build tool, or code analyzer check such things for you, even better. Try to bring it up to speed so it can automatically check—or even apply—your chosen style.

Comment Module Declarations

Opinions on code documentation, like Javadoc or inline comments, vary wildly, but whatever your team’s position on comments is, extend it to module declarations. If you like abstractions to have a sentence or two explaining their meaning and importance, add such a Javadoc comment to each module. Even if that’s not your style, most people agree that it’s good to document why