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How to Serialize classes, then read them with a modified version of that same class in Java

I am developing a Minecraft plugin which uses a class that I made called customPlayer. When I save the plugin data from a running instance, I put all of these objects into a HashMap<String,customPlayer> and save them with ObjectOutputStream. Loading these classes back into the same version of the plugin works great, but my problem arises when I modify the class and try to read the object using that modified class (usually associated with a new version of my plugin).

I thought about it for a bit, and thought I came up with a clever solution. My idea was to just include the old class files as an External Library inside the new version of the plugin, cross my fingers and hope it worked. It didn’t.

Is there a better way to do this? I’m new to serialization and this kind of stuff, so any suggestions would be greatly appreciated. Below I will include a few Screenshots of the customPlayer class and the crash log of the server. Ideally any solution that is presented should be able to be used easily with future modifications to the class (Updates to the Jar downloaded Via a Github repo).

Instance Variables and Constructor of



Is there a better way to do this?

There certainly is. Stop using Serialization and ObjectOutputStream. These classes are a disaster (even OpenJDK core team effectively agrees with this assessment). The output they generate is not particularly efficient (it’s more bytes than is needed), it is not human readable, nor (easily) read by anything except java code, and it results in such hairy situations as you ran into.

Instead use e.g. Jackson to turn your objects into JSON, or use google’s protobuf to turn it into efficient binary blobs.

You can read this JSON or these binary blobs in any language you want and you’ll have your pick of the litter as far as libraries go. You will need to write some explicit code to ‘save’ an object (turn it into JSON / protobuf), and to ‘read’ one, but now you are free to change your code.

If you insist on continuing with serialization, you need to add a field named serialVersionUID, and set up readObject and writeObject. it’s convoluted rocket science that’s hard to get right. The details are in the javadoc of

Do yourself a favour though. Don’t do it.