diff --git a/src/Pure/Admin/build_jdk.scala b/src/Pure/Admin/build_jdk.scala
--- a/src/Pure/Admin/build_jdk.scala
+++ b/src/Pure/Admin/build_jdk.scala
@@ -1,166 +1,169 @@
 /*  Title:      Pure/Admin/build_jdk.scala
     Author:     Makarius
 
 Build Isabelle jdk component using downloads from Azul.
 */
 
 package isabelle
 
 
 import java.nio.file.Files
 import java.nio.file.attribute.PosixFilePermission
 
 
 object Build_JDK {
   /* platform information */
 
   sealed case class JDK_Platform(name: String, url_template: String) {
     override def toString: String = name
 
     def url(base_url: String, jdk_version: String, zulu_version: String): String =
       base_url + "/" + url_template.replace("{V}", jdk_version).replace("{Z}", zulu_version)
   }
 
   val platforms: List[JDK_Platform] =
     List(
       JDK_Platform("arm64-darwin", "zulu{Z}-jdk{V}-macosx_aarch64.tar.gz"),
       JDK_Platform("arm64-linux", "zulu{Z}-jdk{V}-linux_aarch64.tar.gz"),
       JDK_Platform("x86_64-darwin", "zulu{Z}-jdk{V}-macosx_x64.tar.gz"),
       JDK_Platform("x86_64-linux", "zulu{Z}-jdk{V}-linux_x64.tar.gz"),
       JDK_Platform("x86_64-windows", "zulu{Z}-jdk{V}-win_x64.zip"))
 
 
   /* build jdk */
 
   val default_base_url = "https://cdn.azul.com/zulu/bin"
   val default_jdk_version = "17.0.5"
   val default_zulu_version = "17.38.21-ca"
 
   def build_jdk(
     target_dir: Path = Path.current,
     base_url: String = default_base_url,
     jdk_version: String = default_jdk_version,
     zulu_version: String = default_zulu_version,
     progress: Progress = new Progress,
   ): Unit = {
+    if (Platform.is_windows) error("Cannot build on Windows")
+
+
     /* component */
 
     val component = "jdk-" + jdk_version
     val component_dir =
       Components.Directory.create(target_dir + Path.basic(component), progress = progress)
 
 
     /* download */
 
     for (platform <- platforms) {
       Isabelle_System.with_tmp_dir("download", component_dir.path.file) { dir =>
         val url = platform.url(base_url, jdk_version, zulu_version)
         val name = Library.take_suffix(_ != '/', url.toList)._2.mkString
         val file = dir + Path.basic(name)
         Isabelle_System.download_file(url, file, progress = progress)
         Isabelle_System.extract(file, dir)
 
         val jdk_dir = File.get_dir(dir, title = url)
         val platform_dir = component_dir.path + Path.basic(platform.name)
         Isabelle_System.move_file(jdk_dir, platform_dir)
       }
     }
 
 
     /* permissions */
 
     for (file <- File.find_files(component_dir.path.file, include_dirs = true)) {
       val path = file.toPath
       val perms = Files.getPosixFilePermissions(path)
       perms.add(PosixFilePermission.OWNER_READ)
       perms.add(PosixFilePermission.GROUP_READ)
       perms.add(PosixFilePermission.OTHERS_READ)
       perms.add(PosixFilePermission.OWNER_WRITE)
       if (file.isDirectory) {
         perms.add(PosixFilePermission.OWNER_WRITE)
         perms.add(PosixFilePermission.OWNER_EXECUTE)
         perms.add(PosixFilePermission.GROUP_EXECUTE)
         perms.add(PosixFilePermission.OTHERS_EXECUTE)
       }
       Files.setPosixFilePermissions(path, perms)
     }
 
 
     /* settings */
 
     File.write(component_dir.settings,
       """# -*- shell-script -*- :mode=shellscript:
 
 case "$ISABELLE_PLATFORM_FAMILY" in
   linux)
     ISABELLE_JAVA_PLATFORM="$ISABELLE_PLATFORM64"
     ISABELLE_JDK_HOME="$COMPONENT/$ISABELLE_JAVA_PLATFORM"
     ;;
   windows)
     ISABELLE_JAVA_PLATFORM="$ISABELLE_WINDOWS_PLATFORM64"
     ISABELLE_JDK_HOME="$COMPONENT/$ISABELLE_JAVA_PLATFORM"
     ;;
   macos)
     if [ -n "$ISABELLE_APPLE_PLATFORM64" -a -d "$COMPONENT/$ISABELLE_APPLE_PLATFORM64" ]
     then
       ISABELLE_JAVA_PLATFORM="$ISABELLE_APPLE_PLATFORM64"
     else
       ISABELLE_JAVA_PLATFORM="$ISABELLE_PLATFORM64"
     fi
     ISABELLE_JDK_HOME="$COMPONENT/$ISABELLE_JAVA_PLATFORM"
     ;;
 esac
 """)
 
 
     /* README */
 
     File.write(component_dir.README,
       """This is OpenJDK """ + jdk_version + """ based on downloads by Azul, see also
 https://www.azul.com/downloads/?package=jdk
 
 The main license is GPL2, but some modules are covered by other (more liberal)
 licenses, see legal/* for details.
 
 Linux, Windows, macOS all work uniformly, depending on platform-specific
 subdirectories.
 """)
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("build_jdk", "build Isabelle jdk component using downloads from Azul",
       Scala_Project.here,
       { args =>
         var target_dir = Path.current
         var base_url = default_base_url
         var jdk_version = default_jdk_version
         var zulu_version = default_zulu_version
 
         val getopts = Getopts("""
 Usage: isabelle build_jdk [OPTIONS]
 
   Options are:
     -D DIR       target directory (default ".")
     -U URL       base URL (default: """" + default_base_url + """")
     -V NAME      JDK version (default: """" + default_jdk_version + """")
     -Z NAME      Zulu version (default: """" + default_zulu_version + """")
 
   Build Isabelle jdk component using downloads from Azul.
 """,
           "D:" -> (arg => target_dir = Path.explode(arg)),
           "U:" -> (arg => base_url = arg),
           "V:" -> (arg => jdk_version = arg),
           "Z:" -> (arg => zulu_version = arg))
 
         val more_args = getopts(args)
         if (more_args.nonEmpty) getopts.usage()
 
         val progress = new Console_Progress()
 
         build_jdk(target_dir = target_dir, base_url = base_url,
           jdk_version = jdk_version, zulu_version = zulu_version, progress = progress)
       })
 }