Deploy Spring Boot applications on Heroku

1. Download Heroku CLI: https://devcenter.heroku.com/articles/deploying-spring-boot-apps-to-heroku
2. Commands (without the preceding $ sign):
   $ heroku login
$ git init
$ git add .
$ git commit -m "Initial commit"
$ heroku create <app-name>
$ git push heroku master
$ heroku open

Now the website should work if you do not have any databases in it.

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Enable MySQL Database:

1. Add Postgres database addon with this command: $ heroku addons:create heroku-postgresql
2. Add this dependency in the maven pom file (used for Postgres database addon that will enable our database to work):

<dependency>
    <groupId>org.postgresql</groupId>
    <artifactId>postgresql</artifactId>
    <version>9.4-1201-jdbc4</version>
</dependency>

3. Then edit the connection attributes in application.properties file to contain:

spring.datasource.url=${JDBC_DATABASE_URL}
spring.datasource.username=${JDBC_DATABASE_USERNAME}
spring.datasource.password=${JDBC_DATABASE_PASSWORD}
spring.jpa.show-sql=false
spring.jpa.generate-ddl=true
spring.jpa.hibernate.ddl-auto=create

Important note:
It's important that at first time you leave this last line and set it create so that your database gets created. After that change it to update and commit and push once more. (i.e. $ git add . , $ git commit -m "Edit application.properties" & $ git push heroku master).

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This tutorial is based on the following two articles (and others):

https://devcenter.heroku.com/articles/deploying-spring-boot-apps-to-heroku
https://devcenter.heroku.com/articles/connecting-to-relational-databases-on-heroku-with-java