Java Bytecode Fundamentals Anton Arhipov [Estonia] http://goo.gl/GO6Mm Java User Group Latvia March 2011 http://goo.gl/77MhT

2. Java Bytecode
Fundamentals
JUG.LV 2011, Riga

3. whoami
Anton Arhipov
ZeroTurnaround
JRebel
http://arhipov.blogspot.com
@antonarhipov
@javarebel

4. who-are-you?

5. 1+2

6. +
1+2
1 2

7. +
1+2
1 2
12+

8. +
1+2
1 2
12+

9. +
1+2
1 2
12+ PUSH 1
1

10. +
1+2
1 2
12+ PUSH 1
PUSH 2
2
1

11. +
1+2
1 2
12+ PUSH 1
PUSH 2
3
ADD

12. +
1+2
1 2
12+ ICONST_1
ICONST_2
3
IADD

13. ?=1+2

14. Byte Code
One-byte instructions
256 possible opcodes
~200 in use

15. Byte Code
One-byte instructions
256 possible opcodes
~200 in use

17. The Master Plan
javap
Stack Machine
Objects and Methods
Flow Control

18. javap
Java class file disassembler
Used with no options shows class
structure only
Methods, superclass, interfaces, etc
-c – shows the bytecode
-private – shows all classes and members
-s – prints internal types signatures
-l – prints lines numbers and local variable
tables

19. C:workjugclasses>javap Hello -c
Compiled from "Hello.java"
public class Hello extends java.lang.Object{
public Hello();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
public static void main(java.lang.String[]);
Code:
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3; //String Hello, World!
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V

20. C:workjugclasses>javap Hello -verbose
Compiled from "Hello.java“
public class Hello extends java.lang.Object
SourceFile: "Hello.java"
minor version: 0
major version: 50
Constant pool:
const #1 = Method #6.#20; // java/lang/Object."<init>":()V
const #2 = Field #21.#22; // java/lang/System.out:Ljava/io/PrintStream;
const #3 = String #23; // Hello, World!
const #4 = Method #24.#25; // java/io/PrintStream.println:(Ljava/lang/String;)V
const #5 = class #26; // Hello
const #6 = class #27; // java/lang/Object
const #7 = Asciz <init>;
const #8 = Asciz ()V;

21. C:workjugclasses>javap Hello -verbose
…
public Hello();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 1: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LHello;

22. C:workjugclasses>javap Hello -verbose
…
public static void main(java.lang.String[]);
Code:
Stack=2, Locals=1, Args_size=1
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3; //String Hello, World!
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 4: 0
line 5: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args [Ljava/lang/String;

23. Stack Machine
JVM is a stack-based machine
Each thread has a stack
Stack stores frames
Frame is created on method invocation
Frame:
Operand stack
Array of local variables

24. Frame

25. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

26. 0 1 2 3 4
aload_0 getfield 00 02 areturn
public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

27. 0 1 2 3 4
2A B4 00 02 B0
public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

28. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

29. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

30. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

31. Stack Operations
dup A
pop B
swap
dup_x1
dup_x2

32. Stack Operations
dup A
pop A
swap B
dup_x1
dup_x2

33. Stack Operations
dup A
pop B
swap
dup_x1
dup_x2

34. Stack Operations
dup B
pop A
swap
dup_x1
dup_x2

35. Stack Operations
dup B
pop A
swap B
dup_x1
dup_x2

36. Stack Operations
dup B
pop A
swap B
dup_x1 B
dup_x2 A

37. Local Variables

38. Local Variables
public int calculate(int);
Code:
Stack=2, Locals=2, Args_size=2
…
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLocalVariables;
0 5 1 value I

39. Local Variables
public int calculate(int);
Code:
Stack=2, Locals=2, Args_size=2
…
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLocalVariables;
0 5 1 value I

40. Local Variables
public int calculate(int);
Code:
Stack=2, Locals=2, Args_size=2
…
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLocalVariables;
0 5 1 value I

41. Local Variables
The table
public int calculate(int);
Code:
maps
Stack=2, Locals=2, Args_size=2 numbers to
…
names
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLocalVariables;
0 5 1 value I

42. Local Variables
public int calculate(int);
Code:
Stack=2, Locals=2, Args_size=2 Sized explicitly
…
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LLocalVariables;
0 5 1 value I

43. Local Variables
Local Variables Stack
var value depth value
ldc "Hello"
0 0
astore_0
1 iconst_1 1
astore_1
2 aload_0 2
3 3
4 4

44. Local Variables
Local Variables Stack
var value depth value
ldc "Hello"
0 0 "Hello"
astore_0
1 iconst_1 1
astore_1
2 aload_0 2
3 3
4 4

45. Local Variables
Local Variables Stack
var value depth value
ldc "Hello"
0 "Hello" 0
astore_0
1 iconst_1 1
astore_1
2 aload_0 2
3 3
4 4

