GoCD recently added support for notification extension point. I've started building slack notification plugin (its a WIP here). As part of that I wrote a Java client for Slack Webhooks. Although I found a java library here, which said it was published but I couldn't find it anywhere on sonatype / maven central. I can't even publish it, so I took that as an inspiration and wrote my own implementation on https://github.com/ashwanthkumar/slack-java-webhook.
(webhookUrl) .icon( ) .sendToUser( ) .displayName( ) .push( ( ) bold( ));
It gets posted in the slack channel like below
libraryDependencies += "in.ashwanthkumar" % "slack-java-webhook" % "0.0.3"