diff --git a/src/Tools/Setup/isabelle/setup/Build_Scala.java b/src/Tools/Setup/isabelle/setup/Build_Scala.java
--- a/src/Tools/Setup/isabelle/setup/Build_Scala.java
+++ b/src/Tools/Setup/isabelle/setup/Build_Scala.java
@@ -1,243 +1,255 @@
 /*  Title:      Tools/Setup/isabelle/setup/Build_Scala.java
     Author:     Makarius
 
 Build Isabelle/Scala modules.
 */
 
 package isabelle.setup;
 
 
 import java.io.BufferedOutputStream;
 import java.io.File;
 import java.io.IOException;
 import java.math.BigInteger;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.StandardCopyOption;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.util.ArrayList;
 import java.util.Comparator;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Locale;
 import java.util.Properties;
 import java.util.jar.Attributes;
 import java.util.jar.JarEntry;
 import java.util.jar.JarOutputStream;
 import java.util.jar.Manifest;
 import java.util.stream.Stream;
 
 import scala.tools.nsc.MainClass;
 
 
 public class Build_Scala
 {
     /** component directory context **/
 
     public static class Context
     {
         private final Path component_dir;
         private Properties props;
 
         public Context(Path dir) throws IOException
         {
             component_dir = dir;
             props = new Properties();
             Path path = component_dir.resolve("etc/scala.props");
             if (Files.exists(path)) { props.load(Files.newBufferedReader(path)); }
         }
 
         @Override public String toString() { return component_dir.toString(); }
 
         public String component_name() { return component_dir.toFile().getName(); }
         public String name() { return props.getProperty("name", component_name()); }
         public String description() { return props.getProperty("description", name()); }
 
         public String lib_name() { return props.getProperty("lib", "lib") + "/" + name(); }
         public String jar_name() { return lib_name() + ".jar"; }
         public String shasum_name() { return lib_name() + ".shasum"; }
 
         public String main() { return props.getProperty("main", ""); }
 
-        public String[] sources() { return props.getProperty("sources", "").split("\\s+"); }
-        public String[] resources() { return props.getProperty("resources", "").split("\\s+"); }
-        public String[] services() { return props.getProperty("services", "").split("\\s+"); }
+        private List<String> get_list(String name)
+        {
+            List<String> list = new LinkedList<String>();
+            for (String s : props.getProperty(name, "").split("\\s+")) {
+                if (!s.isEmpty()) { list.add(s); }
+            }
+            return List.copyOf(list);
+        }
+        public List<String> sources() { return get_list("sources"); }
+        public List<String> resources() { return get_list("resources"); }
+        public List<String> services() { return get_list("services"); }
 
         public Path path(String file) { return component_dir.resolve(file); }
         public boolean exists(String file) { return Files.exists(path(file)); }
 
         public String item_name(String s)
         {
             int i = s.indexOf(':');
             return i > 0 ? s.substring(0, i) : s;
         }
 
         public String item_target(String s)
         {
             int i = s.indexOf(':');
             return i > 0 ? s.substring(i + 1) : s;
         }
 
         public String shasum(String file)
             throws IOException, NoSuchAlgorithmException
         {
             if (exists(file)) {
                 MessageDigest sha = MessageDigest.getInstance("SHA");
                 sha.update(Files.readAllBytes(path(file)));
                 String digest = String.format(Locale.ROOT, "%040x", new BigInteger(1, sha.digest()));
                 return digest + " *" + file + "\n";
             }
             else { return ""; }
         }
     }
 
 
     /** compile sources **/
 
     public static void compile_sources(
         Path target_dir, List<Path> deps, String options, List<Path> sources)
     {
         ArrayList<String> args = new ArrayList<String>();
         args.add("-d");
         args.add(target_dir.toString());
         args.add("-bootclasspath");
         args.add(Environment.join_paths(deps));
-        for (String s : options.split("\\s+")) { args.add(s); }
+        for (String s : options.split("\\s+")) {
+          if (!s.isEmpty()) { args.add(s); }
+        }
         args.add("--");
         for (Path p : sources) { args.add(p.toString()); }
 
         MainClass main = new MainClass();
         boolean ok = main.process(args.toArray(String[]::new));
         if (!ok) throw new RuntimeException("Failed to compile sources");
     }
 
 
     /** create jar **/
 
     public static void create_jar(Path dir, String main, Path jar)
         throws IOException
     {
         Files.deleteIfExists(jar);
 
