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 FundamentalsJUG.LV 2011, Riga

3. whoamiAnton ArhipovZeroTurnaroundJRebelhttp://arhipov.blogspot.com@antonarhipov@javarebel

4. who-are-you?

5. 1 + 2

6. +1 + 212

7. +1 + 2121 2 +

8. +1 + 2121 2 +

9. +1 + 2121 2 + 1PUSH 1

10. +1 + 2121 2 + 2PUSH 1PUSH 21

11. +1 + 2121 2 + 3PUSH 1PUSH 2ADD

12. +1 + 2121 2 + 3ICONST_1ICONST_2IADD

13. ? = 1 + 2

14. Byte CodeOne-byte instructions256 possible opcodes~200 in use

15. Byte CodeOne-byte instructions256 possible opcodes~200 in use

17. The Master PlanjavapStack MachineObjects and MethodsFlow Control

18. javapJava class file disassemblerUsed 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:\work\jug\classes>javap Hello -cCompiled from "Hello.java"public class Hello extends java.lang.Object{public Hello();Code:0: aload_01: invokespecial #1; //Method java/lang/Object."<init>":()V4: returnpublic 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:\work\jug\classes>javap Hello -verboseCompiled 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>":()Vconst #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;)Vconst #5 = class#26; // Helloconst #6 = class#27; // java/lang/Objectconst #7 = Asciz<init>;const #8 = Asciz()V;

21. C:\work\jug\classes>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: returnLineNumberTable: line 1: 0 LocalVariableTable: Start Length Slot Name Signature 0 5 0 this LHello;

22. C:\work\jug\classes>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 MachineJVM is a stack-based machineEach thread has a stackStack stores framesFrame is created on method invocationFrame:Operand stackArray 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: areturnLocalVariableTable: Start Length Slot Name Signature 0 5 0 this LGet;

26. 0 1 2 3 4areturnaload_00002getfieldpublic java.lang.String getName(); Code: Stack=1, Locals=1, Args_size=1 0: aload_0 1: getfield #2; //Field name:Ljava/lang/String; 4: areturnLocalVariableTable: Start Length Slot Name Signature 0 5 0 this LGet;

27. 0 1 2 3 4B02A0002B4public java.lang.String getName(); Code: Stack=1, Locals=1, Args_size=1 0: aload_0 1: getfield #2; //Field name:Ljava/lang/String; 4: areturnLocalVariableTable: 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: areturnLocalVariableTable: 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: areturnLocalVariableTable: 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: areturnLocalVariableTable: Start Length Slot Name Signature 0 5 0 this LGet;

31. Stack OperationsAduppopswapdup_x1dup_x2B

32. Stack OperationsAduppopswapdup_x1dup_x2AB

33. Stack OperationsAduppopswapdup_x1dup_x2B

34. Stack OperationsBduppopswapdup_x1dup_x2A

35. Stack OperationsBduppopswapdup_x1dup_x2AB

36. Stack OperationsBduppopswapdup_x1dup_x2ABBA

37. Local Variables

38. Local Variablespublic int calculate(int);Code: Stack=2, Locals=2, Args_size=2 … LocalVariableTable: Start Length Slot Name Signature0 5 0 this LLocalVariables;0 5 1 value I

39. Local Variablespublic int calculate(int);Code: Stack=2, Locals=2, Args_size=2 … LocalVariableTable: Start Length Slot Name Signature0 5 0 this LLocalVariables;0 5 1 value Inumbered from 0

40. Local Variablesinstance methodshave this at 0public int calculate(int);Code: Stack=2, Locals=2, Args_size=2 … LocalVariableTable: Start Length Slot Name Signature0 5 0 this LLocalVariables;0 5 1 value I

41. Local VariablesThe table maps numbers to namespublic int calculate(int);Code: Stack=2, Locals=2, Args_size=2 … LocalVariableTable: Start Length Slot Name Signature0 5 0 this LLocalVariables;0 5 1 value I

42. Local VariablesSized explicitlypublic int calculate(int);Code: Stack=2, Locals=2, Args_size=2 … LocalVariableTable: Start Length Slot Name Signature0 5 0 this LLocalVariables;0 5 1 value I

43. Local VariablesStackLocal Variablesvaluevarvaluedepthldc"Hello"astore_0iconst_1astore_1aload_00011223344

44. Local VariablesStackLocal Variablesvaluevarvaluedepthldc"Hello"astore_0iconst_1astore_1aload_000"Hello"11223344

45. Local VariablesStackLocal Variablesvaluevarvaluedepthldc"Hello"astore_0iconst_1astore_1aload_0"Hello"0011223344

46. Local VariablesStackLocal Variablesvaluevarvaluedepthldc"Hello"astore_0iconst_1astore_1aload_0"Hello"00111223344

47. Local VariablesStackLocal Variablesvaluevarvaluedepthldc"Hello"astore_0iconst_1astore_1aload_0"Hello"00111223344

48. Local VariablesStackLocal Variablesvaluevarvaluedepthldc"Hello"astore_0iconst_1astore_1aload_0"Hello"00"Hello"111223344

49. Local Variables & StackloadStackLocal Variables Tablestore

50. Object Initializationnew <init><clinit>Instance initialization methodClass 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: newpublic 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>":()V12: putfield #3; //Field o:Ljava/lang/Object;15: return

54. Object Initialization: newpublic 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>":()V12: 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_010:iconst_2 11:putfield #3; //Field c:I 14:aload_015:iload_116:putfield #4; //Field b:I19:return

58. Method Invocationinvokestaticinvokeinterfaceinvokevirtualinvokespecialinvokedynamic

59. Parameter Passing

60. Parameter Passingparameterreturnvalue

61. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3;10: areturnvarvalueStack1valuedepth21323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn51623

62. Local Variablespublic java.lang.Object execute();0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStack1valuedepth21this323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn51623

63. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; //createRandomValue() 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStack1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn51623

64. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2;4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStackobjectref1valuedepth21323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn51623

65. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStackobjectref1valuedepth21this323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn51623

66. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStackobjectref1valuedepth21objectref32this3private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn51623

67. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3;//incValue10: areturnvarvalueStackobjectref1valuedepth21323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn5objectref1623

68. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue10: areturnvarvalueStackobjectref1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8;4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn5objectref1623

69. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue10: areturnvarvalueStackobjectref1valuedepth21X323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8;// Integer.intValue:()4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn5objectref1623

70. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue10: areturnvarvalueStackobjectref1valuedepth21132X3private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn5objectref1623

71. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue10: areturnvarvalueStackobjectref1valuedepth21X + 1323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd6: invokestatic #7; 9: areturn5objectref1623

72. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue10: areturnvarvalueStackobjectref1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7;//Integer.valueOf9: areturn5objectref1623

73. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue10: areturnvarvalueStackobjectref1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7;9: areturn5objectref1623

74. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStackobjectref1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7;9: areturn51623

75. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStack1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7;9: areturn51623

76. Local Variablespublic java.lang.Object execute(); 0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturnvarvalueStack1valuedepth21objectref323private java.lang.Integer incValue(java.lang.Integer);4varvalue0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7;9: areturn51623

77. Flow Control

78. Flow ControlGOTO

79. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn123

80. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturna123

81. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn101a23

82. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn101a23

83. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn123

84. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn100123

85. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn100123

86. Stackvaluedepthpublic int decide(int); Code: 0: iload_1 1: bipush 10 3: if_icmpge 8 6: iconst_0 7: ireturn 8: bipush 10010: ireturn123

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