The document discusses Scala under the hood by explaining key concepts like the JVM, bytecode, val vs var vs def, and objects. Some key points: 1. Scala compiles to JVM bytecode like Java and uses frames on the stack for method calls. 2. val declares immutable fields, var mutable fields, and def defines methods. 3. Objects in Scala are singleton objects that behave like static members in Java, with the object's methods called on its MODULE$ field.

1. SCALA UNDER THE HOOD
TZOFIA SHIFTAN / OVEROPS

3. AGENDA
1. JVM 101
2. THE COOL STUFF

4. JVM 101

5. bytecode
Noun
Java bytecode is the instruction set of the
Java virtual machine
Wikipedia

6. JVM
Example.classExample.scala
scalac scala
javap
class file

7. javap

8. $ javap Example.class
Compiled from "Example.scala"
public class Example {
public int b();
public int bar(int);
public Example();
}
javap

9. $ javap -p Example.class
Compiled from "Example.scala"
public class Example {
private int a;
private final int b;
private int a();
private void a_$eq(int);
public int b();
private void foo();
public int bar(int);
public Example();
}
javap

10. $ javap -c Example.class
Compiled from "Example.scala"
public class Example {
public int b();
Code:
0: aload_0
1: getfield #21 // Field b:I
4: ireturn
public int bar(int);
Code:
0: iconst_3
1: iload_1
2: imul
3: ireturn
...
javap

11. JVM
THREADTHREAD THREAD
STACK STACK STACK
FRAME
FRAME
FRAME
FRAME
FRAME
FRAME
FRAME
FRAME
FRAME

12. FRAME
OPERAND STACKLOCAL VARIABLES
this
0 1 2

13. def add(a : Int, b : Int) = a+b
iload_0
iload_1
iadd
ireturn
add(2,5)
FRAME
LOCAL VARIABLES OPERAND STACK
a b
2 5

14. def add(a : Int, b : Int) = a+b
iload_0
iload_1
iadd
ireturn
add(2,5)
FRAME
LOCAL VARIABLES OPERAND STACK
a b
2 5
2

15. def add(a : Int, b : Int) = a+b
iload_0
iload_1
iadd
ireturn
add(2,5)
FRAME
LOCAL VARIABLES OPERAND STACK
a b
2 5
2
5

16. def add(a : Int, b : Int) = a+b
iload_0
iload_1
iadd
ireturn
add(2,5)
FRAME
LOCAL VARIABLES OPERAND STACK
a b
2 5
7
2
5
7
+
=
2
5

17. FRAME
LOCAL VARIABLES OPERAND STACK
a b
2 5
7
def add(a : Int, b : Int) = a+b
iload_0
iload_1
iadd
ireturn
add(2,5)

18. THE COOL STUFF

19. val vs var vs def

20. val
var
def
class Var {
var a = 1
}
class Val {
val a = 1
}
class Def {
def a = 1
}

21. javap -p Var
javap -p Val
javap -p Def
val
var
def

22. private final int a;
public int a();
public Val();
private int a;
public int a();
public void a_$eq(int);
public Var();
public int a();
public Def();
val
var
def

23. private final int a;
public int a();
public Val();
private int a;
public int a();
public void a_$eq(int);
public Var();
public int a();
public Def();
val
var
def

24. private final int a;
public int a();
public Val();
private int a;
public int a();
public void a_$eq(int);
public Var();
public int a();
public Def();
val
var
def

25. private final int a;
public int a();
public Val();
private int a;
public int a();
public void a_$eq(int);
public Var();
public int a();
public Def();
val
var
def

26. public int a();
aload_0
getfield #13 // Field a:I
ireturn
public int a();
iconst_1
ireturn
return this.a
return 1
val
var
def

27. private final int a;
public int a();
public Val();
private int a;
public int a();
public void a_$eq(int);
public Var();
public int a();
public Def();
val
var
def

28. public void a_$eq(int);
aload_0
iload_1
putfield #13 // Field a:I
return
this.a = a
val
var
def

29. private final int a;
public int a();
public Val();
private int a;
public int a();
public void a_$eq(int);
public Var();
public int a();
public Def();
val
var
def

30. public Va[r|l]();
aload_0
invokespecial #19 // Method java/lang/Object."<init>":()V
aload_0
iconst_1
putfield #13 // Field a:I
return
public Def();
aload_0
invokespecial #16 // Method java/lang/Object."<init>":()V
return
val
var
def
this.a = 1
return

