title: How to use system properties to substitute placeholders in Typesafe Config file?
body: <p>I need to refer to <strong>java.io.tmpdir</strong> in my application.conf file</p>
  
  <p>I printed content of my config with </p>
  
  <pre><code>val c = ConfigFactory.load()
  System.err.println(c.root().render())
  </code></pre>
  
  <p>and it renders it like</p>
  
  <pre><code># dev/application.conf: 1
  "myapp" : {
      # dev/application.conf: 47
      "db" : {
          # dev/application.conf: 49
          "driver" : "org.h2.Driver",
          # dev/application.conf: 48
          "url" : "jdbc:h2:file:${java.io.tmpdir}/db;DB_CLOSE_DELAY=-1"
      }
   ...
   }
  # system properties
  "java" : {
      # system properties
      "io" : {
          # system properties
          "tmpdir" : "/tmp"
      },
  ....
  </code></pre>
  
  <p>So I guess that forward-reference does not work. Is there any way to get my options loaded after system properties, so config parser will correctly substitute values?</p>