46. Local Variables
Local Variables Stack
var value depth value
ldc "Hello"
0 "Hello" 0 1
astore_0
1 iconst_1 1
astore_1
2 aload_0 2
3 3
4 4

47. Local Variables
Local Variables Stack
var value depth value
ldc "Hello"
0 "Hello" 0
astore_0
1 1 iconst_1 1
astore_1
2 aload_0 2
3 3
4 4

48. Local Variables
Local Variables Stack
var value depth value
ldc "Hello"
0 "Hello" 0 "Hello"
astore_0
1 1 iconst_1 1
astore_1
2 aload_0 2
3 3
4 4

49. Local Variables & Stack
load
Local
Variables Stack
Table
store

50. Object Initialization
new
<init>
Instance initialization method
<clinit>
Class and interface
initialization method

51. Object Initialization: static {}
static {};
Code:
0: iconst_1
1: putstatic #2; //Field a:I
4: iconst_2
5: putstatic #3; //Field b:I
8: return

52. Object Initialization: static {}
<clinit>
static {};
Code:
0: iconst_1
1: putstatic #2; //Field a:I
4: iconst_2
5: putstatic #3; //Field b:I
8: return

53. Object Initialization: new
public Initializer();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: new #2; //class java/lang/Object
8: dup
9: invokespecial #1; //Method java/lang/Object."<init>":()V
12: putfield #3; //Field o:Ljava/lang/Object;
15: return

54. Object Initialization: new
public Initializer();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0
5: new #2; //class java/lang/Object
8: dup
9: invokespecial #1; //Method java/lang/Object."<init>":()V
12: putfield #3; //Field o:Ljava/lang/Object;
15: return

55. Object Initialization: {}

56. Object Initialization: {}

57. Object Initialization: {}
public Initializer(int);
Code:
0: aload_0
1: invokespecial #1; // ..<init>
4: aload_0
5: iconst_1
6: putfield #2; //Field a:I
9: aload_0
10: iconst_2
11: putfield #3; //Field c:I
14: aload_0
15: iload_1
16: putfield #4; //Field b:I
19: return

58. Method Invocation
invokestatic
invokeinterface
invokevirtual
invokespecial
invokedynamic

59. Parameter Passing

60. Parameter Passing
parameter
return
value

61. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1
4: astore_1 depth value
2 5: aload_0
6: aload_1 1
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

62. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 this
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

63. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2; //createRandomValue()
1
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

64. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

65. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 this
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

66. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3;
10: areturn 2 this
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

67. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1
3 7: invokespecial #3; //incValue
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

68. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3; //incValue
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

69. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 X
3 7: invokespecial #3; //incValue
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; // Integer.intValue:() 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

70. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 1
3 7: invokespecial #3; //incValue
10: areturn 2 X
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

71. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 X+1
3 7: invokespecial #3; //incValue
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

72. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3; //incValue
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7; //Integer.valueOf
9: areturn

73. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3; //incValue
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 objectref 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

74. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1 objectref
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

75. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

76. Local Variables
public java.lang.Object execute();
var value 0: aload_0
Stack
1: invokespecial #2;
1
4: astore_1 depth value
2 5: aload_0
6: aload_1 1 objectref
3 7: invokespecial #3;
10: areturn 2
3
private java.lang.Integer incValue(java.lang.Integer); 4
var value
0: aload_1
1 1: invokevirtual #8; 5
4: iconst_1 6
2
5: iadd
3
6: invokestatic #7;
9: areturn

77. Flow Control

78. Flow Control
GOTO

79. Stack
depth value
public int decide(int);
Code: 1
0: iload_1
1: bipush 10 2
3: if_icmpge 8
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

80. Stack
depth value
public int decide(int);
Code:
0: iload_1
1 a
1: bipush 10 2
3: if_icmpge 8
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

81. Stack
depth value
public int decide(int);
Code:
0: iload_1
1 10
1: bipush
3: if_icmpge 8
10 2 a
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

82. Stack
depth value
public int decide(int);
Code:
0: iload_1
1 10
1: bipush
3: if_icmpge 8
10 2 a
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

83. Stack
depth value
public int decide(int);
Code: 1
0: iload_1
1: bipush 10 2
3: if_icmpge 8
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

84. Stack
depth value
public int decide(int);
Code:
0: iload_1
1 100
1: bipush 10 2
3: if_icmpge 8
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

85. Stack
depth value
public int decide(int);
Code:
0: iload_1
1 100
1: bipush 10 2
3: if_icmpge 8
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

86. Stack
depth value
public int decide(int);
Code: 1
0: iload_1
1: bipush 10 2
3: if_icmpge 8
6: iconst_0 3
7: ireturn
8: bipush 100
10: ireturn

87. ant.arhipov@gmail.com
http://arhipov.blogspot.com
@antonarhipov
@javarebel