Heroku workshop - Java

Reading Configuration Variables from Code

Now you have seen how to set configuration variables, you need to know how to access them from your application.

In Java its a simple call to System.getenv to return the value. This returns a string so you man need to convert it to an Integer value:


Take a look at the sample Java app. In the src/main/java/Main.java file you can see how the server is bound to the value

public static void main(String[] args) throws Exception{
      Server server = new Server(Integer.valueOf(System.getenv("PORT")));
      ServletContextHandler context = new ServletContextHandler(ServletContextHandler.SESSIONS);
      context.addServlet(new ServletHolder(new Main()),"/*");

Add a configuration variable for energy conversion and update the code to use that variable

First set the configuration variable on your Heroku app:

heroku config:set ENERGY="20 GeV"

We are going to use a library to calculate the value of energy, so edit the pom.xml file and add the following dependency


Now edit the src/main/java/Main.java file. First lets add some libraries

import static javax.measure.unit.SI.KILOGRAM;
    import javax.measure.quantity.Mass;
    import org.jscience.physics.model.RelativisticModel;
    import org.jscience.physics.amount.Amount;

Still in the src/main/java/Main.java file, edit the showHome method to calculate E=mc^2 using the value from the ENERGY environment variable

private void showHome(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException {

  System.out.println("Calculating with Einstein");   

  // Energy is compatible with mass (E=mc2)

  String energy = System.getenv().get("ENERGY");

  Amount mass = Amount.valueOf(energy).to(KILOGRAM);
  resp.getWriter().print("E=mc^2: " + energy + " = " + mass);  

Add the changes to your local repository and push those changes to Heroku.

git add pom.xml src/main/java/Main.java 
    git commit -m "using configuration variable to calculate energy"
    git push heroku master

Application - calculating energy


All the changes above are also in a branch of the Git repository you initially cloned. So, as an alternative to the above, you can checkout the branch reading-configuration-variables and merge it into the master branch. Then push the change merged into master to Heroku.

git checkout reading-configuration-variables
git checkout master
git merge reading-configuration-variables
git push heroku master