How to read an environment variable in Quarkus

I'm trying to read an environment variable in my Quarkus application using @ConfigProperty, but didn't manage to do that. I thought that just specifying variable name should be enough, but it isn't working:

@ConfigProperty(name = "MY_ENV_VAR")
String envVar;

I've also tried to specify a property in application.properties

my.var=${MY_ENV_VAR:defaultValue}

and read it like

@ConfigProperty(name = "my.var")
String myVar;

but the variable is always null.

Did I miss something? What is the correct way to read an environment variable in Quarkus?
I'm using Quarkus 1.2.0.Final

We've found the source of the problem. I was trying to read a field annotated with @ConfigProperty in the constructor, but the properties are being injected only after calling a constructor.

A working example:

package org.acme.config;

import org.eclipse.microprofile.config.inject.ConfigProperty;

import javax.enterprise.context.ApplicationScoped;

@ApplicationScoped
public class EnvGreetingService {

    private final String varA;

    @ConfigProperty(name = "ENV_VAR_B")
    String varB;

    public EnvGreetingService(@ConfigProperty(name = "ENV_VAR_A") String varA) {
        System.out.println("constructor varA=" + varA);
        System.out.println("constructor varB=" + varB);
        this.varA = varA;
    }

    public void greetWithEnvVar() {
        System.out.println("method varA=" + varA);
        System.out.println("method varB=" + varB);
    }
}

Note: it is not required to map environment variables in application.properties

And console output would be:

constructor varA=a
constructor varB=null
method varA=a
method varB=b

So, if you want to use application properties or environment variables in the constructor, you should inject it in constructor arguments.