Javasm & JAL

If you can't understand it, you don't belong here - Mike Gancarz

Javasm is the IntelliJ IDEA plugin for the **JAL (JVM Assembly Language) ** — a custom-designed assembly language for the Java Virtual Machine.
It makes exploring the internals of the JVM not just possible, but genuinely fun.

JAL is a fresh take on Java assembly — a spiritual successor to projects like Jasmin, but with modern features and design.

Unlike Jasmin, which stopped development around 2022 and only supports outdated Java versions, JAL supports modern JVM features, including automatic generation of StackMapFrames, which are mandatory from Java 1.6 and up. Jasmin users may remember VerifyErrors haunting their dreams — JAL eliminates them.

• Named local variables
  Instead of remembering slot numbers, you can name your variables:

  istore 0 [->example]  
  iload example
  

• Structured exception handling with labels
  Try-catch-finally blocks are declared with jump labels:

  tryStart: [~tryEnd, java/lang/Exception: catchStart, java/lang/Error: catchStart2 ->finallyStart]  
  tryEnd:  
    
  catchStart:  
  catchStart2:  
  finallyStart:
  

• Readable member references
  Method and field calls separate class and member names for clarity:

  invokevirtual java/io/PrintStream->println(Ljava/lang/String;)V
  

public class HelloWorld {
    public static main([Ljava/lang/String;)

    V {
        // Print "Hello, World!"
        getstatic java/lang / System -> out:Ljava / io / PrintStream;
        ldc "Hello, World!"
        invokevirtual java/io / PrintStream -> println(Ljava / lang / String;)V

        // Return from main
        return
    }
}

public class FizzBuzz {
    public static main([Ljava/lang/String;)

    V {
        // int i = 1;
        iconst_1
                istore_1  // Store the value of i in local variable 1

        LoopStart:
        // if (i >= 101) goto LoopEnd;
        iload_1
        bipush 101
        if_icmpge LoopEnd

        // if (i % 15 = 0)
        iload_1
        bipush 15
        irem
        ifne NotFizzBuzz

        // System.out.println("FizzBuzz");
        getstatic java/lang / System -> out:Ljava / io / PrintStream;
        ldc "FizzBuzz"
        invokevirtual java/io / PrintStream -> println(Ljava / lang / String;)V
    goto LoopIncrement

        NotFizzBuzz:
        // else if (i % 3 == 0)
        iload_1
                iconst_3
        irem
        ifne NotFizz

        // System.out.println("Fizz");
        getstatic java/lang / System -> out:Ljava / io / PrintStream;
        ldc "Fizz"
        invokevirtual java/io / PrintStream -> println(Ljava / lang / String;)V
    goto LoopIncrement

        NotFizz:
        // else if (i % 5 == 0)
        iload_1
                iconst_5
        irem
        ifne PrintNumber

        // System.out.println("Buzz");
        getstatic java/lang / System -> out:Ljava / io / PrintStream;
        ldc "Buzz"
        invokevirtual java/io / PrintStream -> println(Ljava / lang / String;)V
    goto LoopIncrement

        PrintNumber:
        getstatic java/lang / System -> out:Ljava / io / PrintStream;
        iload_1
        invokevirtual java/io / PrintStream -> println(I) V

        LoopIncrement:
        // i++
        iinc 1 1
    goto LoopStart

        LoopEnd:
        // return;
        return
    }
}

One of the biggest pain points in JVM bytecode writing is StackMapFrame management.
JAL's compiler calculates and inserts StackMapFrames automatically — no more manual frame declaration, no more hair-pulling VerifyErrors.

Javasm supercharges your JAL development inside IntelliJ IDEA:

• ✅ Instruction name completion
  Start typing and get autocompletion for all instructions.

• 📄 Hover documentation
  Hover over instructions to see inline documentation.

• 🌀 Label navigation
  Ctrl+Click a label to jump to its declaration.

• 🐞 Debugger integration
  Full integration with IntelliJ's standard JVM debugger (JDWP).
  Breakpoints, step-over, step-into — all supported.

• 📊 Frame and Stack Viewer
  A custom tool window that shows:

  • Stack state at the selected instruction
  • Local variable states
  • Visualises the stack during live debugging sessions

This plugin contains JAL implementation V1.3.1!

No manual build needed — just plug and play.

MIT Licence. See the LICENCE file for details.

❤️ A project by someone who enjoys poking around the JVM bytecode guts.