postinst is in openjdk-11-jdk 10.0.1+10-3ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 | #!/bin/sh
set -e
multiarch=x86_64-linux-gnu
priority=1101
basedir=/usr/lib/jvm/java-11-openjdk-amd64
mandir=$basedir/man
jdiralias=java-1.11.0-openjdk-amd64
srcext=1.gz
dstext=1.gz
jdk_tools='appletviewer jconsole'
case "$1" in
configure)
    # obsolete tool
    if update-alternatives --list apt 2>/dev/null; then
	update-alternatives --remove-all apt || true
    fi
    if [ -z "$2" ]; then
	update_alternatives=y
    fi
    if [ -n "$multiarch" ] && [ -n "$2" ]; then
	for i in $jdk_tools; do
	    if [ -z "$(update-alternatives --list $i 2>/dev/null | grep ^$basedir/)" ]; then
		update_alternatives=y
		break
	    fi
	done
    fi
    if [ "$update_alternatives" != y ] && [ $priority -gt 1060 ]; then
	for i in $jre_tools; do
	    oldp=$(update-alternatives --query java | awk -v b=$basedir '/^Alternative:/ && $2~b {p=1} /^Priority:/ && p {print $2; exit}')
	    if [ -n "$oldp" ] && [ "$oldp" -le 1060 ]; then
		update_alternatives=y
		break
	    fi
	done
    fi
    if [ "$update_alternatives" = y ]; then
    if [ -n "$multiarch" ] && [ "$DPKG_MAINTSCRIPT_ARCH" != $(dpkg --print-architecture) ]; then
	priority=$(expr $priority - 1)
    fi
    for i in $jdk_tools; do
	unset slave1 slave2 || true
        if [ -e $mandir/man1/$i.$srcext ]; then
	    slave1="--slave \
		/usr/share/man/man1/$i.$dstext \
                $i.$dstext \
                $mandir/man1/$i.$srcext"
	fi
        if false && [ -e $mandir/ja/man1/$i.$srcext ]; then
	    slave2="--slave \
		/usr/share/man/ja/man1/$i.$dstext \
                ${i}_ja.$dstext \
                $mandir/ja/man1/$i.$srcext"
	fi
        update-alternatives \
            --install \
            /usr/bin/$i \
            $i \
            $basedir/bin/$i \
            $priority \
	    $slave1 $slave2
    done
    fi # update alternatives
    ;;
esac
exit 0
 |