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procyon
Procyon is a suite of Java metaprogramming tools, including a rich reflection API, a LINQ-inspired expression tree API for runtime code generation, and a Java decompiler.
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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
I suspect you're thinking of a decompiler like the one that's part of Procyon (https://github.com/mstrobel/procyon/wiki/Java-Decompiler). As bitnotno observed, the result may not be especially helpful, and - especially if things are well documented - rewriting the code may well result in additional benefits as far as improving code structure and readability are concerned.