How Scala code is expressed in the JVM
9 likes8,454 views
- The document discusses how Scala code is compiled to Java Virtual Machine (JVM) bytecodes. It begins with an introduction to Koichi Sakata, the presenter, and an overview that the session will look at JVM bytecodes for beginners. - The presentation then asks if attendees know what happens when Scala code is compiled. It is explained that Scala code, like code from other JVM languages, is compiled to class files that contain JVM bytecodes. - An example Java class file is opened in a binary editor to show the JVM bytecode format. It is explained that class files have a defined format with elements like magic number, version, and bytecode instructions.
![Hello
World
(Java)
public
class
HelloWorld
{
public
static
void
main(String[]
args)
{
System.out.println("Hello,
world!");
}
}
§ Compiled
into...
§ HelloWorld.class](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-9-320.jpg)
![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.](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-14-320.jpg)
![...
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](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-15-320.jpg)
![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](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-24-320.jpg)
![public
static
void
main(java.lang.String[]);
Code:
0:
getstatic
#2
3:
ldc
#3
5:
invokevirtual
#4
8:
return](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-29-320.jpg)
![public
static
void
main(java.lang.String[]);
Code:
0:
getstatic
#2
3:
ldc
#3
5:
invokevirtual
#4
8:
return
-‐-‐
#0
=
index
of
Constant
pool](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-36-320.jpg)
![Hello
World
(Scala)
object
HelloWorld
{
def
main(args:
Array[String])
{
println("Hello,
world!")
}
}
§ Compiled
into...
§ HelloWorld.class
§ HelloWorld$.class](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-40-320.jpg)
![public
static
void
main(java.lang.String[]);
0:
getstatic
#16
//
Field
HelloWorld$.
MODULE$:LHelloWorld$;
3:
aload_0
4:
invokevirtual
#18
//
Method
HelloWorld$.main:
([Ljava/lang/String;)V
7:
return
-‐-‐
4:
in
Java
HelloWorld$.MODULE$.main(arg);](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-42-320.jpg)
![public
void
main(java.lang.String[]);
0:
getstatic
#19
//
Field
scala/Predef$.MODULE$:
Lscala/Predef$;
3:
ldc
#21
//
String
Hello,
world!
5:
invokevirtual
#25
//
Method
scala/Predef$.println:
(Ljava/lang/Object;)V
8:
return](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-44-320.jpg)
![trait
Similarity
{
def
isSimilar(x:
Any):
Boolean
def
isNotSimilar(x:
Any):
Boolean
=
!isSimilar(x)
}
class
Point(xc:
Int,
yc:
Int)
extends
Similarity
{
var
x:
Int
=
xc
var
y:
Int
=
yc
def
isSimilar(obj:
Any)
=
obj.isInstanceOf[Point]
&&
obj.asInstanceOf[Point].x
==
x
}](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-48-320.jpg)
![object
MatchTest1
{
def
matchTest(x:
Int):
String
=
x
match
{
case
1
=>
"one"
case
2
=>
"two"
case
_
=>
"many"
}
def
main(args:
Array[String])
{
println(matchTest(3))
}
}](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-71-320.jpg)
![object
CurryTest
{
def
filter(xs:
List[Int],
p:
Int
=>
Boolean):
List[Int]
=
if
(xs.isEmpty)
xs
else
if
(p(xs.head))
xs.head
::
filter(xs.tail,
p)
else
filter(xs.tail,
p)
def
modN(n:
Int)(x:
Int)
=
((x
%
n)
==
0)](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-76-320.jpg)
![def
main(args:
Array[String])
{
val
nums
=
List(1,
2,
3,
4,
5,
6,
7,
8)
println(filter(nums,
modN(2)))
println(filter(nums,
modN(3)))
}
}](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-77-320.jpg)
![def
main(args:
Array[String])
{
val
nums
=
List(1,
2,
3,
4,
5,
6,
7,
8)
println(filter(nums,
modN(2)))
println(filter(nums,
modN(3)))
}
}](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-82-320.jpg)
![public
final
class
CurryTest$
...
public
void
main(java.lang.String[]);
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
scala/Predef$.println:
(Ljava/lang/Object;)V](https://image.slidesharecdn.com/scalamatsuri2016-160130065908/85/How-Scala-code-is-expressed-in-the-JVM-84-320.jpg)
How Scala code is expressed in the JVM
- 1. Koichi Sakata （阪田 浩一） @jyukutyo How Scala code is expressed in the JVM -‐ ScalaMatsuri 2016 -‐
- 2. 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
- 3. I ♥ JVM
- 4. 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
- 5. 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？
- 6. 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
- 7. 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
- 8. 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
- 9. Hello World (Java) public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, world!"); } } § Compiled into... § HelloWorld.class
- 10. 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
- 11. 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
- 12. you will close the editor right away すぐにエディタを閉じるでしょう
- 13. 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
- 14. 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.
