This document discusses Java bytecode fundamentals including: - Bytecode is composed of one-byte instructions with ~200 opcodes in use - The javap tool can be used to disassemble classes and view bytecode - The Java Virtual Machine is stack-based and each thread has a stack frame containing an operand stack and local variables - Bytecode instructions manipulate the stack and local variables to implement method calls and object initialization

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 1 2 +

8. + 1 + 2 1 2 1 2 +

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

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

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

12. + 1 + 2 1 2 1 2 + 3 ICONST_1 ICONST_2 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:orkuglasses>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:orkuglasses>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:orkuglasses>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:orkuglasses>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 areturn aload_0 00 02 getfield 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 B0 2A 00 02 B4 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 A dup pop swap dup_x1 dup_x2 B

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

33. Stack Operations A dup pop swap dup_x1 dup_x2 B

34. Stack Operations B dup pop swap dup_x1 dup_x2 A

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

36. Stack Operations B dup pop swap dup_x1 dup_x2 A B B 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 numbered from 0

40. Local Variables instance methods have this at 0 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 maps numbers to names 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

42. Local Variables Sized explicitly 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

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

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

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

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

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

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

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

50. Object Initialization new <init> <clinit> Instance initialization method 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(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturn var value Stack 1 value depth 2 1 3 2 3 private java.lang.Integer incValue(java.lang.Integer); 4 var value 0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn 5 1 6 2 3

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

77. Flow Control

78. Flow Control GOTO

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

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

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

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

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

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

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

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

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