Spring component scan filter
I currently live in jar/dependency hell. At one point there was about 5 jars for everything and statements like "We need to break things apart" swung the pendulum all the way to the other side. Left or the right, you pick :). Recently I was working on a project where we have old spring xml configuration meshed with new java config. There was some conflicting configuration classes so I needed to use a filter to customize scanning.