Passing environment variables through jar file which app uses - jar

Question

I am currently trying out on the docker link between my app and db containers. I've checked on my app container and environment variables are automatically set when I link the containers together.
What I want to do is for my config file, which is packaged into a jar file, to receive the environment variables and set the required values to it. Any advice or help?
And this is how I create a config file in my jar file to connect to MySQL
database { url="jdbc:mysql://${MYSQL_PORT_3306_TCP_ADDR}:${MYSQL_PORT_3306_TCP_PORT}/mydb" driver="com.mysql.jdbc.Driver"}

Answer 1

Updating the config file inside the jar could be quite overkill.
It think you have several choices
read the config environment variable directly in you program
use variable either directly or generate the config file there
create launch script (details of this depends of you guest os in docker how to do it; sh/bash for linux etc..)
that script can generate new config file from environment and put it on classpath before jar so you program sees it.
EDIT: added example
You can save this kind of launcher script on docker image which dynamically creates configuration before launching actual program.
#!/bin/bash
# some default values for testing even without links to other container
MYSQL_PORT_3306_TCP_ADDR=${MYSQL_PORT_3306_TCP_ADDR:-127.0.0.1}
MYSQL_PORT_3306_TCP_PORT=${MYSQL_PORT_3306_TCP_PORT:-3306}
cat << EOF > /opt/yourprogram/dbconfig.conf
database { url="jdbc:mysql://${MYSQL_PORT_3306_TCP_ADDR}:${MYSQL_PORT_3306_TCP_PORT}/mydb" driver="com.mysql.jdbc.Driver"
}
EOF
scala -classpath /opt/yourprogram YourProgram

Answer 2

What I did is that I wrote the sh file in my directory /tmp/restcore-1.0-SNAPSHOT/bin like this:
#!/bin/bash echo "database{url="jdbc:mysql://"${MYSQL_PORT_3306_TCP_ADDR}":"${MYSQL_PORT_3306_TCP_PORT}"/mydb" driver="com.mysql.jdbc.Driver" }" > myconf.conf
jar uf /tmp/restcore-SNAPSHOT/lib/com.organization.restcore-1.0-SNAPSHOT.jar /tmp/restcore-1.0-SNAPSHOT/bin/myconf.conf
After building the Dockerfile and running the sh file in CMD, I use cat myconf.conf to check the config file and I'll be able to see the environment set.