How Scala code is expressed in the JVM

Koichi
Sakata （阪田浩一）
@jyukutyo
How
Scala
code
is

expressed
in
the
JVM

-‐
ScalaMatsuri
2016
-‐

関ジャバです！
2
@jyukutyo
produced three Java framework books
one was translated into Korean
JUG Leader (KansaiJUG)
blog：Fight the Future
http://jyukutyo.hatenablog.com/
FURYU CORPORATION
age: 36
Koichi Sakata (阪田浩一)
フリューの阪田です関ジャバの会長です

this
session
is

for
beginners

about
JVM

byte
codes
このセッションは
JVMのバイトコード初心者向けです

Do
you
know

what
happens

when
we
compile

Scala
code?
Scalaコードをコンパイルする時
何が起こるかご存じでしょうか？

Java
Scala
JRuby
Groovy
C
o
m
p
i
l
e
r
Class

file
cafe babe
0000 0032
0017 0100
JVM
Class

file
Class

file
Class

file
Class

file
Class

file § Runs
§ Interprets
§ Translates
bytecode
into
Native
Machine
Code

Have
you
ever

opened

a
class
file?
クラスファイルを開いたことはありますか

First,

let's
try
opening

a
class
file

from
a
simple

Java
code
Javaのクラスファイルを開いてみましょう

Hello
World
(Java)

public
class
HelloWorld
{

public
static
void
main(String[]
args)
{

System.out.println("Hello,
world!");

}

}

§ Compiled
into...

§ HelloWorld.class

open

HelloWorld.class

file

with
binary
editor

e.g.)
hexl-‐find-‐file

(Emacs)
クラスファイルをバイナリエディタで開く

cafe
babe
0000
0032
0017
0100
0a48
656c

6c6f
576f
726c
6407
0001
0100
106a
6176

612f
6c61
6e67
2f4f
626a
6563
7407
0003

0100
1048
656c
6c6f
576f
726c
642e
7363

616c
6101
001e
4c73
6361
6c61
2f72
6566

6c65
6374
2f53
6361
6c61
5369
676e
6174

7572
653b
0100
0562
7974
6573
0100
ef06

0115
3a51
2101
0209
0215
0921
0253
336d

593e
3c76
4e1d
3765
1505
1911
6102
1f66

5b42
2418
5050
0201
2109
3171
2144
0103

0d15
4121
0123
010a
0529
4155
0d1c
3770

2f3e
1448
0e5a
0a03
0f29
0122
6103
080e

0331
5111
2144
0106
670e
0c47
2e59
0503

1f31
1161
2111
387a
2516
3407
2242
0908

you
will

close
the
editor

right
away
すぐにエディタを閉じるでしょう

class
file

has
a
format
クラスファイルには
フォーマットがあります

ClassFile
{

u4

magic;

u2

minor_version;

u2

major_version;

u2

constant_pool_count;

cp_info

constant_pool[constant_pool_count-‐1];

u2

access_flags;

u2

this_class;

u2

super_class;

u2

interfaces_count;

u2

interfaces[interfaces_count];

...

-‐-‐
u1
represents
one-‐byte
quantity,

Hexadecimal
2
digit.

...

u2

fields_count;

field_info

fields[fields_count];

u2

methods_count;

method_info

methods[methods_count];

u2

attributes_count;

attribute_info
attributes[attributes_count];

}

-‐-‐
u1
represents
one-‐byte
quantity,

Hexadecimal
2
digit.
u1が1バイト
16進数では2桁になります

cafe
babe
0000
0032

ClassFile
{

u4

magic;

u2

minor_version;

u2

major_version;

"magic"
is
always

"0xCAFEBABE"

major:
0x0032
=
50

minor:
0x0000
=
0

50.0
means
Java
SE
6

51.0
is
7,
52.0
is
8...

but,
this
is

purgatory
でもこれは苦行です

Never
mind!

We
have

a
handy
tool!
便利なツールがあります！

javap
command

The
Java
Class
File
Disassembler

§ included
in
JDK

§ `javap
-‐v
[FQCN]`

§ Scala
REPL

§ scala>
:javap
[FQCN]

§ equivalent
to
`javap
-‐v`

http://docs.oracle.com/javase/7/docs/technotes/tools/windows/javap.html
javap - Java クラスファイル逆アセンブラ

so,
a
class
file

can
be

disassembled

to
Java
Classes
クラスファイルはJavaのクラスに
逆アセンブルできるのです

javap
-‐v
HelloWorld

public
class
HelloWorld

minor
version:
0

major
version:
52

Constant
pool:

#1
=
Methodref

#6.#15

//
java/lang/Object."<init>":()V

#2
=
Fieldref

#16.#17

//
java/lang/System.out:Ljava/io/
PrintStream;

#3
=
String

#18

//
Hello,
world!

