1. Java Bytecode for
Discriminating Developers
JavaZone’11, Oslo

2. whoami
Anton Arhipov
Java dev. / Product Lead
JRebel
http://arhipov.blogspot.com
@antonarhipov
@javarebel

3. Java… Do you speak it!?

4. Java… Do you speak it!?

5. The Master Plan
Examples
Bytecode 101

6. THE INTRO

7. 1+2

8. +
1+2
1 2

9. +
1+2
1 2
12+

10. +
1+2
1 2
12+

11. +
1+2
1 2
12+ PUSH 1
1

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

13. +
1+2
1 2
12+ PUSH 1
PUSH 2
3
ADD

14. +
1+2
1 2
12+ ICONST_1
ICONST_2
3
IADD

15. ?=1+2

16. TAXONOMY

17. Bytecode
One-byte instructions
256 possible opcodes
200+ in use

18. Bytecode
One-byte instructions
256 possible opcodes
200+ in use

19. TYPE OPERATION

20. TYPE OPERATION
<TYPE> ::= b, s, c, i, l, f, d, a

21. TYPE OPERATION
<TYPE> ::= b, s, c, i, l, f, d, a
Operations with constant values (ldc, iconst_1)

22. TYPE OPERATION
<TYPE> ::= b, s, c, i, l, f, d, a
Operations with constant values (ldc, iconst_1)
Local variables and stack interaction (load/store)
Array operations (aload, astore)
Math (add, sub, mul, div)
Boolean/bitwise operations (iand, ixor)
Comparisons (cmpg, cmpl, ifne, ifeq)
Conversions (l2d, i2l)

23. Bytecode Taxonomy
Stack
Manipulation

24. Bytecode Taxonomy
Stack Flow
Manipulation Control

25. Bytecode Taxonomy
Stack Flow
Manipulation Control
Object
Model

26. Bytecode Taxonomy
Stack Flow
Manipulation Control
Object
Arithmetics
Model

27. Bytecode Taxonomy
Stack Flow
Manipulation Control
monitorenter
monitorexit
Object
Arithmetics
Model

28. TOOLING

29. javap -c -verbose Clazz

30. HELLO, WORLD!

31. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
}

32. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c
}

33. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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

34. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c
} Compiled from "Hello.java"
public class Hello extends java.lang.Object{
public Hello();
Code: the default constructor
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return

35. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c
} Compiled from "Hello.java"
public class Hello extends java.lang.Object{
public Hello();
Code: push this to stack
0: aload_0
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: return

36. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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
invoke <init> on this

37. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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

38. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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

39. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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
get static field
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

40. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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
load string to the stack

41. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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
invoke method with parameter

42. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>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

43. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c
What’s #1,#2, etc ?
} 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

44. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -verbose
}

45. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -verbose
}

46. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -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;

47. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -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;

48. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -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;

49. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -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;

50. public class Hello {
public static void main(String[] args) {
System.out.println ( “Hello, World!” );
}
C:workgeeconclasses>javap Hello -c -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;

51. STACK
MACHINE

52. Stack Machine

53. Stack Machine
JVM is a stack-based machine

54. Stack Machine
JVM is a stack-based machine
Each thread has a stack

55. Stack Machine
JVM is a stack-based machine
Each thread has a stack
Stack stores frames

56. Stack Machine
JVM is a stack-based machine
Each thread has a stack
Stack stores frames
Frame is created on method
invocation

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

58. Local variables
0 1 2 … N
Operand stack
#1
Constant
Pool

59. public class Get {
String name;
public String getName() {
return name;
}
}
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;

60. public class Get {
String name;
public String getName() {
return name;
}
}
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;

61. 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;

62. 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;