31. getter setter evaluation
val 1
var 1
def multiple

32. lazy val

33. val
lazy val
class Val {
val a = 1
}
class LazyVal {
lazy val a = 1
}

34. javap -p Val
javap -p LazyVal
val
lazy val

35. private final int a;
public Val();
public int a();
private int a;
public LazyVal();
private volatile boolean bitmap$0;
private int a$lzycompute();
public int a();
val
lazy val

36. private final int a;
public Val();
public int a();
private int a;
public LazyVal();
private volatile boolean bitmap$0;
private int a$lzycompute();
public int a();
val
lazy val

37. private final int a;
public Val();
public int a();
private int a;
public LazyVal();
private volatile boolean bitmap$0;
private int a$lzycompute();
public int a();
val
lazy val

38. public Val();
aload_0
invokespecial #19 // Method java/lang/Object."<init>":()V
aload_0
iconst_1
putfield #13 // Field a:I
return
public LazyVal();
aload_0
Invokespecial #28 // Method java/lang/Object."<init>":()V
return
super()
val
lazy val

39. private final int a;
public Val();
public int a();
private int a;
public LazyVal();
private volatile boolean bitmap$0;
private int a$lzycompute();
public int a();
val
lazy val

40. private int a$lzycompute();
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: getfield #16 // Field bitmap$0:Z
8: ifne 21
11: aload_0
12: iconst_1
13: putfield #18 // Field a:I
16: aload_0
17: iconst_1
18: putfield #16 // Field bitmap$0:Z
21: aload_1
22: monitorexit
23: goto 29
26: aload_1
27: monitorexit
28: athrow
29: aload_0
30: getfield #18 // Field a:I
33: ireturn

41. private int a$lzycompute();
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: getfield #16 // Field bitmap$0:Z
8: ifne 21
11: aload_0
12: iconst_1
13: putfield #18 // Field a:I
16: aload_0
17: iconst_1
18: putfield #16 // Field bitmap$0:Z
21: aload_1
22: monitorexit
23: goto 29
26: aload_1
27: monitorexit
28: athrow
29: aload_0
30: getfield #18 // Field a:I
33: ireturn
synchronize start

42. private int a$lzycompute();
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: getfield #16 // Field bitmap$0:Z
8: ifne 21
11: aload_0
12: iconst_1
13: putfield #18 // Field a:I
16: aload_0
17: iconst_1
18: putfield #16 // Field bitmap$0:Z
21: aload_1
22: monitorexit
23: goto 29
26: aload_1
27: monitorexit
28: athrow
29: aload_0
30: getfield #18 // Field a:I
33: ireturn
bitmap$0 ? 0

43. private int a$lzycompute();
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: getfield #16 // Field bitmap$0:Z
8: ifne 21
11: aload_0
12: iconst_1
13: putfield #18 // Field a:I
16: aload_0
17: iconst_1
18: putfield #16 // Field bitmap$0:Z
21: aload_1
22: monitorexit
23: goto 29
26: aload_1
27: monitorexit
28: athrow
29: aload_0
30: getfield #18 // Field a:I
33: ireturn
this.a = 1
this.bitmap$0 = 1
synchronize end
return this.a
bitmap$0 == 0

44. private int a$lzycompute();
0: aload_0
1: dup
2: astore_1
3: monitorenter
4: aload_0
5: getfield #16 // Field bitmap$0:Z
8: ifne 21
11: aload_0
12: iconst_1
13: putfield #18 // Field a:I
16: aload_0
17: iconst_1
18: putfield #16 // Field bitmap$0:Z
21: aload_1
22: monitorexit
23: goto 29
26: aload_1
27: monitorexit
28: athrow
29: aload_0
30: getfield #18 // Field a:I
33: ireturn
synchronize end
return this.a
bitmap$0 == 1

45. private int a$lzycompute() {
LazyVal lazyVal = this;
synchronized (lazyVal) {
if (!this.bitmap$0) {
this.a = 1;
this.bitmap$0 = true;
}
}
return this.a;
}

46. private int a$lzycompute() {
LazyVal lazyVal = this;
synchronized (lazyVal) {
if (!this.bitmap$0) {
this.a = 1;
this.bitmap$0 = true;
}
}
return this.a;
}

47. private int a$lzycompute() {
LazyVal lazyVal = this;
synchronized (lazyVal) {
if (!this.bitmap$0) {
this.a = 1;
this.bitmap$0 = true;
}
}
return this.a;
}

48. private final int a;
public Val();
public int a();
private int a;
public LazyVal();
private volatile boolean bitmap$0;
private int a$lzycompute();
public int a();
val
lazy val

49. public int a();
0: aload_0
1: getfield #16 // Field bitmap$0:Z
4: ifne 14
7: aload_0
8: invokespecial #24 // Method a$lzycompute:()I
11: goto 18
14: aload_0
15: getfield #18 // Field a:I
18: ireturn