#4
=
Methodref

#19.#20

//
java/io/PrintStream.println:(Ljava/
lang/String;)V

Constant
pool
is

a
table
of

symbolic

information
コンスタントプールは
シンボル情報のテーブル

public
static
void

main(java.lang.String[]);

Code:

0:
getstatic

#2

3:
ldc

#3

5:
invokevirtual
#4

8:
return

JVM
instructions

aaload aastore aconst_null aload
anewarray areturn arraylength astore
athrow baload bastore bipush
caload castore checkcast d2f
d2i d2l dadd daload
dastore dcmp<op> dconst_<d> ddiv
dload dmul dneg ...

about
200

instructions

Since
first

release
of
Java,

added
instruction

is
...
Javaが最初にリリースされてから
追加されたJVM命令は...

Initial
design

of
JVM

is
excellent!
JVMは初期設計から優秀だった！

instructions
of
method
call

invokevirtual
invoke
instance
method;

dispatched
based
on
class
invokeinterface invoke
interface
method
invokestatic invoke
static
method
invokespecial
invoke
method;

superclass,
private,

constructor...
invokedynamic invoke
dynamic
method

public
static
void


Code:

0:
getstatic

#2

3:
ldc

#3

5:
invokevirtual
#4

8:
return

-‐-‐
#0
=
index
of
Constant
pool

public
class
HelloWorld

Constant
pool:

#1
=
Methodref

#6.#15

//
java/lang/Object."<init>":()V

#2
=
Fieldref

#16.#17

//
java/lang/System.out:Ljava/io/
PrintStream;

#3
=
String

#18

//
Hello,
world!

#4
=
Methodref

#19.#20

//
java/io/PrintStream.println:(Ljava/
lang/String;)V

...

OK!

It's
Scala's
turn!

Hello
World
(Scala)

object
HelloWorld
{

def
main(args:
Array[String])
{

println("Hello,
world!")

}

}

§ Compiled
into...

§ HelloWorld.class

§ HelloWorld$.class

public
final
class
HelloWorld

Constant
pool:

#15
=
NameAndType

#13:#14

//
MODULE$:LHelloWorld$;

#16
=
Fieldref

#12.#15

//
HelloWorld$.MODULE$:LHelloWorld$;

#17
=
NameAndType

#9:#10

//
main:([Ljava/lang/String;)V

#18
=
Methodref

#12.#17

//
HelloWorld$.main:

([Ljava/lang/String;)V

...

public
static
void


0:
getstatic

#16

//
Field
HelloWorld$.

MODULE$:LHelloWorld$;

3:
aload_0

4:
invokevirtual
#18

//
Method
HelloWorld$.main:

([Ljava/lang/String;)V

7:
return

-‐-‐
4:
in
Java

HelloWorld$.MODULE$.main(arg);

public
final
class
HelloWorld$

-‐-‐
omit
Constant
pool

public
static
{};

0:
new

#2

//
class
HelloWorld$

3:
invokespecial
#12

//
Method
"<init>":()V

6:
return

-‐-‐
<init>
means
constructor

public
void

0:
getstatic

#19

//
Field
scala/Predef$.MODULE$:

Lscala/Predef$;

3:
ldc

#21

//
String
Hello,
world!

5:
invokevirtual
#25

//
Method
scala/Predef$.println:

(Ljava/lang/Object;)V

8:
return

The
following
samples

are
quoted

from
tutorials

in
Scala
official
site

http://docs.scala-‐lang.org/tutorials/
これ以降で使用するサンプルは
Scalaの公式サイトのチュートリアルです

trait
Similarity
{

def
isSimilar(x:
Any):
Boolean

def
isNotSimilar(x:
Any):
Boolean
=

!isSimilar(x)

}

class
Point(xc:
Int,
yc:
Int)

extends
Similarity
{

var
x:
Int
=
xc

var
y:
Int
=
yc

def
isSimilar(obj:
Any)
=

obj.isInstanceOf[Point]
&&

obj.asInstanceOf[Point].x
==
x

}

§ Compiled
into...

§ Similarity.class

§ Similarity$class.class

§ Point.class

public
interface
Similarity

public
abstract
boolean

isSimilar(java.lang.Object);

public
abstract
boolean

isNotSimilar(java.lang.Object);

public
abstract
class
Similarity$class

public
static
boolean

isNotSimilar(Similarity,j.l.Object);

0:
aload_0

1:
aload_1

2:
invokeinterface
#13,

2

//
InterfaceMethod

Similarity.isSimilar:

(Ljava/lang/Object;)Z

7:
ifeq

14

10:
iconst_0

11:
goto

15

14:
iconst_1

15:
ireturn

a
trait
is

divided
into

an
interface
and

an
abstract
class
トレイトはインタフェースと
抽象クラスに分かれます

abstract
class
Term

case
class
Var(name:
String)

extends
Term

case
class
Fun(arg:
String,
body:
Term)

extends
Term

case
class
App(f:
Term,
v:
Term)

extends
Term

§ Compiled
into...

