Hello Learners!

We discovered a fascinating library which allows us to test our architecture: [ArchUnit](https://www.archunit.org/)

We want to share a few cool use cases for the library. We hope you find them intriguing!

# Test Layered Architecture

layeredArchitecture()
    .layer("Controllers").definedBy("io.pivotal.archunitdemo.controller..")
    .layer("Services").definedBy("io.pivotal.archunitdemo.service..")
    .layer("Persistence").definedBy("io.pivotal.archunitdemo.persistence..")

    .whereLayer("Controllers").mayNotBeAccessedByAnyLayer()
    .whereLayer("Services").mayOnlyBeAccessedByLayers("Controllers")
    .whereLayer("Persistence").mayOnlyBeAccessedByLayers("Services")

    .check(classes);

# Prevent Cyclic Dependencies

slices()
    .matching("..(domain_package).(*)..").namingSlices("$2 of $1")
    .should().beFreeOfCycles()
    .check(classes)

Note: slices are sets of related packages. For example, the above test will ensure all packages under `domain_package` are free of cycles.

# Introducing New Rules To Existing Codebase

Say you want to specify an architecture rule, but you inherit a legacy codebase which has multiple violations of that rule.

You can record these violations and use `FreezingArchRule` to disregard past violations and only enforce the rule in the future.

freeze(noClasses().should().dependOnClassesThat().resideInAPackage("..service.."))
    .check(classes)

# Prevent calls to `System.out.println()` and `System.err.println()`
You can also use ArchUnit as a linter.

noClasses().should(ACCESS_STANDARD_STREAMS).check(classes)

# Visualizing Architecture
Do you have a component diagram that you use to communicate your architecture? How up to date is it?

ArchUnit can turn your [PlantUML](https://plantuml.com/) diagrams into tests!

Now you can be confident that your architecture and your diagram are always in sync.

For example, if you have the following architecture diagram:


then you can ask ArchUnit to run this diagram as a test like so:

val plantUmlDiagram = ArchUnitTest::class.java.getResource("archunit_demo.puml")
classes().should(adhereToPlantUmlDiagram(plantUmlDiagram, consideringOnlyDependenciesInDiagram()))
    .check(classes)

# Intrigued?

The above are only a few examples of what you can achieve with ArchUnit.

For more examples, please visit [ArchUnit Test Examples](https://github.com/TNG/ArchUnit/tree/master/archunit-example/example-plain/src/test/java/com/tngtech/archunit/exampletest) and the [official docs](https://www.archunit.org/userguide/html/000_Index.html).

Have a great weekend!