--- icedtea-web-1.8.8/acinclude.m4	2025-06-22 23:23:07.948458965 +0200
+++ icedtea-web-1.8.8/acinclude.m4	2025-06-22 23:24:13.072043812 +0200
@@ -882,10 +882,12 @@
   fi
   if test "${JAVA_VERSION}" -eq "8"; then
     HAVE_JAVA8="yes"
+  elif test "$JAVA_VERSION" -ge "21"; then
+    HAVE_JAVA21="yes"
   elif test "$JAVA_VERSION" -ge "9"; then
     HAVE_JAVA9="yes"
   fi
-  if test -z "$HAVE_JAVA8" -a -z "$HAVE_JAVA9"; then
+  if test -z "$HAVE_JAVA8" -a -z "$HAVE_JAVA9" -a -z "$HAVE_JAVA21"; then
     AC_MSG_ERROR([JDK8 or newer is required, detected was: $JAVA_VERSION])
   fi
   if ! test -z "$HAVE_JAVA8"; then
@@ -898,6 +900,7 @@
   AM_CONDITIONAL([HAVE_JAVA8], test x"${HAVE_JAVA8}" = "xyes")
   AM_CONDITIONAL([HAVE_JAVA9], test x"${HAVE_JAVA9}" = "xyes")
   AM_CONDITIONAL([HAVE_JAVA17], test x"${HAVE_JAVA17}" = "xyes")
+  AM_CONDITIONAL([HAVE_JAVA21], test x"${HAVE_JAVA21}" = "xyes")
 ])
 
 AC_DEFUN_ONCE([IT_FIND_KEYTOOL],
--- icedtea-web-1.8.8/configure.ac	2025-06-22 23:23:07.948921028 +0200
+++ icedtea-web-1.8.8/configure.ac	2025-06-22 23:23:29.972854328 +0200
@@ -148,11 +148,14 @@
 dnl IT575 - Plugin depends on com.sun/jndi.toolkit.url.UrlUtil
 dnl IT576 - Plugin depends on sun.applet.AppletImageRef
 dnl IT578 - Remove need for patching AppletPanel for Plugin/Webstart
-if test x"$HAVE_JAVA9" = xyes ; then
+if test x"$HAVE_JAVA9" = xyes -o x"$HAVE_JAVA21" = xyes ; then
   JAVA_BASE=java.base
   JAVA_DESKTOP=java.desktop
   JAVA_NAMING=java.naming
 fi
+if test x"$HAVE_JAVA21" = xyes ; then
+  JDK_JARTOOL=jdk.jartool
+fi
 IT_CHECK_FOR_CLASS(JAVA_NET_COOKIEMANAGER, [java.net.CookieManager], [some.pkg], [])
 IT_CHECK_FOR_CLASS(JAVA_NET_HTTPCOOKIE, [java.net.HttpCookie], [some.pkg], [])
 IT_CHECK_FOR_CLASS(JAVA_NET_COOKIEHANDLER, [java.net.CookieHandler], [some.pkg], [])
@@ -162,6 +165,9 @@
 IT_CHECK_FOR_CLASS(SUN_SECURITY_X509_X500NAME, [sun.security.x509.X500Name], [some.pkg], [$JAVA_BASE])
 IT_CHECK_FOR_CLASS(SUN_SECURITY_ACTION_GETPROPERTYACTION, [sun.security.action.GetPropertyAction], [some.pkg], [$JAVA_BASE])
 # the classname cant be substitued by variable, as it is substituted to inner class
+if test x"$HAVE_JAVA21" = xyes ; then
+  IT_CHECK_FOR_CLASS(HEXDUMPENCODER, [sun.security.util.HexDumpEncoder], [some.pkg], [$JAVA_BASE])
+else
 if test x"$HAVE_JAVA9" = xyes ; then
   IT_CHECK_FOR_CLASS(HEXDUMPENCODER, [sun.security.util.HexDumpEncoder], [some.pkg], [$JAVA_BASE])
   IT_CHECK_FOR_CLASS(SUN_MISC_JARINDEX, [jdk.internal.util.jar.JarIndex], [some.pkg], [$JAVA_BASE])
@@ -170,6 +176,7 @@
   IT_CHECK_FOR_CLASS(SUN_MISC_JARINDEX, [sun.misc.JarIndex], [some.pkg], [$JAVA_BASE])
   IT_CHECK_FOR_CLASS(SUN_MISC_LAUNCHER, [sun.misc.Launcher], [some.pkg], [$JAVA_BASE])
 fi
+fi
 IT_CHECK_FOR_CLASS(SUN_SECURITY_VALIDATOR_VALIDATOREXCEPTION, [sun.security.validator.ValidatorException], [some.pkg], [$JAVA_BASE])
 IT_CHECK_FOR_CLASS(SUN_NET_WWW_PROTOCOL_JAR_URLJARFILE, [sun.net.www.protocol.jar.URLJarFile], [some.pkg], [$JAVA_BASE])
 IT_CHECK_FOR_CLASS(SUN_NET_WWW_PROTOCOL_JAR_URLJARFILECALLBACK, [sun.net.www.protocol.jar.URLJarFileCallBack], [some.pkg], [$JAVA_BASE])
--- icedtea-web-1.8.8/Makefile.am	2025-06-22 23:23:07.949358377 +0200
+++ icedtea-web-1.8.8/Makefile.am	2025-06-22 23:23:29.973243659 +0200
@@ -289,6 +289,14 @@
  -bottom '<font size="-1"> <a href="http://icedtea.classpath.org/bugzilla">Submit a bug or feature</a></font>'
 if HAVE_JAVA9
 JAVADOC_OPTS+=-source $(IT_LANGUAGE_SOURCE_VERSION)
+JAVA_EXPORTS=--add-exports=java.base/sun.security.action=ALL-UNNAMED
+else
+if HAVE_JAVA21
+JAVADOC_OPTS+=-source $(IT_LANGUAGE_SOURCE_VERSION)
+JAVA_EXPORTS=--add-exports=java.base/sun.security.action=ALL-UNNAMED
+else
+JAVA_EXPORTS=
+endif
 endif
 if JAVADOC_SUPPORTS_J_OPTIONS
 JAVADOC_MEM_OPTS=-J-Xmx1024m -J-Xms128m
--- icedtea-web-1.8.8/netx/net/sourceforge/jnlp/jdk89acesses/JarIndexAccess.java	2025-06-22 22:37:35.533723369 +0200
+++ icedtea-web-1.8.8/netx/net/sourceforge/jnlp/jdk89acesses/JarIndexAccess.java	2025-06-22 23:23:29.973643895 +0200
@@ -28,7 +28,7 @@
                 jarIndexClass = Class.forName("jdk.internal.util.jar.JarIndex");
             } catch (ClassNotFoundException exx) {
                 OutputController.getLogger().log(exx);
-                throw new RuntimeException("JarIndex not found!");
+                jarIndexClass = JarIndexDummy.class;
             }
         }
     }
@@ -71,4 +71,17 @@
         return (LinkedList<String>) o;
     }
 
+    /**
+     * Because starting of JAVA 21 the JarIndex is no longer supported we provide a dummy implementation which always returns {@code null}.
+     */
+    @SuppressWarnings("unused")
+    private static class JarIndexDummy {
+        public static Object getJarIndex(JarFile jar) {
+            return null;
+        }
+
+        public Object get(String fileName) {
+            return null;
+        }
+    }
 }