diff --git a/lib/Tools/java_monitor b/lib/Tools/java_monitor
--- a/lib/Tools/java_monitor
+++ b/lib/Tools/java_monitor
@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 #
 # Author: Makarius
 #
 # DESCRIPTION: monitor another Java process
 
-isabelle java "-Dapple.awt.application.name=Java Monitor" isabelle.Java_Monitor "$@"
+isabelle java "-Dapple.awt.application.name=Java Monitor" --add-opens jdk.jconsole/sun.tools.jconsole=ALL-UNNAMED isabelle.Java_Monitor "$@"