- 15. ... 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
- 16. Let's try again
- 17. 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
- 18. cafe babe 0000 0032 ClassFile { u4 magic; u2 minor_version; u2 major_version;
- 19. "magic" is always "0xCAFEBABE"
- 20. major: 0x0032 = 50 minor: 0x0000 = 0 50.0 means Java SE 6 51.0 is 7, 52.0 is 8...
- 21. Yes!
- 22. but, this is purgatory でもこれは苦行です
- 23. Never mind! We have a handy tool! 便利なツールがあります！
- 24. 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
- 25. 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
- 27. 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
- 28. 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
- 29. public static void main(java.lang.String[]); Code: 0: getstatic #2 3: ldc #3 5: invokevirtual #4 8: return
- 30. 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 ...
- 32. 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...
- 33. only ONE!
- 34. Initial design of JVM is excellent! JVMは初期設計から優秀だった！
- 35. 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
- 36. public static void main(java.lang.String[]); Code: 0: getstatic #2 3: ldc #3 5: invokevirtual #4 8: return -‐-‐ #0 = index of Constant pool
- 37. 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 ...
- 38. OK! It's Scala's turn!
- 39. Hello world
- 40. Hello World (Scala) object HelloWorld { def main(args: Array[String]) { println("Hello, world!") } } § Compiled into... § HelloWorld.class § HelloWorld$.class
- 41. 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 ...
- 42. public static void main(java.lang.String[]); 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);
- 43. 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
- 44. public void main(java.lang.String[]); 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
- 45. you can read it!
- 46. 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
- 47. Traits
- 48. 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 }
- 49. § Compiled into... § Similarity.class § Similarity$class.class § Point.class
- 50. javap -‐v
- 51. public interface Similarity public abstract boolean isSimilar(java.lang.Object); public abstract boolean isNotSimilar(java.lang.Object);
- 52. 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
- 53. 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
- 54. Case Classes
- 55. 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
- 56. § Compiled into... § Term.class § App.class § App$.class § Fun.class § Fun$.class § Var.class § Var$.class
- 57. javap -‐v
- 58. 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); ...
- 59. 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
- 60. Fun and Var looks the same as App FunとVarも同様の内容です
- 61. 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
- 62. Pattern Matching
- 63. 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")) }
- 64. § Compiled into... § MatchTest2.class § MatchTest2$.class § MatchTest2$delayedInit$body.class
- 65. javap -‐v
- 66. 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:
- 67. 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
- 68. 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 ...
- 69. Compiled into if statements if文へコンパイルしています
- 70. in case of another Pattern Matching... 違うパターンマッチの場合
- 71. 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)) } }
- 72. javap -‐v
- 73. 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
- 74. Compiled into a switch statement switch文にコンパイルしています
- 75. Currying
- 76. 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)
- 77. 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))) } }
- 78. § Compiled into... § CurryTest.class § CurryTest$.class § CurryTest$$anonfun$main$1.class § CurryTest$$anonfun$main$2.class
- 79. javap -‐v
- 80. public final class CurryTest$$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
- 81. public final class CurryTest$$anonfun$main$2 extends scala.runtime.AbstractFunction1$mcZI$sp implements scala.Serializable public boolean apply$mcZI$sp(int); 3: iconst_3 4: iload_1 5: invokevirtual #33 // Method CurryTest$.modN:(II)Z 8: ireturn
- 82. def main(args: Array[String]) { val nums = List(1, 2, 3, 4, 5, 6, 7, 8) println(filter(nums, modN(2))) println(filter(nums, modN(3))) } }
- 83. each partial application creates a new class 部分適用の箇所ごとに クラスを生成しています
- 84. public final class CurryTest$ ... public void main(java.lang.String[]); 63: invokespecial #82 // Method CurryTest$$anonfun$main$1. "<init>":()V 66: invokevirtual #41 // Method filter:(Lscala/collection/ immutable/List;Lscala/Function1;) Lscala/collection/immutable/List; 69: invokevirtual #86 // Method scala/Predef$.println: (Ljava/lang/Object;)V
- 85. public final class CurryTest$$anonfun$main$1 extends scala.runtime.AbstractFunction1$mcZI$sp implements scala.Serializable
- 87. 81: invokespecial #89 // Method CurryTest$$anonfun$main$2. "<init>":()V 84: invokevirtual #41 // Method filter:(Lscala/collection/ immutable/List;Lscala/Function1;) Lscala/collection/immutable/List; 87: invokevirtual #86 // Method scala/Predef$.println: (Ljava/lang/Object;)V
- 88. § 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
- 89. Thank you!