Are Cyclic Module Dependencies Allowed in Java 9?

Are cyclic module dependencies allowed in Java 9?

In Java 9, the introduction of the Java Platform Module System (JPMS) fundamentally changed how classes and packages are organized. One of the critical aspects of this new module system is its stance on cyclic module dependencies, which are relationships where two or more modules depend on each other. This article explores whether cyclic module dependencies are permissible in Java 9, the implications of such designs, and alternatives for managing dependencies in a modular application.

module A {
    requires B;
}

module B {
    requires A; // This creates a cyclic dependency, which is not allowed
}

Causes

• Cyclic dependencies can lead to classes being confused about which module to load first, leading to potential runtime issues.
• They can complicate module management and increase maintenance difficulty due to interdependencies.

Solutions

• To avoid cyclic dependencies, you can refactor your codebase by creating new modules that hold common functionalities, allowing various modules to share them without direct dependencies.
• Implement interfaces in one module and the implementation in another, which helps to decouple the modules and avoid cycles.

Mistake: Not understanding the implications of needing two modules to have mutual dependencies, leading to design flaws.

Solution: Carefully plan module designs to avoid cyclic dependencies, for example, by establishing clear ownership of functionality.

Mistake: Overlooking the need to refactor existing code to adapt to the modular structure of Java 9.

Solution: Review the current architecture and identify opportunities to create clear separations between modules.