Serve Index File Instead of Directory Listing if Available

[maltejur]

This will serve a index file (for example index.html) instead of a directory listing, if that index file is available. That index file can be customized, for example:

auto server = HttpServer.builder()
      .setListener(8080)
      .resource("/", "./resources", "custom_index.html")

This behaviour can also be completely disabled again by passing null to indexFile:

auto server = HttpServer.builder()
      .setListener(8080)
      .resource("/", "./resources", null)