Comprehensive Bytecode Basics

To put bytecode simply, it is a set of instructions that can do two different actions: push or pull.

Here is a very basic piece of bytecode with its Java counterpart:

The different between a pushing or pulling instruction is simple:

It may be simpler to sort by pulling instructions and indent what it is pulling, as it will look more readable to a human. (Hint: basic Abstract Syntax Tree)

Reading through it will give you a better sense of it, but as a very small rundown, here are some:

You can use this concept to create a very basic Abstract Syntax Tree. Putting this simple concept into a recursive pattern adding to a tree would yield code similar to the indented instructions above.

Here is a very basic piece of code that requires a recursive structure:

So let’s disect this using the JVM Specification and write down what we know:

Now we can use this to create our more human-readable structure! Remember to start at the “lowest” pulling instruction. Every time you hit a pulling instruction, to pull data recursively.

After this, you now have a queryable structure that can be used for analysis. You now also have a bit more of an understanding of the basics of bytecode!

Let me know if there’s anything else you’d like to see covered!

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