The URL Looks Wrong! Who's the Culprit, Java or Spring?

While processing message landing URLs in Kakao Pay's Notification Feed service, an incident occurred where the host information of a specific URL disappeared. This began when logic was added to remove a specific parameter from the URL in order to make the landing behavior of the OS notification center and the notification feed different.

The cause of the incident was a behavioral mismatch between the java.net.URI class and Spring's UriComponentsBuilder class. When using the UriComponentsBuilder.fromUri(URI(this)) approach, it was found that during the conversion of a specific URL format (e.g., kakaopay://payweb_tab) into a java.net.URI object, the host information was processed as null.

As a result, UriComponentsBuilder received incorrect information from the URI object and generated a response missing key elements of the URL. To resolve this, the logic was improved to use the fromUriString method, which handles the string directly without going through a URI object in between.