Fix root build file selection (#429)

Author: ashakirin

components/sbm-core/src/main/java/org/springframework/sbm/build/api/ApplicationModules.java

@@ -44,9 +44,9 @@ public Stream<Module> stream() {
 
     public Module getRootModule() {
         return modules.stream()
-                .sorted((m2, m1) -> m1.getBuildFile().getAbsolutePath().toString().compareTo(m2.getBuildFile().getAbsolutePath().toString()))
+                .filter(m -> m.getBuildFile().isRootBuildFile())
                 .findFirst()
-                .orElse(modules.get(0));
+                .orElseThrow(() -> new RootBuildFileNotFoundException("Module with root build file is missing"));
     }
 
     public List<Module> list() {

components/sbm-core/src/test/java/org/springframework/sbm/build/api/ApplicationModulesTest.java

@@ -22,8 +22,12 @@
 import org.springframework.sbm.project.resource.TestProjectContext;
 
 import java.nio.file.Path;
+import java.util.List;
+import java.util.Optional;
+import java.util.stream.Stream;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 class ApplicationModulesTest {
 
@@ -115,17 +119,22 @@ void shouldBeRecognizedAsMultiModuleProject() {
     }
 
     @Test
-    void rootModule() {
+    void shouldFindRootModule() {
         Module rootModule = sut.getRootModule();
         assertThat(rootModule.getModulePath()).isEqualTo(Path.of(""));
         assertThat(rootModule.getBuildFile().getCoordinates()).isEqualTo("org.example:parent:1.0-SNAPSHOT");
-//        assertThat(rootModule.getModules().get(0).getBuildFile().getCoordinates()).isEqualTo("org.example:module1:1.0-SNAPSHOT");
-//        assertThat(rootModule.getModules().get(1).getBuildFile().getCoordinates()).isEqualTo("org.example:module2:1.0-SNAPSHOT");
         assertThat(rootModule.getDeclaredModules()).hasSize(2);
         assertThat(rootModule.getDeclaredModules().get(0)).isEqualTo("org.example:module1:1.0-SNAPSHOT");
         assertThat(rootModule.getDeclaredModules().get(1)).isEqualTo("org.example:module2:1.0-SNAPSHOT");
     }
 
+    @Test
+    void shouldNotFindRootModuleForMissingRootBuildFile() {
+        ApplicationModules applicationModules = new ApplicationModules(List.of());
+
+        assertThatThrownBy(applicationModules::getRootModule).isInstanceOf(RootBuildFileNotFoundException.class);
+    }
+
     @Test
     void getModule() {
         Module parentModule = sut.findModule("org.example:parent:1.0-SNAPSHOT").get();