latjug. java bytecode fundamentals

Java Bytecode

Fundamentals

JUG.LV 2011, Riga

whoami

http://arhipov.blogspot.com

@antonarhipov

@javarebel

Anton Arhipov ZeroTurnaround

JRebel

who-are-you?

1 + 2

1 + 2 1 2

+

1 + 2 1 2

+

1 2 +

1 + 2 1 2

+

1 2 +

1 + 2 1 2

+

1 2 + PUSH 1 1

1 + 2 1 2

+

1 2 + PUSH 1

PUSH 2 2 1

1 + 2 1 2

+

1 2 + PUSH 1

PUSH 2

ADD

3

1 + 2 1 2

+

1 2 + ICONST_1

ICONST_2

IADD

3

? = 1 + 2

Byte Code

One-byte instructions

256 possible opcodes

~200 in use

Byte Code

One-byte instructions

256 possible opcodes

~200 in use

The Master Plan

javap

Stack Machine

Objects and Methods

Flow Control

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

C:\work\jug\classes>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

C:\work\jug\classes>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;

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 machine

Each thread has a stack

Stack stores frames

Frame is created on method invocation

Frame:

Operand stack

Array of local variables

Frame

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;

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;

aload_0 getfield 00 02 areturn

0 1 2 3 4

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;

2A B4 00 02 B0

0 1 2 3 4

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;

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;

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;

Stack Operations

A B

dup

pop

swap

dup_x1

dup_x2

Stack Operations

A

B

dup

pop

swap

dup_x1

dup_x2

A

Stack Operations

dup

pop

swap

dup_x1

dup_x2

A B

Stack Operations

dup

pop

swap

dup_x1

dup_x2

A B

Stack Operations

dup

pop

swap

dup_x1

dup_x2

A B

B

Stack Operations

dup

pop

swap

dup_x1

dup_x2

A B

B B A

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

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

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

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

The table

maps

numbers to

names

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

Sized explicitly

Local Variables

ldc "Hello"

astore_0

iconst_1

astore_1

aload_0

depth value

Stack

var value

Local Variables

0

1

2

3

4

0

1

2

3

4

Local Variables

ldc "Hello"

astore_0

iconst_1

astore_1

aload_0

depth value

Stack

var value

Local Variables

"Hello" 0

1

2

3

4

0

1

2

3

4

Local Variables

ldc "Hello"

astore_0

iconst_1

astore_1

aload_0

depth value

Stack

var value

Local Variables

"Hello" 0

1

2

3

4

0

1

2

3

4

Local Variables

ldc "Hello"

astore_0

iconst_1

astore_1

aload_0

depth value

Stack

var value

Local Variables

"Hello" 0

1

2

3

4

0

1

2

3

4

1

Local Variables

ldc "Hello"

astore_0

iconst_1

astore_1

aload_0

depth value

Stack

var value

Local Variables

"Hello" 0

1

2

3

4

0

1

2

3

4

1

Local Variables

ldc "Hello"

astore_0

iconst_1

astore_1

aload_0

depth value

Stack

var value

Local Variables

"Hello" 0

1

2

3

4

0

1

2

3

4

1

"Hello"

Local Variables & Stack

Stack Local

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: 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: {}

Object Initialization: {}

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

invokestatic

invokeinterface

invokevirtual

invokespecial

invokedynamic

Parameter Passing

Parameter Passing

parameter

return

value

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

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();

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

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();

objectref

Stack

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

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();

objectref

Stack

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

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();

this

objectref

Stack

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

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();

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

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();

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

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();

objectref

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

0: aload_1

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

4: iconst_1

5: iadd

6: invokestatic #7;

9: areturn

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

public java.lang.Object execute();

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

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();

objectref

objectref

1

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

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();

objectref

objectref

X + 1

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

0: aload_1

1: invokevirtual #8;

4: iconst_1

5: iadd

6: invokestatic #7; //Integer.valueOf

9: areturn

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

public java.lang.Object execute();

objectref

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

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();

objectref

objectref

objectref

Stack

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

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();

objectref

objectref

Stack

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

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();

objectref

Stack

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

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();

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

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

a

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

10 a

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

10 a

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

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

100

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

100

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

ant.arhipov@gmail.com

http://arhipov.blogspot.com

@antonarhipov

@javarebel