Java Bytecode Explained

1. Lifting The Veil – Reading Java Byte
Code
Alexander Shopov

2. Alexander Shopov
By day: Software Engineer at Cisco
By night: OSS contributor
Coordinator of Bulgarian Gnome TP
Contacts:
E-mail: ash@kambanaria.org
Jabber: al_shopov@jabber.minus273.org
LinkedIn: http://www.linkedin.com/in/alshopov
Google: Just search “al_shopov“

3. Please Learn And Share
License: CC-BY v3.0
Creative Commons Attribution v3.0

4. Disclaimer
My opinions, knowledge and experience!
Not my employer's.

5. Contents
● Why read?
● How to read?
● JVM Internals;
● JVM Data Types;
● JVM Opcodes.
● Let's read some code.
● What next?

6. Why Read Byte code?
● Understand your platform
● It is interesting and not too hard
● How does Java function? How does X function?
● Job interviews
● Catch compiler bugs/optimizations
● Learn to read before you write
● Source may not correspond to binary
● C/C++ people know their assembler
● Java language evolution vs. Java platform evolution

7. Bad News And Good News
Bad: Good:
We will be Easiest
reading assembler
assembler in world

8. What Is The JVM?
● Stack based, byte oriented virtual machine
without registers easily implementable on 32 bit
hardware.
● 206 (<256) instructions that are easy to group
and there is no need to remember them all
● Some leeway in implementations (even with
Oracle)

9. Dramatis Personæ
● The JVM
● The threads
● The frames
● The stacks – LIFO
● The local variables – array of slots
● The runtime constant pool – array of values
● The bytecode – the instructions
● Class files – serialized form of constants and byte
code

10. Enter JVM
JVM OS process

11. Thread A
Thread B
Thread C
Enter Threads
Thread D

12. Enter Frames
F4
Thread C
Thread D
Thread A
Thread B
F3 F3
F2 F2 F2
F1 F1 F1 F1
F0 F0 F0 F0

13. Enter Frames, Really!
F0
F0 F2
F3 F3
F0
F2 F1
F1 F1
F1 F0 F4
F2

14. What Is A Frame Actually?
F0

15. Let's Peek Inside A Frame
F0

16. Enter Local Variables
0 1 2 3 4 5 6 …
Local variables
F0

17. Enter Stack
0 1 2 3 4 5 6 …
Local variables
F0
Stack

18. Enter Pool Of Constants
0 1 2 3 4 5 6 …
Local variables
F0
Pool of
constants
Stack

19. Where Is The Code?
0 1 2 3 4 5 6 …
Local variables
F0
Pool of
constants
Stack

20. Where Is The Code?
JVM (heap)
0 1 2 3 4 5 6 …
Local variables
F0
Pool of
constants
Stack

21. Where Is The Code?
JVM (heap)
0 1 2 3 4 5 6 … Class
PC
Local variables Method code
F0
Class
Pool of
constants
Stack

22. Where is the code?
JVM (heap)
0 1 2 3 4 5 6 … Class
6
PC
Local variables Method code
F0
Class
Pool of
constants
Stack

23. Load
JVM (heap)
0 1 2 3 4 5 6 … Class
6
PC
Local variables Method code
F0
Class
Pool of
constants
6
Stack

24. And…
JVM (heap)
0 1 2 3 4 5 6 … Class
6
PC
Local variables Method code
F0
Class
Pool of
8 constants
6
Stack

25. Store
JVM (heap)
0 1 2 3 4 5 6 … Class
6 8
Local variables PC Method code
F0
Class
Pool of
8 constants
6
Stack

26. JVM Datatypes
● Primitive types
● Java { numeric – integral: byte (±8), short (±16),
int (±32), long (±64), char (+16), floating point:
float (±32), double (±64); boolean (int or byte) }
● returnAddress – pointers to the opcodes of JVM
(jumps - loops)
● Reference types
● class, array, interface
● null

27. JVM Datatypes Descriptors
Java type Type descriptor
boolean Z
char C
byte B
short S
int I
float F
long J
double D
Object Ljava/lang/Object;
byte[] [B
String[][] [[Ljava/lang/String;
void V

28. JVM Method Descriptors
Source Code Method Method Descriptor
declaration
void m1(int i, double d, (IDF)V
float f)
byte[] m2(String s) (Ljava/lang/String;)[B
Object m3(int[][][] i) ([[[I)Ljava/lang/Object;
boolean[] m4()

