java bytecode fundamentals - jug.lv

Java Bytecode Fundamentals

JUG.LV 2011, Riga

whoami

http://arhipov.blogspot.com@antonarhipov

@javarebel

Anton ArhipovZeroTurnaround

JRebel

http://arhipov.blogspot.com/

who-are-you?

1 + 21 2

+

1 + 21 2

+

1 2 +

1 + 21 2

+

1 2 + PUSH 1 1

1 + 21 2

+

1 2 + PUSH 1PUSH 2

21

1 + 21 2

+

1 2 + PUSH 1PUSH 2ADD

3

1 + 21 2

+

1 2 + ICONST_1ICONST_2IADD

3

? = 1 + 2

Byte Code

One-byte instructions256 possible opcodes~200 in use

The Master Plan

javapStack MachineObjects and MethodsFlow Control

javap

Java 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

C:\work\jug\classes>javap Hello -cCompiled 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

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;

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: return LineNumberTable: line 1: 0

LocalVariableTable: Start Length Slot Name Signature 0 5 0 this LHello;

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;

Stack Machine

JVM is a stack-based machineEach thread has a stackStack stores framesFrame is created on method invocationFrame:

Operand stackArray of local variables

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;


aload_0 getfield

00 02 areturn

0 1 2 3 4


2A B4 00 02 B0

0 1 2 3 4

Stack Operations

AB

duppopswapdup_x1dup_x2

Stack Operations

A

B


A

Stack Operations


AB

Stack Operations


AB

B

Stack Operations


AB

BBA

Local Variables

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

Local Variables



numbered

from 0

Local Variables



instance methodshave this at 0

Local Variables



The table maps

numbers to names

Local Variables



Sized explicitly

Local Variables

ldc "Hello"astore_0iconst_1astore_1aload_0

depth value

Stack

var

value

Local Variables

0

1

2

3

4

0

1

2

3

4

Local Variables


depth value

Stack

var

value

Local Variables

"Hello"0

1

2

3

4

0

1

2

3

4

Local Variables


depth value

Stack

var

value

Local Variables

"Hello"0

1

2

3

4

0

1

2

3

4

1

Local Variables


depth value

Stack

var

value

Local Variables

"Hello"0

1

2

3

4

0

1

2

3

4

1

"Hello"

Local Variables & Stack

StackLocal

Variables Table

store

load

Object Initialization

new <init> <clinit>

Instance initialization method

Class and interface

initialization method

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

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

<clinit>

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

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

Method Invocation

invokestaticinvokeinterfaceinvokevirtualinvokespecialinvokedynamic

Parameter Passing

Parameter Passing

parameter

returnvalue

0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3;10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

Stack

1

2

3

var

value

0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn

private java.lang.Integer incValue(java.lang.Integer);

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

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




this

Stack

0: aload_0 1: invokespecial #2; //createRandomValue()

4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; 10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

Stack


depth value

1

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4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

Stack


depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




this

objectref

Stack


depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




this

objectref

objectref

Stack

0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue

10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

objectref

Stack


10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

objectref

objectref

Stack


10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value

0: aload_1 1: invokevirtual #8; // Integer.intValue:()

4: iconst_1 5: iadd 6: invokestatic #7; 9: areturn



objectref

objectref

X

Stack

0: aload_0 1: invokespecial #2; 4: astore_1 5: aload_0 6: aload_1 7: invokespecial #3; //incValue 10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

objectref

1

X

Stack


10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

objectref

X + 1

Stack


10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value

0: aload_1 1: invokevirtual #8; 4: iconst_1 5: iadd 6: invokestatic #7; //Integer.valueOf 9: areturn



objectref

objectref

objectref

Stack


10: areturn

depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

objectref

objectref

Stack


depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

objectref

Stack


depth value

1

2

3

4

5

6

1

2

3

var

value

Local Variables

1

2

3

var

value




objectref

Stack

Flow Control

Flow Control

GOTO

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

depth value

Stack


1

2

3

depth value

Stack

a


1

2

3

depth value

Stack

10a


1

2

3

depth value

Stack


1

2

3

depth value

Stack

100


1

2

3

depth value

Stack

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java bytecode fundamentals - jug.lv

Technology

stack operationsaduppopswapdup

stack operationsbduppopswapdup

stack machinejvm

string hello

string getnamecode

args ljavalangstring23

javapublic class hello

stackbased machineeach