Simple example connecting to Mysql and Postgres databases in Java

Uses Environment variables for the username and password. Useful for container like environments.

Setup

1. Need to download the mysql java JDBC connector if not got already: https://dev.mysql.com/downloads/connector/j/ https://jdbc.postgresql.org/download.html
2. Then add the java JDBC connector to the class path e.g. :

Postgresql example

 export CLASSPATH=./postgresql-42.2.5.jar:$CLASSPATH
 # Set user and password
 export DB_USER=root
 export DB_PASS=password
 # Then you can run:
 javac Postgres.java
 java Postgres

Mysql example

 export CLASSPATH=/usr/share/java/mysql-connector-java-8.0.15.jar:$CLASSPATH
 # Or copy /mysql-connector-java to current directory
 export CLASSPATH=./mysql-connector-java-8.0.15.jar:$CLASSPATH 

 # Set user and password
 export DB_USER=root
 export DB_PASS=password
 # Then you can run:
 javac Mysql.java
 java Mysql

Docker run

Spin up docker container, connect to postgres database using environment vars and then drop to a sh shell.

 docker build -t my-java-app .
 docker run -it --rm --name my-running-app myjava
 docker run --network="host" --env DB_USER=postgres --env DB_PASS=password -it --rm --name my-running-app myjava

Kubernetes

Install MicroK8s

MicroK8s is a snap that you can run locally to simulate a kubernetes environment on your laptop.

snap install microk8s --classic --channel=1.12/stable

You can read more on the MicroK8s release channels in the Release Channels and Upgrades doc.

At any point you can check MicroK8s' availability with:

microk8s.status

During installation you can use the --wait-ready flag to wait for the kubernetes services to initialise:

microk8s.status --wait-ready

In order to install MicroK8s make sure

• port 8080 is not used and
• if you have AppArmor enabled (check with sudo apparmor_status) you do not have any other dockerd installed. You can use the dockerd coming with MicroK8s.

microk8s.enable dns dashboard

Run on kubernetes

The following will deploy to Kubernetes.

Build the image and push to local docker registry:

microk8s.docker build -t myjava .
microk8s.docker tag myjava localhost:32000/myjava
microk8s.docker push localhost:32000/myjava

Deploy the app to kubernetes

microk8s.kubectl create -f k8.yaml

Note that the default Postgres.java file referes to "postgres-service" as the host for the postgres instance. This is refering to the postgres-service defines in k8.yaml. This resolves to the postgres instance within a kubernetes pod (an nslookup postgres-service resolves to the ip of the postgres container.

View kubernetes pod->container logs

First get pod name:

microk8s.kubectl get pods

View the logs (e.g. "org.postgresql.util.PSQLException: FATAL: database "fred_db" does not exist"):

microk8s.kubectl logs <pod-name>

If you see that error, your java app has connected to Postgres correctly using the secrets, and via the postgres service to the running database.

To delete your kubernetes deplpoyment, manually or run the following file:

./delete.sh # from this repo