29. JVM Method Descriptors
Source Code Method Method Descriptor
declaration
void m1(int i, double d, (IDF)V
float f)
byte[] m2(String s) (Ljava/lang/String;)[B
Object m3(int[][][] i) ([[[I)Ljava/lang/Object;
boolean[] m4() ()[B
(Ljava/lang/Object;Ljava/lang/Long;)J

30. JVM Method Descriptors
Source Code Method Method Descriptor
declaration
void m1(int i, double d, (IDF)V
float f)
byte[] m2(String s) (Ljava/lang/String;)[B
Object m3(int[][][] i) ([[[I)Ljava/lang/Object;
boolean[] m4() ()[B
long m5(Object, Long) (Ljava/lang/Object;Ljava/lang/Long;)J

31. 206 instructions
DON'T PANIC!

32. Level 1 – Do Nothing/1
● nop

33. Level 2 – Load Constants/20
● aconst_null,
● iconst_m1, iconst_0, iconst_1, iconst_2, iconst_3,
iconst_4, iconst_5
● lconst_0, lconst_1,
● fconst_0, fconst_1, fconst_2
● dconst_0, dconst_1
● bipush, sipush – 1, 2 bytes
● ldc, ldc_w, ldc2_w – load from index in constant
pool 1,2,2 bytes for index

34. Level 3 – Load Variables/33
● iload, lload, fload, dload, aload
● iload_0, iload_1, iload_2, iload_3, lload_0,
lload_1, lload_2, lload_3, fload_0, fload_1,
fload_2, fload_3, dload_0, dload_1, dload_2,
dload_3, aload_0, aload_1, aload_2, aload_3
● iaload, laload, faload, daload, aaload, baload,
caload, saload – consume reference to array
and int index in it

35. Level 4 – Conversions/15
● i2l, i2f, i2d, l2i, l2f, l2d, f2i, f2l, f2d, d2i, d2l, d2f,
i2b, i2c, i2s

36. Level 6 – Maths/37
● iadd, ladd, fadd, dadd, isub, lsub, fsub, dsub,
imul, lmul, fmul, dmul, idiv, ldiv, fdiv, ddiv, irem,
lrem, frem, drem, ineg, lneg, fneg, dneg, ishl,
lshl, ishr, lshr, iushr, lushr, iand, land, ior, lor,
ixor, lxor
● Iinc - increment local variable #index by signed
byte const

37. Level 7 – Stores/33
● istore, lstore, fstore, dstore, astore, istore_0,
istore_1, istore_2, istore_3, lstore_0, lstore_1,
lstore_2, lstore_3, fstore_0, fstore_1, fstore_2,
fstore_3, dstore_0, dstore_1, dstore_2,
dstore_3, astore_0, astore_1, astore_2,
astore_3, iastore, lastore, fastore, dastore,
aastore, bastore, castore, sastore

38. Level 8 – No-branch Comparisons/5
● lcmp, fcmpl, fcmpg, dcmpl, dcmpg (beware
NaN)

39. Level 9 – Objects/15
● getstatic, putstatic
● getfield, putfield
● invokevirtual, invokespecial, invokestatic,
invokeinterface
● new, newarray, anewarray
● arraylength
● athrow
● checkcast, instanceof (difference is treatment of
null)

40. Level 10 – Return/6
● ireturn, lreturn, freturn, dreturn, areturn, return

41. 165 of 206
81%

42. We Have Enough Mana/Resources!
Let's dive in bytecode!

43. Enter Bytecode
javap – your only true friend now
javap -classpath PATH -p -c -l -s CLASS

44. Example 1
public static int whatIsThis(int, int, int);
Signature: (III)I
Code:
0: iload_0
1: iload_1
2: iadd
3: istore_3
4: iload_3
5: iload_2
6: iadd
7: istore_3
8: iload_3
9: ireturn

45. Example 1
public static int whatIsThis(int, int, int);
Signature: (III)I
Code:
0: iload_0
1: iload_1
2: iadd
3: istore_3
4: iload_3
5: iload_2
public static int whatIsThis
6: iadd (int a, int b, int c) {
7: istore_3 int result = a + b;
8: iload_3 result += c;
9: ireturn return result;
}