50. public int a();
0: aload_0
1: getfield #16 // Field bitmap$0:Z
4: ifne 14
7: aload_0
8: invokespecial #24 // Method a$lzycompute:()I
11: goto 18
14: aload_0
15: getfield #18 // Field a:I
18: ireturn
bitmap$0 ? 0

51. public int a();
0: aload_0
1: getfield #16 // Field bitmap$0:Z
4: ifne 14
7: aload_0
8: invokespecial #24 // Method a$lzycompute:()I
11: goto 18
14: aload_0
15: getfield #18 // Field a:I
18: ireturn
this.a$lzycompute()
return ^
bitmap$0 == 0

52. public int a();
0: aload_0
1: getfield #16 // Field bitmap$0:Z
4: ifne 14
7: aload_0
8: invokespecial #24 // Method a$lzycompute:()I
11: goto 18
14: aload_0
15: getfield #18 // Field a:I
18: ireturn
bitmap$0 == 1 return this.a

53. evaluation
val c’tor
lazy val lzycompute

54. object

55. object Hello {
def main(args: Array[String]) {
println("Hello, Scala Swarm!")
}
}

56. Hello.scala
Hello.class
Hello$.class
scalac

57. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

58. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

59. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

60. public static void main(java.lang.String[]);
getstatic #16 // Field Hello$.MODULE$:LHello$;
aload_0
invokevirtual #18 // Method Hello$.main:([Ljava/lang/String;)V
return
Hello$.MODULE$.main(args)

61. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

62. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

63. public static {};
new #2 // class Hello$
invokespecial #12 // Method "<init>":()V
return
new Hello$()

64. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

65. private Hello$();
aload_0
invokespecial #29 // Method java/lang/Object."<init>":()V
aload_0
putstatic #31 // Field MODULE$:LHello$;
return
MODULE$ = this

66. Compiled from "Hello.scala"
public final class Hello {
public static void main(java.lang.String[]);
}
Compiled from "Hello.scala"
public final class Hello$ {
public static Hello$ MODULE$;
public static {};
private Hello$();
public void main(java.lang.String[]);
}

67. public void main(java.lang.String[]);
getstatic #20 // Field scala/Predef$.MODULE$:Lscala/Predef$;
ldc #22 // String Hello, Scala Swarm!
invokevirtual #26 // Method scala/Predef$.println:(Ljava/lang/Object;)V
return
println(“Hello, Scala Swarm!”)

68. public final class Hello$ {
public static Hello$ MODULE$;
public static {
new Hello$();
}
private Hello$() {
MODULE$ = this;
}
public void main(String[] args) {
println("Hello, Scala Swarm!");
}
}
public final class Hello {
public static void main(final String[] args) {
Hello$.MODULE$.main(args);
}
}

69. public final class Hello$ {
public static Hello$ MODULE$;
public static {
new Hello$();
}
private Hello$() {
MODULE$ = this;
}
public void main(String[] args) {
println("Hello, Scala Swarm!");
}
}
public final class Hello {
public static void main(final String[] args) {
Hello$.MODULE$.main(args);
}
}

70. public final class Hello$ {
public static Hello$ MODULE$;
public static {
new Hello$();
}
private Hello$() {
MODULE$ = this;
}
public void main(String[] args) {
println("Hello, Scala Swarm!");
}
}
public final class Hello {
public static void main(final String[] args) {
Hello$.MODULE$.main(args);
}
}

71. public final class Hello$ {
public static Hello$ MODULE$;
public static {
new Hello$();
}
private Hello$() {
MODULE$ = this;
}
public void main(String[] args) {
println("Hello, Scala Swarm!");
}
}
public final class Hello {
public static void main(final String[] args) {
Hello$.MODULE$.main(args);
}
}

72. public final class Hello$ {
public static Hello$ MODULE$;
public static {
new Hello$();
}
private Hello$() {
MODULE$ = this;
}
public void main(String[] args) {
println(“Hello, Scala Swarm!");
}
}
public final class Hello {
public static void main(final String[] args) {
Hello$.MODULE$.main(args);
}
}

73. public final class Hello$ {
public static Hello$ MODULE$;
public static {
new Hello$();
}
private Hello$() {
MODULE$ = this;
}
public void main(String[] args) {
println("Hello, Scala Swarm!");
}
}
public final class Hello {
public static void main(final String[] args) {
Hello$.MODULE$.main(args);
}
}

74. Hello, Scala Swarm!

75. SUMMARY
Given a problem, consider the following steps:
1. javap
2. repeat

76. tzofia@overopshq.com
@tzofias