There is *plenty* Java-based open source software out there. Many of the Apache projects are java-based, for example:
Here's another vast amount of Java-based open source software:
Anyway, I suspect that pointing you to any number of Java based free software projects won't help convince you, but I thought I'd give it another shot.
Note that I'm not even a Java programmer myself (Python is my language of choice). But some of these Java-based projects have plenty of visibility.
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