How is obfuscation done in Java?

Question

Today I came across an obfuscated class (well a lot of obfuscated classes in a jar) and I do not have a clue on how this kind of obfuscation is done.

An example:

  protected void a(ChannelHandlerContext ☃, ByteBuf ☃, ByteBuf ☃)
    throws Exception
  {
    int ☃ = ☃.readableBytes();
    if (☃ < this.c)
    {
      ☃.b(0);
      ☃.writeBytes(☃);
    }
    else
    {
      byte[] ☃ = new byte[☃];
      ☃.readBytes(☃);

      ☃.b(☃.length);

      this.b.setInput(☃, 0, ☃);
      this.b.finish();
      while (!this.b.finished())
      {
        int ☃ = this.b.deflate(this.a);
        ☃.writeBytes(this.a, 0, ☃);
      }
      this.b.reset();
    }
  }

}

As you see above, all the parameter variables are a snow-man. How can this be undone? Also how is it done in the first place; how is the JVM able to "process" those and execute the code without any problem?

To clarify, I am not going to use this code, it is just for educational purposes. I am taking the Computer Science course at school so since we are learning Java and talking of limitations such as decompilations. I am interested in learning more, so I decided to have a look into bigger projects especially servers. This piece of code is pulled out of the Spigot server for Minecraft (A game) that is a fork of Bukkit server for Minecraft that was supposed to be open source.

Answer 1

First of all, you should note that it is the parameters which have this unicode and not the methods. Why is this important? Parameters do not need to have names specified, as they are mostly indexed by a number reference. However it can be specified and I assume that most java runtimes do in fact not check this name as it is not needed for execution. In the opposite, class names, method names, and field names are however needed.

About you mentioning Spigot, Spigot is indeed open source. However you most likely decompiled a class which is originally from the original Mojang Minecraft server, which is not open source and is indeed obfuscated.

Edit: In the case you want to investigate these classes, I recently found a tool called Bytecode Viewer, which is available at https://github.com/Konloch/bytecode-viewer This tool has multiple decompilers as well as some options to view a more bytecode like version of the class file. An example of a function I found contains the following bytecode data:

     <localVar:index=1 , name=☃ , desc=D, sig=null, start=L1, end=L2>
     <localVar:index=3 , name=☃ , desc=D, sig=null, start=L1, end=L2>
     <localVar:index=5 , name=☃ , desc=D, sig=null, start=L1, end=L2>

Indeed as is visible, the unicode name has been set the same, but it does not matter as in the end the indexes (1,3,5) are used to reference these variables.

Answer 2

protected void a(ChannelHandlerContext ☃, ByteBuf ☃, ByteBuf ☃)

This isn't valid. You cannot have multiple parameters with the same name. It could be that you are not reading the unicode text with the right text format.