Specialization of MapPropertySource designed for use with system environment variables. Compensates for constraints in Bash and other shells that do not allow for variables containing the period character and/or hyphen character; also allows for uppercase variations on property names for more idiomatic shell use.

For example, a call to getProperty("foo.bar") will attempt to find a value for the original property or any 'equivalent' property, returning the first found:

• foo.bar - the original name
• foo_bar - with underscores for periods (if any)
• FOO.BAR - original, with upper case
• FOO_BAR - with underscores and upper case

Any hyphen variant of the above would work as well, or even mix dot/hyphen variants.

The same applies for calls to containsProperty(String), which returns true if any of the above properties are present, otherwise false.

This feature is particularly useful when specifying active or default profiles as environment variables. The following is not allowable under Bash:

spring.profiles.active=p1 java -classpath ... MyApp

However, the following syntax is permitted and is also more conventional:

SPRING_PROFILES_ACTIVE=p1 java -classpath ... MyApp

Enable debug- or trace-level logging for this class (or package) for messages explaining when these 'property name resolutions' occur.

This property source is included by default in StandardEnvironment and all its subclasses.