64. STACK
JUGGLING

65. dup A
pop B
swap
dup_x1
dup2_x1

66. dup A
pop A
swap B
dup_x1
dup2_x1

67. dup A
pop B
swap
dup_x1
dup2_x1

68. dup B
pop A
swap
dup_x1
dup2_x1

69. dup B
pop A
swap B
dup_x1
dup2_x1

70. dup B
pop A
swap B
dup_x1 B
dup2_x1 A

71. How do you
swap doubles?

72. dup2_x2

73. dup2_x2
dconst_0
0.0

74. dup2_x2
dconst_0
dconst_1 1.0
0.0

75. dup2_x2
dconst_0
dconst_1 1.0
swap
0.0

76. dup2_x2
dconst_0
dconst_1 1.0
swap not allowed!
0.0

77. dup2_x2
dconst_0
dconst_1 1.0
swap2
0.0

78. dup2_x2
dconst_0
dconst_1 1.0
doesn’t
swap2 exist
0.0

79. dup2_x2
dconst_0
dconst_1 1.0
dup2_x2
0.0
1.0

80. dup2_x2
dconst_0
dconst_1 0.0
dup2_x2
pop2 1.0

81. dup2_x2
dconst_0
dconst_1 0.0
dup2_x2
pop2 1.0
profit! 

82. LOCAL
VARIABLES

83. Local Variables
public int calculate(int value) {
return value + 42;
}

84. Local Variables
public int calculate(int value) {
return value + 42;
}
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

85. Local Variables
public int calculate(int value) {
return value + 42;
}
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

86. Local Variables
public int calculate(int value) {
return value + 42;
}
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

87. Local Variables
public int calculate(int value) {
return value + 42;
}
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

88. Local Variables
public int calculate(int value) {
return value + 42;
}
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

89. 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

90. 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

91. 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

92. 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

93. 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

94. 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

95. load
Local
Variables Stack
Table
store

96. OBJECTS

97. Object Initialization
new
0xBB
<init>
Instance initialization method
<clinit>
Class and interface
initialization method

98. Object Initialization: static {}
public class Initializer {
static int a;
static int b;
static { a = 1; }
static { b = 2; } static {};
Code:
} 0: iconst_1
1: putstatic #2; //Field a:I
4: iconst_2
5: putstatic #3; //Field b:I
8: return

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

100. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
}
}

101. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
public Initializer(); }
Code:
}

102. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
public Initializer(); }
Code:
0: aload_0 }

103. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
public Initializer(); }
Code:
0: aload_0 }
1: invokespecial #1; //Method java/lang/Object."<init>":()V

104. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
public Initializer(); }
Code:
0: aload_0 }
1: invokespecial #1; //Method java/lang/Object."<init>":()V
4: aload_0

105. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
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

106. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
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;

107. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
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

108. Object Initialization: new
public class Initializer {
Object o;
public Initializer() {
o = new Object();
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

109. Object Initialization: {}
public class Initializer {
int a;
int b;
int c;
{ a = 1; }
public Initializer(int b) {
this.b = b;
}
{ c = 2; }
}

110. Object Initialization: {}
public class Initializer {
int a;
int b;
int c;
{ a = 1; }
public Initializer(int b) {
this.b = b;
}
{ c = 2; }
}

111. Object Initialization: {}
public class Initializer {
int a;
int b;
int c;
{ a = 1; }
public Initializer(int b) {
this.b = b;
}
{ c = 2; }
}

112. Object Initialization: {}
public class Initializer {
int a;
int b;
int c;
{ a = 1; }
public Initializer(int b) {
this.b = b;
}
{ c = 2; }
}

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

116. There’s no initializer

117. METHOD INVOCATION
&
PARAMETER PASSING
METHOD
INVOCATION

118. Java Method Invocation

119. Java Method Invocation
invokestatic
invokespecial
invokevirtual
invokeinterface
invokedynamic

120. Java Method Invocation
invokestatic
invokespecial
invokevirtual
invokeinterface Integer.valueOf(“42”)
invokedynamic

121. Java Method Invocation
invokestatic
invokespecial
<init>
invokevirtual
invokeinterface
invokedynamic
private void foo();
super.method();

122. Java Method Invocation
invokestatic class A
invokespecial A/method1
A/method2
invokevirtual
invokeinterface
invokedynamic

