Reverse Engineering for Beginners

CHAPTER 54. JAVA CHAPTER 54. JAVA

public int my_rand();

flags: ACC_PUBLIC

Code:

stack=2, locals=1, args_size=1

0: getstatic #2 // Field rand_state:I

3: getstatic #3 // Field RNG_a:I

6: imul

7: putstatic #2 // Field rand_state:I

10: getstatic #2 // Field rand_state:I

13: getstatic #4 // Field RNG_c:I

16: iadd

17: putstatic #2 // Field rand_state:I

20: getstatic #2 // Field rand_state:I

23: sipush 32767

26: iand

27: ireturn

It just loads all the values from the object’ fields, does the operations and updatesrand_state’s value using theputstatic
instruction. At offset 20,rand_stateis reloaded again (because it was dropped from the stack before, byputstatic).
This looks like non-efficient code, but be sure, theJVMis usually good enough to optimize such things really well.

54.8 Conditional jumps.

Now let’s proceed to conditional jumps.

public class abs
{
public static int abs(int a)
{
if (a<0)
return -a;
return a;
}
}

public static int abs(int);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=1, locals=1, args_size=1

0: iload_0

1: ifge 7

4: iload_0

5: ineg

6: ireturn

7: iload_0

8: ireturn

ifgejumps to offset 7 if the value atTOSis greater or equal to 0. Don’t forget, anyifXXinstruction pops the value (to be
compared) from the stack.

inegjust negates value atTOS.

Another example:

public static int min (int a, int b)

{

if (a>b)

return b;

return a;

}

We get:

public static int min(int, int);

flags: ACC_PUBLIC, ACC_STATIC

Code:

stack=2, locals=2, args_size=2