diff --git a/Admin/build b/Admin/build
--- a/Admin/build
+++ b/Admin/build
@@ -1,88 +1,88 @@
 #!/usr/bin/env bash
 #
 # Administrative build for Isabelle source distribution.
 
 ## directory layout
 
 if [ -z "$ISABELLE_HOME" ]; then
   unset CDPATH
   ISABELLE_HOME="$(cd "$(dirname "$0")"; cd "$(pwd -P)"; cd ..; pwd)"
   ISABELLE_TOOL="$ISABELLE_HOME/bin/isabelle"
 fi
 
 
 ## diagnostics
 
 PRG="$(basename "$0")"
 
 function usage()
 {
   cat <<EOF
 
 Usage: $PRG [MODULES]
 
   Produce Isabelle distribution modules from current repository sources.
   The MODULES list may contain any of the following:
 
     all             all modules below
     browser         graph browser
     jars            Isabelle/Scala
     jars_fresh      fresh build of jars
 
 EOF
   exit 1
 }
 
 function fail()
 {
   echo "$1" >&2
   exit 2
 }
 
 
 ## process command line
 
 [ "$#" -eq 0 ] && usage
 
 MODULES="$@"; shift "$#"
 
 
 ## modules
 
 function build_all ()
 {
   build_browser
   build_setup build
 }
 
 
 function build_browser ()
 {
   pushd "$ISABELLE_HOME/lib/browser" >/dev/null
   "$ISABELLE_TOOL" env ./build || exit $?
   popd >/dev/null
 }
 
 
 function build_setup ()
 {
   rm -rf \
     "$ISABELLE_HOME/lib/classes/Pure.jar" \
     "$ISABELLE_HOME/lib/classes/Pure.shasum" \
     "$ISABELLE_HOME/src/Tools/jEdit/dist"
-  "$ISABELLE_TOOL" java isabelle.setup.Setup "$@"
+  env ISABELLE_SETUP_CLASSPATH_SKIP=true "$ISABELLE_TOOL" java isabelle.setup.Setup "$@"
 }
 
 
 ## main
 
 for MODULE in $MODULES
 do
   case $MODULE in
     all) build_all;;
     browser) build_browser;;
     jars) build_setup build;;
     jars_fresh) build_setup build_fresh;;
     *) fail "Bad module $MODULE"
   esac
 done
diff --git a/lib/Tools/java b/lib/Tools/java
--- a/lib/Tools/java
+++ b/lib/Tools/java
@@ -1,13 +1,17 @@
 #!/usr/bin/env bash
 #
 # Author: Makarius
 #
 # DESCRIPTION: invoke Java within the Isabelle environment
 
 eval "declare -a JAVA_ARGS=($ISABELLE_JAVA_SYSTEM_OPTIONS)"
 
-classpath "$ISABELLE_SETUP_CLASSPATH"; unset ISABELLE_SETUP_CLASSPATH
+if [ -z "$ISABELLE_SETUP_CLASSPATH_SKIP" -o "$ISABELLE_SETUP_CLASSPATH_SKIP" = "false" ]
+then
+  classpath "$ISABELLE_SETUP_CLASSPATH"; unset ISABELLE_SETUP_CLASSPATH
+fi
+
 classpath "$CLASSPATH"; unset CLASSPATH
 
 isabelle_java java "${JAVA_ARGS[@]}" \
   -classpath "$(platform_path "$ISABELLE_CLASSPATH")" "$@"