Take abstract syntax tree for method type signatures and constants

• serialize it as a graph

To see it, use

javap -verbose

Cafebabe does most of these things for you

• can unzip Scala source code to see how

Documentation is in JVM Spec, Class File Format

Example

The most complex part is constant pool, which is an acyclic directed graph whose nodes have a tag and content. The tag indicates the nature of the cell, and the content can be e.g. a tuple of pointers to other cells, or a zero-terminated sequence of characters. See constant pool definition.

class Factorial {
    public int fact(int num){
	int num_aux;
	if (num < 1)
	    num_aux = 1 ;
	else 
	    num_aux = num * (this.fact(num-1));
	return num_aux;
    }
}
class Calls {
    public static void main(String[] args) {
	Factorial f = new Factorial();
	System.out.println("Result is: " + f.fact(6));
    }
}

Result of javap -v:

Compiled from "Calls.java"
class Calls extends java.lang.Object
  SourceFile: "Calls.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method	#14.#23;	//  java/lang/Object."<init>":()V
const #2 = class	#24;	//  Factorial
const #3 = Method	#2.#23;	//  Factorial."<init>":()V
const #4 = Field	#25.#26;	//  java/lang/System.out:Ljava/io/PrintStream;
const #5 = class	#27;	//  java/lang/StringBuilder
const #6 = Method	#5.#23;	//  java/lang/StringBuilder."<init>":()V
const #7 = String	#28;	//  Result is: 
const #8 = Method	#5.#29;	//  java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
const #9 = Method	#2.#30;	//  Factorial.fact:(I)I
const #10 = Method	#5.#31;	//  java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
const #11 = Method	#5.#32;	//  java/lang/StringBuilder.toString:()Ljava/lang/String;
const #12 = Method	#33.#34;	//  java/io/PrintStream.println:(Ljava/lang/String;)V
const #13 = class	#35;	//  Calls
const #14 = class	#36;	//  java/lang/Object
const #15 = Asciz	<init>;
const #16 = Asciz	()V;
const #17 = Asciz	Code;
const #18 = Asciz	LineNumberTable;
const #19 = Asciz	main;
const #20 = Asciz	([Ljava/lang/String;)V;
const #21 = Asciz	SourceFile;
const #22 = Asciz	Calls.java;
const #23 = NameAndType	#15:#16;//  "<init>":()V
const #24 = Asciz	Factorial;
const #25 = class	#37;	//  java/lang/System
const #26 = NameAndType	#38:#39;//  out:Ljava/io/PrintStream;
const #27 = Asciz	java/lang/StringBuilder;
const #28 = Asciz	Result is: ;
const #29 = NameAndType	#40:#41;//  append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
const #30 = NameAndType	#42:#43;//  fact:(I)I
const #31 = NameAndType	#40:#44;//  append:(I)Ljava/lang/StringBuilder;
const #32 = NameAndType	#45:#46;//  toString:()Ljava/lang/String;
const #33 = class	#47;	//  java/io/PrintStream
const #34 = NameAndType	#48:#49;//  println:(Ljava/lang/String;)V
const #35 = Asciz	Calls;
const #36 = Asciz	java/lang/Object;
const #37 = Asciz	java/lang/System;
const #38 = Asciz	out;
const #39 = Asciz	Ljava/io/PrintStream;;
const #40 = Asciz	append;
const #41 = Asciz	(Ljava/lang/String;)Ljava/lang/StringBuilder;;
const #42 = Asciz	fact;
const #43 = Asciz	(I)I;
const #44 = Asciz	(I)Ljava/lang/StringBuilder;;
const #45 = Asciz	toString;
const #46 = Asciz	()Ljava/lang/String;;
const #47 = Asciz	java/io/PrintStream;
const #48 = Asciz	println;
const #49 = Asciz	(Ljava/lang/String;)V;

{
Calls();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:	aload_0
   1:	invokespecial	#1; //Method java/lang/Object."<init>":()V
   4:	return
  LineNumberTable: 
   line 11: 0


public static void main(java.lang.String[]);
  Code:
   Stack=4, Locals=2, Args_size=1
   0:	new	#2; //class Factorial
   3:	dup
   4:	invokespecial	#3; //Method Factorial."<init>":()V
   7:	astore_1
   8:	getstatic	#4; //Field java/lang/System.out:Ljava/io/PrintStream;
   11:	new	#5; //class java/lang/StringBuilder
   14:	dup
   15:	invokespecial	#6; //Method java/lang/StringBuilder."<init>":()V
   18:	ldc	#7; //String Result is: 
   20:	invokevirtual	#8; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   23:	aload_1
   24:	bipush	6
   26:	invokevirtual	#9; //Method Factorial.fact:(I)I
   29:	invokevirtual	#10; //Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
   32:	invokevirtual	#11; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   35:	invokevirtual	#12; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   38:	return
  LineNumberTable: 
   line 13: 0
   line 14: 8
   line 15: 38
}