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HOCON conditional substitution with surrounding text

I have no way currently to test this syntax myself, thus the question:

Will substitution/overriding of a value still happen if it's surrounded by text?

For example, I know that:

foo.baseUrl = "http://foo:1234/" foo.baseUrl = ${?FOO_BASE_URL}

will overwrite foo.baseURL if the ENV var FOO_BASE_URL exists.

What happens here?

foo.baseUrl = "http://foo:1234/" foo.baseUrl = "http://${?FOO_BASE_URL}:1234/"

does the override still happen if the ENV var exists? does putting it within a string negate this?

Solution

  • Just ran a quick test, and the second value will replace the first, missing the middle part. So if FOO_BASE_URL is not defined, and application.conf contains the following (note omission of " in the second line, variable substitution is not performed within double quotes):

    foo.baseUrl = "http://foo:1234/"
foo.baseUrl = http://${?FOO_BASE_URL}:1234/
foo.baseUrl = ${?FOO_BASE_URL}

    The value of foo.baseUrl will be http://:1234/. The not-found variable is considered an empty string in the second line. The relevant quote from the Substitution section of the docs:

    If a substitution with the ${?foo} syntax is undefined:

    • if it is the value of an object field then the field should not be created. If the field would have overridden a previously-set value for the same field, then the previous value remains.
    • if it is an array element then the element should not be added.
    • if it is part of a value concatenation with another string then it should become an empty string; if part of a value concatenation with an object or array it should become an empty object or array.
    • foo : ${?bar} would avoid creating field foo if bar is undefined. foo : ${?bar}${?baz} would also avoid creating the field if both bar and baz are undefined.