concept cyclic dependency in category java

This is an excerpt from Manning's book The Java Module System. Login to get full access to this book.

Then you have monitor.rest, which also depends on the statistics module because it handles statistics. Beyond that, it uses the Spark micro framework to create the REST endpoint. When modularizing the application in section 2.2.1, I made a point that MonitorServer doesn’t depend on Monitor. That comes in handy now because it means monitor.rest doesn’t depend on monitor; this is great because monitor depends on monitor.rest, and the module system forbids declaring cyclic dependencies. Finally, monitor depends on all the other modules because it creates most instances and pipes the results from one into the other.

Figure 3.5 Getting dependency cycles past the compiler isn’t easy. Here it’s done by picking two unrelated modules, persistence and rest (both depend on statistics), and then adding dependencies from one to the other. It’s important to compile rest against the old persistence so the cycle doesn’t show and compilation passes. In a final step, both original modules can be replaced with the newly compiled ones that have the cyclic dependency between them.

We’ll also look into using services to break cyclic dependencies between modules (section 10.3.5). Last but not least—and this is particularly interesting for those who plan to use services on different Java versions—we discuss how services work across plain and modular JARs (section 10.3.6).