postinst is in gcj-4.6-jre-headless 4.6.3-1ubuntu2.
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  | #! /bin/sh -e
prio=1046
update-alternatives --quiet \
    --install /usr/bin/java java /usr/bin/gij-4.6 $prio \
    --slave /usr/share/man/man1/java.1.gz java.1.gz /usr/share/man/man1/gij-4.6.1.gz
update-alternatives --quiet \
    --install /usr/bin/rmiregistry rmiregistry /usr/bin/grmiregistry-4.6 $prio \
    --slave /usr/share/man/man1/rmiregistry.1.gz rmiregistry.1.gz /usr/share/man/man1/grmiregistry-4.6.1.gz
update-alternatives --quiet \
    --install /usr/bin/keytool keytool /usr/bin/gkeytool-4.6 $prio \
    --slave /usr/share/man/man1/keytool.1.gz keytool.1.gz /usr/share/man/man1/gkeytool-4.6.1.gz
update-alternatives --quiet \
    --install /usr/bin/orbd orbd /usr/bin/gorbd-4.6 $prio \
    --slave /usr/share/man/man1/orbd.1.gz orbd.1.gz /usr/share/man/man1/gorbd-4.6.1.gz
update-alternatives --quiet \
    --install /usr/bin/rmid rmid /usr/bin/grmid-4.6 $prio \
    --slave /usr/share/man/man1/rmid.1.gz rmid.1.gz /usr/share/man/man1/grmid-4.6.1.gz
case "$1" in
configure)
    if [ ! -f /var/lib/gcj-4.6/classmap.db ]; then
	uname=$(uname -m)
	mkdir -p /var/lib/gcj-4.6
	if gcj-dbtool-4.6 -n /var/lib/gcj-4.6/classmap.db; then
	    case "$uname" in arm*|m68k|parisc*)
	        echo >&2 "gcj-dbtool succeeded unexpectedly"
	    esac
	else
	    case "$uname" in
	    arm*|m68k|parisc*)
	        echo >&2 "ERROR: gcj-dbtool did fail; known problem on $uname";;
	    *)
		exit 2
	    esac
	    touch /var/lib/gcj-4.6/classmap.db
	fi
    fi
esac
exit 0
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