         Manifest manifest = new Manifest();
         Attributes attributes = manifest.getMainAttributes();
         attributes.put(Attributes.Name.MANIFEST_VERSION, "1.0");
         attributes.put(new Attributes.Name("Created-By"),
             System.getProperty("java.version") + " (" + System.getProperty("java.vendor") + ")");
         if (!main.isEmpty()) { attributes.put(Attributes.Name.MAIN_CLASS, main); }
 
         try (JarOutputStream out =
             new JarOutputStream(new BufferedOutputStream(Files.newOutputStream(jar)), manifest))
         {
             for (Path path : Files.walk(dir).sorted().toArray(Path[]::new)) {
                 boolean is_dir = Files.isDirectory(path);
                 boolean is_file = Files.isRegularFile(path);
                 if (is_dir || is_file) {
                     String name = Environment.slashes(dir.relativize(path).toString());
                     JarEntry entry = new JarEntry(is_dir ? name + "/" : name);
                     entry.setTime(path.toFile().lastModified());
                     out.putNextEntry(entry);
                     if (is_file) { out.write(Files.readAllBytes(path)); }
                     out.closeEntry();
                 }
             }
         }
     }
 
 
 
     /** build scala **/
 
     public static void build_scala(Context context, boolean fresh)
         throws IOException, InterruptedException, NoSuchAlgorithmException
     {
         String jar_name = context.jar_name();
         Path jar_path = context.path(jar_name);
         String shasum_name = context.shasum_name();
 
-        String[] sources = context.sources();
-        String[] resources = context.resources();
+        List<String> sources = context.sources();
+        List<String> resources = context.resources();
 
-        if (sources.length == 0) {
+        if (sources.isEmpty()) {
             Files.deleteIfExists(jar_path);
             Files.deleteIfExists(context.path(shasum_name));
         }
         else {
             String shasum_old =
                 context.exists(shasum_name) ? Files.readString(context.path(shasum_name)) : "";
             String shasum_sources;
             {
                 StringBuilder _shasum = new StringBuilder();
                 for (String s : resources) {
                     _shasum.append(context.shasum(context.item_name(s)));
                 }
                 for (String s : sources) { _shasum.append(context.shasum(s)); }
                 shasum_sources = _shasum.toString();
             }
             if (fresh || !shasum_old.equals(context.shasum(jar_name) + shasum_sources)) {
                 System.out.println(
                     "### Building " + context.description() + " (" + jar_path + ") ...");
 
                 String scalac_options = Environment.getenv("ISABELLE_SCALAC_OPTIONS");
                 String isabelle_class_path = Environment.getenv("ISABELLE_CLASSPATH");
 
                 Path build_dir = Files.createTempDirectory("isabelle");
                 try {
                     /* compile sources */
 
                     List<Path> compiler_deps = new LinkedList<Path>();
                     for (String s : isabelle_class_path.split(":", -1)) {
-                        compiler_deps.add(Path.of(Environment.platform_path(s)));
+                        if (!s.isEmpty()) {
+                          compiler_deps.add(Path.of(Environment.platform_path(s)));
+                        }
                     }
 
                     List<Path> compiler_sources = new LinkedList<Path>();
                     for (String s : sources) { compiler_sources.add(context.path(s)); }
 
                     compile_sources(build_dir, compiler_deps, scalac_options, compiler_sources);
 
 
                     /* copy resources */
 
                     for (String s : context.resources()) {
                         String name = context.item_name(s);
                         String target = context.item_target(s);
                         Path file_name = Path.of(name).normalize().getFileName();
                         Path target_path = Path.of(target).normalize();
 
                         Path target_dir;
                         Path target_file;
                         {
                             if (target.endsWith("/") || target.endsWith("/.")) {
                                 target_dir = build_dir.resolve(target_path);
                                 target_file = target_dir.resolve(file_name);
                             }
                             else {
                                 target_file = build_dir.resolve(target_path);
                                 target_dir = target_file.getParent();
                             }
                         }
                         Files.createDirectories(target_dir);
                         Files.copy(context.path(name), target_file,
                             StandardCopyOption.COPY_ATTRIBUTES);
                     }
 
 
                     /* packaging */
 
                     create_jar(build_dir, context.main(), jar_path);
 
                     String shasum = context.shasum(jar_name) + shasum_sources;
                     Files.writeString(context.path(shasum_name), shasum);
                 }
                 finally {
                     try (Stream<Path> walk = Files.walk(build_dir)) {
                         walk.sorted(Comparator.reverseOrder())
                             .map(Path::toFile)
                             .forEach(File::delete);
                     }
                 }
             }
         }
     }
 }