Question

When running javap on a very simple HelloWorld application I have some confusion on the output around the constant pool.

Test Code

public class TestClass {
    public static void main(String[] args) {
        System.out.println("hello world");
    }
}

Javap -c -verbose output (snipped)

// Header + consts 1..22 snipped
const #22 = String      #23;    //  hello world
const #23 = Asciz       hello world;

public static void main(java.lang.String[]);
  Signature: ([Ljava/lang/String;)V
  Code:
   Stack=2, Locals=1, Args_size=1
   0:   getstatic       #16; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   ldc     #22; //String hello world
   5:   invokevirtual   #24; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   8:   return
  // Debug info snipped
}

Ok, so on line 3 we see a pushing of the “hello world” constant onto the stack via #22, but const #23 seems to hold the actual value. I guess I am a little confused with what the #(number) means when it appears on the right-hand-side of the printout.

Oracle/Sun’s man page for javap leaves much to be desired.

Answer 1

All your class, interface, field names and string constants go into the java constant pool.

As per VM Spec ( http://java.sun.com/docs/books/jvms/second_edition/html/ClassFile.doc.html ):

The constant_pool is a table of
structures (§4.4) representing various
string constants, class and interface
names, field names, and other
constants that are referred to within
the ClassFile structure and its
substructures. The format of each
constant_pool table entry is indicated
by its first “tag” byte. The
constant_pool table is indexed from 1
to constant_pool_count-1.

So in terms of constant pool something like below can be viewed as:

const #22 = String      #23;    //  hello world
const #23 = Asciz       hello world;

The value at #22 (index 22) is of type String and its value is null terminated c string (Asciz) hello world is at index 23.