46. Example 2
public static int whatIsThis(int, int, int);
Signature: (III)I
Code:
0: iload_0
1: iload_1
2: iadd
3: iload_2
4: iadd
5: ireturn

47. Example 2
public static int whatIsThis(int, int, int);
Signature: (III)I
Code:
0: iload_0
1: iload_1
2: iadd
3: iload_2
4: iadd
5: ireturn
public static int whatIsThis
(int a, int b, int c) {
result a + b + c;
}

48. Example 3
public static int whatIsThis(int, float, double);
Signature: (IFD)I
Code:
0: iload_0
1: i2f
2: fload_1
3: fadd
4: f2d
5: dload_2
6: dadd
7: d2i
8: ireturn
LineNumberTable:
line 6: 0
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 a I
0 9 1 b F
0 9 2 c D

49. Example 3
public static int whatIsThis(int, float, double);
Signature: (IFD)I
Code:
0: iload_0
1: i2f
2: fload_1
3: fadd
4: f2d
5: dload_2
6: dadd
7: d2i
8: ireturn
LineNumberTable:
line 6: 0
public static int whatIsThis
LocalVariableTable: (int a, float b, double c) {
Start Length Slot Name return (int) (a + b + c);
Signature
0 9 0 } I
a
0 9 1 b F
0 9 2 c D

50. Example 4
public static void main(java.lang.String[]);
Signature: ([Ljava/lang/String;)V
Code:
0: getstatic #16 // Field
java/lang/System.out:Ljava/io/PrintStream;
3: ldc #22 // String BGOUG
5: invokevirtual #24 // Method
java/io/PrintStream.println:(Ljava/lang/String;)V
8: return

51. More verbosity
javap -v -classpath PATH -p -c -l -s
CLASS

52. Example 4
Constant pool:
#1 = Class #2 //
org/kambanaria/readbytecode/bgoug/Example4
#2 = Utf8 org/kambanaria/readbytecode/bgoug/Example4
…
#16 = Fieldref #17.#19 //
java/lang/System.out:Ljava/io/PrintStream;
…
#22 = String #23 // BGOUG
#23 = Utf8 BGOUG
#24 = Methodref #25.#27 //
java/io/PrintStream.println:(Ljava/lang/String;)V
…

53. Example 4
public static void main(java.lang.String[]);
Signature: ([Ljava/lang/String;)V
Code:
0: getstatic #16 // Field
java/lang/System.out:Ljava/io/PrintStream;
3: ldc #22 // String BGOUG
5: invokevirtual #24 // Method
java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
public static void main
(String[] args){
System.out.println("BGOUG");
}
// Hello, BGOUG!

54. Example 5
public char[] whatIsThis();
Code:
0: aload_0
1: getfield #12 // Field content:[C
4: areturn
public static void main(java.lang.String[]);
Code:
0: getstatic #22
java/lang/System.out:Ljava/io/PrintStream;
3: new #1 // class
org/kambanaria/readbytecode/bgoug/Example5
6: dup
7: invokespecial #28 // Method "<init>":()V
10: invokevirtual #29 // Method whatIsThis:()[C
13: invokestatic #31 // Method
java/util/Arrays.toString:([C)Ljava/lang/String;
16: invokevirtual #37 // Method
java/io/PrintStream.println:(Ljava/lang/String;)V
19: return

55. Example 5
public char[] whatIsThis();
public char[] whatIsThis()
Code: { return content; }
0: aload_0
1: getfield #12 // Field content:[C
4: areturn
public static void main(java.lang.String[]);
Code:
0: getstatic #22
java/lang/System.out:Ljava/io/PrintStream;
3: new #1 // class
org/kambanaria/readbytecode/bgoug/Example5
6: dup
7: invokespecial #28 // Method "<init>":()V
10: invokevirtual #29 // Method whatIsThis:()[C
13: invokestatic #31 // Method
java/util/Arrays.toString:([C)Ljava/lang/String;
16: invokevirtual #37 // Method
java/io/PrintStream.println:(Ljava/lang/String;)V
19: return

