Reverse Engineering for Beginners

CHAPTER 54. JAVA CHAPTER 54. JAVA

0: iconst_0

1: istore_1

2: iload_1

3: aload_0

4: arraylength

5: if_icmpge 23

8: getstatic #2 // Field java/lang/System.out:Ljava/io/⤦

ÇPrintStream;

11: aload_0

12: iload_1

13: iaload

14: invokevirtual #3 // Method java/io/PrintStream.println:(I)V

17: iinc 1, 1

20: goto 2

23: return

The incomingreferenceto the array is in the zeroth slot. Thea.lengthexpression in the source code is converted to
anarraylengthinstruction: it takes areferenceto the array and leaves the array size atTOS. ialoadat offset 13 is used
to load array elements, it requires to arrayreferencebe present in the stack (prepared byaload_0at 11), and also an index
(prepared byiload_1at offset 12).

Needless to say, instructions prefixed withamay be mistakenly comprehended asarrayinstructions. It’s not correct. These
instructions works withreferencesto objects. And arrays and strings are objects too.

54.13.2Summing elements of array.

Another example:

public class ArraySum
{
public static int f (int[] a)
{
int sum=0;
for (int i=0; i<a.length; i++)
sum=sum+a[i];
return sum;
}
}

public static int f(int[]);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=3, locals=3, args_size=1

0: iconst_0

1: istore_1

2: iconst_0

3: istore_2

4: iload_2

5: aload_0

6: arraylength

7: if_icmpge 22

10: iload_1

11: aload_0

12: iload_2

13: iaload

14: iadd

15: istore_1

16: iinc 2, 1

19: goto 4

22: iload_1

23: ireturn

LVAslot 0 contains areferenceto the input array. LVAslot 1 contains the local variablesum.

54.13.3The only argument of themain()function is an array too

We’ll be using the only argument of themain()function, which is an array of strings: