The main public repository for Java-ecosystem packages is Maven Central. I discovered that when you installed Java packages using a tool like maven or ant, they were served unencrypted over HTTP, without any sort of cryptographic verification of their contents. Anyone who has control over a wifi router could trick Java developers into downloading compromised Jars and run arbitrary code on their systems.
I tried asking the company that runs Maven Central nicely to change this, but they didn’t budge. So, to prove a point, I wrote dilettante, a man-in-the-middle proxy that would intercept Jars as they are being downloaded and inject cat pictures into them.
It worked, and Maven Central now serves all Jars over SSL by default!