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As the author has forgotten to add a detailed usage listing
No, I haven't :) See readme.txt in your jar for the full documentation.
Current source code and documentation of Fernflower is available here: https://github.com/fesh0r/fernflower
Command line options are listed there, I do not copy-paste them from github.
Online version of Fernflower is now available on this site: http://www.javadecompilers.com/
Try to invoke main class org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler manually
java -cp java-decompiler.jar org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler myclassfile.class .
Also you can find more about internal usage of Fernflower inside IDEA by query in source code https://github.com/JetBrains/intellij-community/search?utf8=%E2%9C%93&q=Fernflower
You can get it looking at JetBrains releases: https://www.jetbrains.com/intellij-repository/releases/
Download the java-decompiler-engine.jar which will actually have the version name, but for simplicity I'll save it as c:\t\dl\jde.jar on my pc.
Then this for example will decompile the jar file for the game Necesse where I have it installed on my PC. The output seems to go into a jar file with the same name in the directory specified, here creating c:\t\Necesse.jar with the .java files in it (command split into lines for readability):
java -cp c:\t\dl\jde.jar
org.jetbrains.java.decompiler.main.decompiler.ConsoleDecompiler
'c:\games\steam\steamapps\common\Necesse\Necesse.jar'
c:\t
Hello! Today I'm happy to announce the release of a project that me and my friends have been working on over the course of the last year, Quiltflower! Originally intended just for use with the QuiltMC toolchain with Minecraft, Quiltflower quickly expanded to be a general purpose java decompiler aiming to create code that is as accurate and clean as possible. If the name sounds familiar it's because Quiltflower is a fork of Fernflower, the (in)famous decompiler that was developed by Stiver, maintained by Jetbrains, and became the default decompiler in Intellij IDEA. Fernflower also quickly found its way into many other tools. After many frustrations with it myself with its decompiled code structuring and quality I decided to do something about it, and here we are! Over the past year, Quiltflower has added support for features such as modern string concatenation, a code formatter, sealed classes, pattern matching, switch expressions, try-with-resources, and more. Quiltflower also focuses on the code quality of the decompiled output, and takes readability very seriously. We'd greatly appreciate it if you'd give it a try, with our Intellij Plugin, as a standalone jar, or on our maven. While it has come a long way it's still a work in progress, and feedback can be reported on our issue tracker.
Here's a comparison of Fernflower and Quiltflower's output.
I'd also like to thank the MinecraftForge Team for creating ForgeFlower, the fork that QuiltFlower was based on, and Lee Benfield for creating CFR and it's truly incredible test suite.