56. Example 5
public static void main (String[] args)
public char[] whatIsThis();
{ System.out.println(
Code:
0: aload_0
Arrays.toString(
1: getfield #12 new Example5(). content:[C
// Field
4: areturn whatIsThis()));}
public static void main(java.lang.String[]);
Code:
0: getstatic #22
java/lang/System.out:Ljava/io/PrintStream;
3: new #1 // class
org/kambanaria/readbytecode/bgoug/Example5
6: dup
7: invokespecial #28 // Method "<init>":()V
10: invokevirtual #29 // Method whatIsThis:()[C
13: invokestatic #31 // Method
java/util/Arrays.toString:([C)Ljava/lang/String;
16: invokevirtual #37 // Method
java/io/PrintStream.println:(Ljava/lang/String;)V
19: return

57. Level 11 – Stack/9
● pop a ➔
● pop2 ba ➔
● dup a ➔aa
● dup_x1 ba ➔aba
● dup_x2 cba ➔acba
● dup2 ba ➔baba
● dup2_x1 cba ➔bacba
● dup2_x2 dcba ➔badcba
● swap ba ➔ab

58. Example 6
public void whatIsThis(java.lang.String);
Code:
0: aload_1
1: ifnonnull 12
4: new #18 // class
java/lang/NullPointerException
7: dup
8: invokespecial #20 // Method
java/lang/NullPointerException."<init>":()V
11: athrow
12: aload_0
13: aload_1
14: putfield #21 // Field
s:Ljava/lang/String;
17: return

59. Example 6
public void whatIsThis(java.lang.String);
Code:
0: aload_1
1: ifnonnull 12
4: new #18 // class
java/lang/NullPointerException
7: dup
8: invokespecial #20 // Method
java/lang/NullPointerException."<init>":()V
11: athrow
12: aload_0
13: aload_1
public void whatIsThis(String s) {
14: putfield #21 // Field
if (null == s) {s:Ljava/lang/String;
17: return throw new NullPointerException();
}
this.s = s;
}

60. Level 12 – conditions, branches,
loops/19
● ifeq, ifne, iflt, ifge, ifgt, ifle
● if_icmpeq, if_icmpne, if_icmplt, if_icmpge,
if_icmpgt, if_icmple
● if_acmpeq, if_acmpne
● ifnull, ifnonnull
● goto, jsr, ret

61. 193 of 206
94%

62. Example 7
public static int parse(java.lang.String);
Code:
0: aload_0
1: invokestatic #16 // Method
java/lang/Integer.parseInt:(Ljava/lang/String;)I
4: ireturn
5: astore_1
6: iconst_0
7: ireturn
Exception table:
from to target type
0 4 5 Class java/lang/NumberFormatException
public static int parse(String s) {
try {
return Integer.parseInt(s);
} catch (NumberFormatException e) {
return 0;
}
}

63. Example 8
public class org.kambanaria.readbytecode.bgoug.Example8 {
static final boolean $assertionsDisabled;
static {};
Code:
0: ldc #1 // class
org/kambanaria/readbytecode/bgoug/Example8
2: invokevirtual #10 // Method
java/lang/Class.desiredAssertionStatus:()Z
5: ifne 12
8: iconst_1
9: goto 13
12: iconst_0
13: putstatic #16 // Field $assertionsDisabled:Z
16: return
public class Example8 {
private static String repeat(String s){
assert s != null;
return s + s;
}
}

64. Example 8
private static java.lang.String repeat(java.lang.String);
Code:
0: getstatic #16 // Field $assertionsDisabled:Z
3: ifne 18
6: aload_0
7: ifnonnull 18
10: new #28 // class java/lang/AssertionError
13: dup
14: invokespecial #30 // Method
java/lang/AssertionError."<init>":()V
17: athrow
18: new #31 // class java/lang/StringBuilder
21: dup
22: aload_0
23: invokestatic #33 // Method java/lang/String.valueOf:
(Ljava/lang/Object;)Ljava/lang/String;
26: invokespecial #39 // Method
java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
29: aload_0
30: invokevirtual #42 // Method
java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
33: invokevirtual #46 // Method
java/lang/StringBuilder.toString:()Ljava/lang/String;
36: areturn

65. Now You Know
Beware Asserts In Public
Methods!

66. Further resources
● Oracle:
The JVM Specification, Java SE 7 Edition
● A. Arhipov:
Java Bytecode For Discriminating Developers
● Wikipedia: Java Bytecode Instruction Listings
● S. H. Park Understanding JVM Internals
● C. McGlone:
Looking "Under the Hood" with javap
● P. Haggar: Java bytecode

67. Presentation background
● Alexander Wilms: Hexagons