Java system properties and environment variables

• System properties are set on the Java command line using the -Dpropertyname=value syntax. They can also be added at runtime using System.setProperty(String key, String value) or via the various System.getProperties().load() methods.
  To get a specific system property you can use System.getProperty(String key) or System.getProperty(String key, String def).

• Environment variables are set in the OS, e.g. in Linux export HOME=/Users/myusername or on Windows SET WINDIR=C:\Windows etc, and, unlike properties, may not be set at runtime.
  To get a specific environment variable you can use System.getenv(String name).