Improve root build file and root module detection for Maven reactors.

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

@@ -21,10 +21,7 @@
 import org.springframework.sbm.build.impl.OpenRewriteMavenBuildFile;
 
 import java.nio.file.Path;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.Optional;
-import java.util.Set;
+import java.util.*;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -43,10 +40,21 @@ public Stream<Module> stream() {
     }
 
     public Module getRootModule() {
+        // 1st try: module explicitly marked as having a root build file
         return modules.stream()
                 .filter(m -> m.getBuildFile().isRootBuildFile())
                 .findFirst()
-                .orElseThrow(() -> new RootBuildFileNotFoundException("Module with root build file is missing"));
+                // 2nd try (fallback): no module marked as root → choose the one
+                // whose build file path is closest to the project root
+                .orElseGet(() -> modules.stream()
+                        .min(Comparator.comparingInt(m -> pathDepth(m.getBuildFile())))
+                        .orElseThrow(() -> new RootBuildFileNotFoundException("Module with root build file is missing")));
+    }
+
+    private int pathDepth(BuildFile buildFile) {
+        Path path = buildFile.getSourcePath();
+        // if for some reason there is no path, treat it as "very deep"
+        return (path == null) ? Integer.MAX_VALUE : path.getNameCount();
     }
 
     public List<Module> list() {

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

@@ -15,17 +15,42 @@
  */
 package org.springframework.sbm.build.api;
 
+import org.jetbrains.annotations.NotNull;
 import org.springframework.sbm.project.resource.ProjectResourceSet;
 import org.springframework.sbm.project.resource.filter.ProjectResourceFinder;
 
+import java.nio.file.Path;
+import java.util.Comparator;
+import java.util.List;
+import java.util.stream.Collectors;
+
 public class RootBuildFileFilter implements ProjectResourceFinder<BuildFile> {
+
     @Override
-    public BuildFile apply(ProjectResourceSet projectResourceSet) {
-        return projectResourceSet.stream()
+    public BuildFile apply(@NotNull ProjectResourceSet projectResourceSet) {
+        // collect all build files (pom.xml, etc.)
+        List<BuildFile> buildFiles = projectResourceSet.stream()
                 .filter(pr -> BuildFile.class.isAssignableFrom(pr.getClass()))
                 .map(BuildFile.class::cast)
-                .filter(bf -> bf.isRootBuildFile())
+                .collect(Collectors.toList());
+
+        if (buildFiles.isEmpty()) {
+            throw new RootBuildFileNotFoundException("Could not find any BuildFile in project.");
+        }
+
+        // 1st try: existing logic – respect explicit isRootBuildFile flag
+        return buildFiles.stream()
+                .filter(BuildFile::isRootBuildFile)
                 .findFirst()
-                .orElseThrow(() -> new RootBuildFileNotFoundException("Could not find BuildFile for root module."));
+                // 2nd try (fallback): no explicit root → choose the build file
+                // whose source path is closest to the project root (smallest depth)
+                .orElseGet(() -> buildFiles.stream()
+                        .min(Comparator.comparingInt(bf -> pathDepth(bf.getSourcePath())))
+                        .orElseThrow(() -> new RootBuildFileNotFoundException("Could not find BuildFile for root module.")));
+    }
+
+    private int pathDepth(Path path) {
+        // defensive: null check, though OpenRewrite usually always has a path
+        return (path == null) ? Integer.MAX_VALUE : path.getNameCount();
     }
-}
+}