There are a couple of changes between stable and testing (libgif4 and tzdata-java) which will make it very hard to use the stable versions of openjdk-7-jre.
Instead, since you're running testing, you should use the version currently available in experimental; add
deb http://ftp.caliu.cat/debian/ experimental main
deb-src http://ftp.caliu.cat/debian/ experimental main
to your repositories, apt-get update, and then you should be able to install openjdk-7-jre:i386.
The actual answer by @cas is good but have some corrections to be applied.
So let's take a fresh installation of Debian 9 and assuming that the contrib non-free repositories are also not enabled.
Step 0 - Enable the contrib non-free repositories
I used sed to find and replace the word main inside /etc/apt/sources.list
sed -i 's/main/main contrib non-free/g' /etc/apt/sources.list
apt-get update
Step 1 - ZFS Installation
Since the last fixes spl-dkms is correctly seen as zfs-dkms dependency so it's recalled automatically and it's not necessary to install it manually before zfs-dkms. The symbolic link is needed due to a bug inside the zfs distribution in Debian, that doesn't look for rm binary in the right position.
apt -y install linux-headers-$(uname -r)
ln -s /bin/rm /usr/bin/rm
apt-get -y install zfs-dkms
Step 2 - ZFS Restart
At this point zfs-dkms is installed but it throws errors in journalctl -xe; to start zfs properly use:
/sbin/modprobe zfs
systemctl restart zfs-import-cache
systemctl restart zfs-import-scan
systemctl restart zfs-mount
systemctl restart zfs-share
Step 3 - YOU MUST CREATE AT LEAST ONE ZPOOL
At this point I discovered that YOU must create a zpool before reboot otherwise zfs will not load the proper modules if there are no zpools. It's a sort of saving resources mechanism ( but even in that case this will still throw errors inside journalctl -xe )
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=864348
" We are not doing this because ZFS modules would taint the kernel, if
there's no zpool available then it shouldn't be loaded. "
If you miss this part you have to start from Step 2
For example, by using the example provided by @cas, you can create this file based zpool or directly create your disk based ones.
truncate -s 100M /root/z1
truncate -s 100M /root/z2
zpool create tank /root/z1 /root/z2
zpool scrub tank
zpool status
then after a reboot everything will work with no errors in journalctl -xe
Best Answer
I got it working (very unsoundly) by adding jessie repo and then installing from it. Include in /etc/apt/sources.list:
Do apt-get update && apt-get install g++-4.9
After installation comment out jessie lines so that it doesn't conflict later with stretch. It would have been much better if Debian had included other g++ versions. Strangely enough, we have many versions of gcc bundled but only one version of g++ (6.0).