How to Create a Data Pool in Jasmin?

Question

Does Jasmin have the ability to specify a constant pool? I need to be able to create a descriptor area of thousands of bytes. The area will contain arbitrary byte data including nulls. Here's an example of a typical assembly directive to declare binary constants:

data  0x00, 0x01, 10, 32 ...

Seems that Java class files contain these areas but I can't find a way in Jasmin to declare the values on such a large scale.

Answer 1

That area in the Java class file is called the Constant Pool and is described in detail in §4.4 of the JVM Spec.

It can't, however contain arbitrary untyped data, but instead contains typed data only.

Supported data types are:

• CONSTANT_Class
• CONSTANT_Fieldref
• CONSTANT_Methodref
• CONSTANT_InterfaceMethodref
• CONSTANT_String
• CONSTANT_Integer
• CONSTANT_Float
• CONSTANT_Long
• CONSTANT_Double
• CONSTANT_NameAndType
• CONSTANT_Utf8

None of those types support arbitrary byte data of arbitrary length. CONSTANT_Utf8 comes closest, but requires its content to be valid UTF-8 data and doesn't allow 0 bytes.

The best way to find out how byte[] are encoded is to define one in Java and decompile the resulting .class file. As far as I know A byte[] is compiled to an initializer block setting its fields.

Confirmed: This Java class:

public class Test {
  byte[] b = { 0, 1, 2, 3, 100 };
}

Decompiles to this (using javac Test.java && javap -v Test):

Compiled from "Test.java"
public class Test extends java.lang.Object
  SourceFile: "Test.java"
  minor version: 0
  major version: 50
  Constant pool:
const #1 = Method   #4.#13; //  java/lang/Object."<init>":()V
const #2 = Field    #3.#14; //  Test.b:[B
const #3 = class    #15;    //  Test
const #4 = class    #16;    //  java/lang/Object
const #5 = Asciz    b;
const #6 = Asciz    [B;
const #7 = Asciz    <init>;
const #8 = Asciz    ()V;
const #9 = Asciz    Code;
const #10 = Asciz   LineNumberTable;
const #11 = Asciz   SourceFile;
const #12 = Asciz   Test.java;
const #13 = NameAndType #7:#8;//  "<init>":()V
const #14 = NameAndType #5:#6;//  b:[B
const #15 = Asciz   Test;
const #16 = Asciz   java/lang/Object;

{
byte[] b;

public Test();
  Code:
   Stack=5, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   iconst_5
   6:   newarray byte
   8:   dup
   9:   iconst_0
   10:  iconst_0
   11:  bastore
   12:  dup
   13:  iconst_1
   14:  iconst_1
   15:  bastore
   16:  dup
   17:  iconst_2
   18:  iconst_2
   19:  bastore
   20:  dup
   21:  iconst_3
   22:  iconst_3
   23:  bastore
   24:  dup
   25:  iconst_4
   26:  bipush  100
   28:  bastore
   29:  putfield    #2; //Field b:[B
   32:  return
  LineNumberTable: 
   line 1: 0
   line 2: 4


}

As you can see, the generated constructor "manually" sets the 5 values into the newly allocated array.