Spring allows to you, to fall-back on system properties, if you not added any corresponding values with that.
That is really a cool feature. If you need to look up,some system properties , you don’t need to do any thing
just configure your bean and Spring will take care that.
These are the some of SYSTEM_PROPTTIRES that you set in your bean
SYSTEM_PROPERTIES_MODE_FALLBACK Check system properties if not resolvable in the specified properties.
SYSTEM_PROPERTIES_MODE_NEVER Never check system properties.
SYSTEM_PROPERTIES_MODE_OVERRIDE Check system properties first, before trying the specified properties.
see here for more information PropertyPlaceholderConfigurer