This app was mentioned in 8 comments, with an average of 3.88 upvotes
The app isn't obfuscated at all and can easily be decompiled using for example show java. There's nothing in there of relevance apart from some of the worst code I've seen, together with ads.
If you want to decompile an app that's installed on your device, using this app is even easier. It decompiles it right on the device, so you don't need to copy the APK to a computer first.
Pretty straightforward, you can even do it directly on android. It might barf depending on whether or not an obfuscator is used and how much it is obfuscated. You might have to switch to a desktop and a proper dex decompiler to get better results.
I don't ever have to use .jar files. As in I never do. So I don't think I could be much of help. However, you could try something like this. But other than at, I don't know. I personally can't help you much. I do hope what I fo have to offer atleast somewhat helps. I know I don't know enough to fully help. I do hope someone who does know how to help you better than me does find your question tho.
I think i got it to work. Sure it can screw your device up but only if you don't look into it. That is true.. But there is an app that makes you see the source code if you want called Show Java. I know what you mean. I am very wary when it comes to adding stuff on my daily driver.
Also you should check this out: https://play.google.com/store/apps/details?id=com.njlabs.showjava&hl=en
There's also this app to show the source code of most apps.
https://play.google.com/store/apps/details?id=com.njlabs.showjava
I used Show Java with the JaDX compiler.
This app might help https://play.google.com/store/apps/details?id=com.njlabs.showjava
You can get your apk and use a decompiler. If you used pro-guard to compile the apk, them good luck. It is very hard to understand decompiled code which was optimized with pro-guard. If you lost the certificarte to sign the apk then it doesnt matter you can recover the code because you wont be able to submit anything to the PlayStore
Edit: a decompiler wont give you the gradle files as they are the build system but it can give you all the source code