I have a Maven project. After I make changes to the project, I package the project up into an Uber Jar file and upload that to my backups. I am wondering, if my computer were to break or I lose all the code to the project, can I get the project back using only the Uber Jar file or would the project be gone forever?
How can I open this Jar file as a project and view all the Java code inside of it?
It would be entirely gone; that jar file contains only class files, not your source files, and you can't 'recover' source files from class files (you can decompile them which is mostly useless for this purpose; all comments, most names, most structure - all gone. You can't feasibly continue your project with this).
As it isn't in there, it's not possible to 'open a jar file and see all java code inside it'.
You'll need to set up backups.
The proper way forward is to first set up version control; this ensures that you don't just have a backup of the latest state of your source files, but every state of it - you can travel back to any time. Protects against code you deleted by accident, and means you can freely remove code you think you no longer need without that nagging feeling of: Yeah but what if.... maybe later? - so stop commenting out stuff you don't need, just get rid of it. It also means if there's a bug, you can time travel to the exact point in time when you added the line, and you can review the changes made by others in a project (presumably you're writing this on your own for now, but at some point you'll work with more people than just yourself!)
Read up on git which is the version control system that 80%+ or so of the community uses, probably best not to spring for exotic options if you aren't familiar. There are a billion tutorials out there to find if you search the web.
Host your git on a site like github or bitbucket which therefore also takes care of backups. Alternatively, host it on your own server (it's not complicated; git is open source). If you can't do that either, just let git write to a local directory and then use e.g. backblaze or something similar to ensure that file is backed up.