Example usage. Suppose you have an application.properties file in your working directory that contains
abc.def=ghi
foo.bar=123
and an environment variable FOO_BAR with value 456, then you can execute the following:
SimpleConfig simpleConfig = new SimpleConfig()
.load("application.properties")
.load(System.getenv())
;
simpleConfig.get("abc.def"); // returns "ghi"
simpleConfig.get("foo.bar"); // returns "456", overridden by env variable
Then suppose you have secrets in a secrets service like Hashicorp Vault. You want to retrieve secrets from Vault and add them to the set of properties; but in order to do so, you want to configure the url of Vault, and be able to configure it statically in a properties file, and override it with an environment value.
Now you're in a bind: you need to use the configuration to obtain further configuration. We got you covered: all you have to do is implement a function that will take the existing configuration, and use it to retrieve further configuration. I call it "refining" the configuration.
Function<ConfigSource, Map<String, String>> refinement = existingConfig -> ... use existingConfig to access e.g. Vault;
simpleConfig.load(refinement);