Sometimes, for one reason or another, you might want to take a look at Minecraft's source code to get a better understanding of the game's inner workings. This is a guide on how to accomplish just that.

To start, you need to obtain the game JAR as well as the deobfuscation mappings. I've compiled links to all of these resources here.

We need the deobfuscation mappings because Minecraft's source code is obfuscated, meaning that the names of classes and methods are set to random, meaningless strings. This was originally done to prevent reverse engineering of the game, but even though it no longer serves this purpose it is still done.

The first thing we're gonna do is rename all the names in the game JAR using the obfuscation mappings. This can be accomplished using a tool called SpecialSource, which can be obtained through Maven. Here's the command to remap the client:

java -jar SpecialSource-1.11.0-shaded.jar \
    -i client.jar \
    -o client-remmaped.jar \
    -m client.txt \
    --kill-lvt

The --kill-lvt flag tells SpecialSource to ignore local variables. We include it since Mojang's official mappings do not include local variable names. If you are on Windows, type this command on one line without the backslashes.

Decompiling the server requires an extra step, since the server.jar you download is actually a launcher program that downloads the real server JAR along with the libraries it needs to run. When you start the server, it will create a folder called versions; the JAR that you want to decompile will be stored here.

Once the JAR is remapped, it's ready to be decompiled. There are a number of decompilers that will all work fine, but my preference is Vineflower. All you need to do is download the latest release, put the decompiler JAR in the same folder as the remapped JAR, and begin the decompilation:

java -jar vineflower-1.10.1.jar <remapped JAR> src/

Once the decompiler finishes, you're all set!

Epilogue §

If you just want one tool to do all of these steps, check out DecompilerMC, which will fetch all the tools and files necessary to decompile any version of Minecraft with released mappings automatically.

For versions 1.12 and below, ModCoderPack can be used to deobfuscate Minecraft with community-submitted mappings. Similarly, MCPConfig has mappings for 1.12 up to 1.19.