Isolating legacy code with ArchUnit tests

// gradle 
dependencies {   
  testImplementation 'com.tngtech.archunit:archunit-junit5:0.20.1' 
}

In our project we decided that all code for the new UI lives in a package called v2. In the test we declare that this v2 package should not rely on legacy code. For this we use ArchUnits ability to assert on package dependencies. In the simplest form this looks as follows

@Test
public void version2PackagesShouldNotDependOnLegacyPackages() {
    // first declare the rule
    var isolatedVersion2 = noClasses()
            .that()
            .resideInAPackage("com.acme.webapp.v2..") // the trailing dot tells ArchUnit to match all sub-packages as well
            .should()
            .dependOnClassesThat().resideInAPackage("com.acme.webapp.legacy..");

    // second define on which classes the rule should be applied
    var webapp = new ClassFileImporter()
            .withImportOption(new ImportOption.DoNotIncludeTests())
            .importPackages("com.acme.webapp");
            
    // finally run the check
    isolatedVersion2.check(webapp);
}

But we cannot completely isolate from legacy code. We do want to make use of logged in users for example. So we need to allow some classes. For that we can create a custom predicate that defines the exceptions.

DescribedPredicate legacyPackagesWithAcceptedDependencies =
    new DescribedPredicate<>("reside in legacy package"){
        String[] acceptedDependencies = { "OrganizationDto", "IdentityContext" };

        @Override
        public boolean apply(JavaClass input) {
            var isAnAcceptedDependency = Arrays
                    .stream(acceptedDependencies)
                    .anyMatch(acceptedDependency -> input.getSimpleName().equals(acceptedDependency));

            return !isAnAcceptedDependency && input.getPackageName().startsWith("com.acme.webapp.legacy");
        }
    };

When put together the whole code looks like this

 package architecture;

  import com.tngtech.archunit.base.DescribedPredicate;
  import com.tngtech.archunit.core.domain.JavaClass;
  import com.tngtech.archunit.core.importer.ClassFileImporter;
  import com.tngtech.archunit.core.importer.ImportOption;
  import org.junit.jupiter.api.Test;

  import java.util.Arrays;

  import static com.tngtech.archunit.lang.syntax.ArchRuleDefinition.noClasses;

  public class LegacyIsolationTests {
      DescribedPredicate legacyPackagesWithAcceptedDependencies =
              new DescribedPredicate<>("reside in legacy package"){
                  String[] acceptedDependencies = { "OrganizationDto", "IdentityContext" };

                  @Override
                  public boolean apply(JavaClass input) {
                      var isAnAcceptedDependency = Arrays
                              .stream(acceptedDependencies)
                              .anyMatch(acceptedDependency -> input.getSimpleName().equals(acceptedDependency));

                      return !isAnAcceptedDependency && input.getPackageName().startsWith("com.acme.webapp.legacy");
                  }
              };

      @Test
      public void version2PackagesShouldNotDependOnLegacyPackages() {

          var isolatedVersion2 = noClasses()
                  .that()
                  .resideInAPackage("com.acme.webapp.v2..")
                  .should()
                  .dependOnClassesThat(legacyPackagesWithAcceptedDependencies);

          var webapp = new ClassFileImporter()
                  .withImportOption(new ImportOption.DoNotIncludeTests())
                  .importPackages("com.acme.webapp");

          isolatedVersion2.check(webapp);
      }
  }