§ Term.class

§ App.class

§ App$.class

§ Fun.class

§ Fun$.class

§ Var.class

§ Var$.class

public
class
App
extends
Term

implements
s.Product,s.Serializable

public
static

s.Option<scala.Tuple2<Term,
Term>>

unapply(App);

public
static
App
apply(Term,
Term);

public
...
tupled();

public
...
curried();

public
Term
f();

public
Term
v();

public
App
copy(Term,
Term);

public
j.l.String
productPrefix();

public
boolean
equals(j.l.Object);

...

public
final
class
App$
extends

s.r.AbstractFunction2<Term,
Term,
App>

implements
scala.Serializable

public
App
apply(Term,
Term);

0:
new

#23

//
class
App

3:
dup

4:
aload_1

5:
aload_2

6:
invokespecial
#26

//
Method
App."<init>":(LTerm;LTerm;)V

9:
areturn

Fun
and
Var
looks

the
same
as
App
FunとVarも同様の内容です

Scala
compiler

generates

many
functions

from
few
snippets

of
code
Scalaコンパイラは少ないコードから
多くの機能を生成しています

object
MatchTest2
extends
App
{

def
matchTest(x:
Any):
Any
=
x
match

{

case
1
=>
"one"

case
"two"
=>
2

case
y:
Int
=>
"scala.Int"

}

println(matchTest("two"))

}

§ Compiled
into...

§ MatchTest2.class

§ MatchTest2$.class

§ MatchTest2$delayedInit$body.class

public
final
class
MatchTest2$

implements
scala.App

7:
invokestatic

#67

//
Method
BoxesRunTime.equals:..

10:
ifeq

19

13:
ldc

#69

//
String
one

15:
astore_3

16:
goto

46

19:
ldc

#71

//
String
two

21:
aload_2

22:
invokevirtual
#74

//
Method
java/lang/Object.equals:

19:
ldc

#71

//
String
two

21:
aload_2

22:
invokevirtual
#74

//
Method
java/lang/Object.equals:

(Ljava/lang/Object;)Z

25:
ifeq

36

28:
iconst_2

32:
astore_3

33:
goto

46

36:
aload_2

37:
instanceof

#76

//
class
java/lang/Integer

40:
ifeq

48

43:
ldc

#78

//
String
scala.Int

45:
astore_3

46:
aload_3

47:
areturn

...

Compiled
into

if
statements
if文へコンパイルしています

in
case
of

another
Pattern

Matching...
違うパターンマッチの場合

object
MatchTest1
{

def
matchTest(x:
Int):
String
=
x

match
{

case
1
=>
"one"

case
2
=>
"two"

case
_
=>
"many"

}

def
main(args:
Array[String])
{

println(matchTest(3))

}

}

3:
tableswitch

{
//
1
to
2

1:
34

2:
29

default:
24

}

24:
ldc

#16

//
String
many

26:
goto

36

29:
ldc

#18

//
String
two

31:
goto

36

34:
ldc

#20

//
String
one

36:
areturn

Compiled
into

a
switch
statement
switch文にコンパイルしています

object
CurryTest
{

def
filter(xs:
List[Int],

p:
Int
=>
Boolean):
List[Int]
=

if
(xs.isEmpty)
xs

else
if
(p(xs.head))

xs.head
::
filter(xs.tail,
p)

else
filter(xs.tail,
p)

def
modN(n:
Int)(x:
Int)
=

((x
%
n)
==
0)

def
main(args:
Array[String])
{

val
nums
=
List(1,
2,
3,
4,

5,
6,
7,
8)

println(filter(nums,
modN(2)))

println(filter(nums,
modN(3)))

}

}

§ Compiled
into...

§ CurryTest.class

§ CurryTest$.class

§ CurryTest$$anonfun$main$1.class

§ CurryTest$$anonfun$main$2.class

public
final
class

CurryTest$$anonfun$main$1
extends

scala.runtime.AbstractFunction1$mcZI$sp

implements
scala.Serializable

public
boolean
apply$mcZI$sp(int);

3:
iconst_2

4:
iload_1

5:
invokevirtual
#33

//
Method
CurryTest$.modN:(II)Z

8:
ireturn

public
final
class

extends


implements
scala.Serializable

public
boolean
apply$mcZI$sp(int);

3:
iconst_3

4:
iload_1

5:
invokevirtual
#33

//
Method
CurryTest$.modN:(II)Z

8:
ireturn

each
partial

application

creates

a
new
class
部分適用の箇所ごとに
クラスを生成しています

public
final
class
CurryTest$

...

public
void

63:
invokespecial
#82

//
Method
CurryTest$$anonfun$main$1.

"<init>":()V

66:
invokevirtual
#41

//
Method
filter:(Lscala/collection/

immutable/List;Lscala/Function1;)

Lscala/collection/immutable/List;

69:
invokevirtual
#86

//
Method


public
final
class

extends


implements
scala.Serializable

CurryTest$$anonfun
$main$2

is
same
$2も同様です

81:
invokespecial
#89

//
Method
CurryTest$$anonfun$main$2.

"<init>":()V

84:
invokevirtual
#41

//
Method
filter:(Lscala/collection/

immutable/List;Lscala/Function1;)

Lscala/collection/immutable/List;

87:
invokevirtual
#86

//
Method


§ Summary

§ JVM
is
excellent!

§ class
file
has
a
format

§ javap
is
useful
to
understand

Scala
at
the
bytecode
level

Try
typing
"javap"

after
you
compile

Scala
code

How Scala code is expressed in the JVM

How Scala code is expressed in the JVM

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