Support for environment variables with square brackets

[jrglee]

The ability to define keys in an array format like service.server[0]=http://server1 is is great when used in conjunction with @ConfigurationProperties.

Unfortunately the square brackets are considered special characters for environment variable names.

• When using Docker, I can use the -e flag to define SERVICE_SERVER[0]=http://server1 and it works
• When using Bash's export like export SERVICE_SERVER[0]=http://server1 I get a not a valid identifier error
• In Kubernetes I can only set environment variables in the C_IDENTIFIER format with is the regexp [A-Za-z_][A-Za-z0-9_]*. There is a related discussion about dashes vs underscores Cannot use dash in environment variable names in a pod definition kubernetes/kubernetes#16863

I chatted with @philwebb yesterday about this issue. While I can find other ways to pass those values to the application, i.e.: use an environment variable to define application args and let the entrypoint in Docker take care of it; it would be great if Spring could handle that out-of-the-box. One idea I discussed with Phil is to use double _ around the index like SERVICE_SERVER__0__.

[philwebb]

I think this is something we should consider around 2.0 when we try to rationalize relaxed binding.

[mbhave]

The new binder supports this by allowing environment variables to be specified using _ for indexes. For eg: HOST_0 is equivalent to host[0].

[mikeloll]

For anyone interested, here are the release notes for the new data binding:

https://github.com/spring-projects/spring-boot/wiki/Relaxed-Binding-2.0