123. Java Method Invocation
invokestatic class A
invokespecial A/method1
A/method2
invokevirtual
invokeinterface class B
invokedynamic

124. Java Method Invocation
invokestatic class A
invokespecial A/method1
A/method2
invokevirtual
invokeinterface class B
A/method1
invokedynamic B/method2
B/method3

125. Java Method Invocation
invokestatic class A
invokespecial A/method1
A/method2
invokevirtual
invokeinterface class B impl X
A/method1
invokedynamic B/method2
B/method3
X/methodX

126. Java Method Invocation
invokestatic class A
invokespecial A/method1
A/method2
invokevirtual
invokeinterface class B impl X
A/method1
invokedynamic B/method2
B/method3
X/methodX
class D impl X
D/method1
X/methodX

127. Java Method Invocation
invokestatic class A
invokespecial A/method1
A/method2
invokevirtual
invokeinterface class B impl X
A/method1
invokedynamic B/method2
B/method3
X/methodX
class D impl X
D/method1
X/methodX

128. Java Method Invocation
invokestatic
invokespecial
invokevirtual
invokeinterface
invokedynamic
Efficient Implementation of Java Interfaces:
Invokeinterface Considered Harmless, Bowen
Alpern, Anthony Cocchi, Stephen Fink, David
Grove, and Derek Lieber, OOPSLA’01

129. Method Invocation
obj.method(param1, param2);

130. Method Invocation
obj.method(param1, param2);
push obj
push param1
push param2
call method

131. Method Invocation
obj.method(param1, param2);
obj
push obj
push param1
push param2
call method

132. Method Invocation
obj.method(param1, param2);
param1
push obj
obj
push param1
push param2
call method

133. Method Invocation
obj.method(param1, param2);
param2
push obj
param1
push param1
obj
push param2
call method

134. Method Invocation
obj.method(param1, param2);
obj?
push obj
push param1
push param2
call method

135. INNER
CLASSES

136. Inner Classes
public class Car {
class Engine {
public void start() {
move();
}
}
private void move() {
}
}

137. Inner Classes
class Car$Engine extends j.l.Object{
final Car this$0;
Car$Engine(Car);
public void start();
Code:
0: aload_0
1: getfield #1; //Field this$0:LCar;
4: invokestatic #3; // Car.access$000:(LCar;)V
7: return
}

138. Inner Classes
public class Car extends j.l.Object{
public Car();
private void move();
class Car$Engine extends j.l.Object{ static void access$000(Car);
final Car this$0; Code:
0: aload_0
Car$Engine(Car); 1: invokespecial #1; // move: ()V;
4: return
public void start(); }
Code:
0: aload_0
1: getfield #1; //Field this$0:LCar;
4: invokestatic #3; // Car.access$000:(LCar;)V
7: return
}

139. Inner Classes
public class Car extends j.l.Object{
public Car();
private void move();
class Car$Engine extends j.l.Object{ static void access$000(Car);
final Car this$0; Code:
0: aload_0
Car$Engine(Car); 1: invokespecial #1; // move: ()V;
4: return
public void start(); }
Code:
0: aload_0
1: getfield #1; //Field this$0:LCar;
4: invokestatic #3; // Car.access$000:(LCar;)V
7: return
}

140. “HOW DO THEY DO THAT?”

141. object Singleton {
def test={}
}

142. object Singleton {
def test={}
}
$> scalac Singleton.scala
Singleton.class Singleton$.class

143. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}

144. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}

145. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}

146. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}
public final class Singleton$ extends java.lang.Object implements
scala.ScalaObject {
public static final Singleton$ MODULE$;
public static {};
Code:
0: new #9; //class Singleton$
3: invokespecial #12; //Method "<init>":()V
6: return
public void test();
private Singleton$();
}

147. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}
public final class Singleton$ extends java.lang.Object implements
scala.ScalaObject {
public static final Singleton$ MODULE$;
public static {};
Code:
0: new #9; //class Singleton$
3: invokespecial #12; //Method "<init>":()V
6: return
public void test();
private Singleton$();
}

148. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}
public final class Singleton$ extends java.lang.Object implements
scala.ScalaObject {
public static final Singleton$ MODULE$;
public static {};
Code:
0: new #9; //class Singleton$
3: invokespecial #12; //Method "<init>":()V
6: return
public void test();
private Singleton$();
}

149. public final class Singleton extends java.lang.Object {
public static final void test();
Code:
0: getstatic #11; //Field Singleton$.MODULE$:LSingleton$;
3: invokevirtual #13; //Method Singleton$.test:()V
6: return
}
public final class Singleton$ extends java.lang.Object implements
scala.ScalaObject {
public static final Singleton$ MODULE$;
public static {};
public void test();
private Singleton$();
Code:
0: aload_0
1: invokespecial #17; //Method java/lang/Object."<init>":()V
4: aload_0
5: putstatic #19; //Field MODULE$:LSingleton$;
8: return

150. object Singleton {
def test={}
}

151. public class Singleton {
public void test(){
Singleton$.MODULE$.test();
}
}
object Singleton {
def test={}
}

152. public class Singleton {
public void test(){
Singleton$.MODULE$.test();
}
}
public final class Singleton$
object Singleton { implements scala.ScalaObject {
def test={} public static final Singleton$ MODULE$;
} static { new Singleton$(); }
private Singleton$(){
MODULE$ = this;
}
public void test() {
}
}

153. CRASH!
BOOM!
BANG!

154. Javassist
Bytecode manipulation made easy
Source-level and bytecode-level API
Uses the vocabulary of Java language
On-the-fly compilation of the injected code
http://www.jboss.org/javassist

155. for(int i = 0; i < 100; i++){
int a = 0;
try {
while (true) {
a++;
foo(a);
if(a > 1) break;
}
} catch (Exception e) {
}
}

156. Javassist
CtMethod method = …
method.setBody(“ for(int i = 0; i < 100; i++){
int a = 0;
try {
while (true) {
a++;
foo(a);
if(a > 1) break;
}
} catch (Exception e) {
}
}”);

157. -Xverify:all
Exception in thread "main" java.lang.VerifyError:
(class: zt/javassist/My, method: test signature:
()V) Inconsistent stack height 0 != 1

158. public void test();
Code:
Stack=2, Locals=4, Args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 42
8: iconst_0
9: istore_2
10: goto 29
13: iinc 2, 1
16: aload_0
17: iload_2
18: invokevirtual #23; //Method zt/javassist/My.foo:(I)V
21: iload_2
22: iconst_1
23: if_icmple 29
26: goto 32
29: goto 13
32: astore_3
33: goto 36
36: iinc 1, 1
39: goto 2
42: return
Exception table:
from to target type
10 32 32 Class java/lang/Exception

159. public void test();
Code:
Stack=2, Locals=4, Args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 42
8: iconst_0
9: istore_2
10: goto 29
13: iinc 2, 1
16: aload_0
17: iload_2
18: invokevirtual #23; //Method zt/javassist/My.foo:(I)V
21: iload_2
No local
22: iconst_1
23: if_icmple 29
variables table
26: goto 32
29: goto 13
32: astore_3
33: goto 36
36: iinc 1, 1
39: goto 2
42: return
Exception table:
from to target type
10 32 32 Class java/lang/Exception

160. public void test();
Code:
Stack=2, Locals=4, Args_size=1
0: iconst_0
1: istore_1
2: iload_1
3: bipush 100
5: if_icmpge 42
8: iconst_0
9: istore_2
10: goto 29
13: iinc 2, 1
16: aload_0
17: iload_2
18: invokevirtual #23; //Method zt/javassist/My.foo:(I)V
21: iload_2
No local
22: iconst_1
23: if_icmple 29
variables table
26: goto 32
29: goto 13
32: astore_3
33: goto 36
36: iinc 1, 1
39: goto 2
42: return
Exception table:
from to target type
10 32 32 Class java/lang/Exception

161. Bytecode is fun!
Know your platform!
Maybe, you can build your own language?
You may need to read bytecode someday.

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