Every Java developer should have a good working knowledge of JVM bytecode. It’s fun, it can help you diagnose problems, improve performance, and even opens the door to building languages of your own. No matter what kind of Java application you work on, you’ll get something out of this talk. We’ll start with bytecode fundamentals. You’ll learn how the most common operations work and see visual representations of how the JVM executes that code. The second part of the talk will introduce Jitescript, a Java library for generating bytecode. You’ll learn how to use Jitescript with some plain old Java code to create your own JVM languages.

1. Programming
JVM Bytecode

2. Joe Kutner
@codefinger
JVM Langs Owner
@Heroku

3. .class.java

4. $ cat Main.class
????3
<init>()VCodeLineNumberTablemain([Ljava/lang/
String;)V
SourceFile Main.java
java/util/ArrayListHello
Mainjava/lang/Objectadd(Ljava/lang/Object;)Z!
*??
*?Y??W?

5. javap

6. class Main {
public static void main(String[] args) {
(new ArrayList<String>()).add("Hello");
}
}

7. Compiled from "Main.java"
public class Main {
public Main();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object...
4: return
public static void main(java.lang.String[]);
Code:
0: new #2 // class java/util/ArrayList
3: dup
4: invokespecial #3 // Method java/util/ArrayList...
7: ldc #4 // String Hello
9: invokevirtual #5 // Method java/util/ArrayList...
12: pop
13: return
}
$ javac Main.java
$ javap -c Main

8. 0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

9. [POSITION]: [OPERATION] [OPERAND]
0: new #2

10. [POSITION]: [OPERATION] [OPERAND]
Location of the instruction in
the raw binary data

11. [POSITION]: [OPERATION] [OPERAND]
A mnemonic that represents one of
the 256 possible opcodes

12. [POSITION]: [OPERATION] [OPERAND]
Arguments to the operation.
The number and type of these
depends on the operation in use.

13. 0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

14. ...
Constant pool:
#1 = Methodref #7.#16 // java/lang/Object...
#2 = Class #17 // java/util/ArrayList
#3 = Methodref #2.#16 // java/util/ArrayList.”<init>”
#4 = String #18 // Hello
#5 = Methodref #2.#19 // java/util/ArrayList.add:(L...
#6 = Class #20 // Main
#7 = Class #21 // java/lang/Object
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 main
#13 = Utf8 ([Ljava/lang/String;)V
#14 = Utf8 SourceFile
#15 = Utf8 Main.java
#16 = NameAndType #8:#9 // "<init>":()V
#17 = Utf8 java/util/ArrayList
$ javap -v -c Main

15. 0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

16. Stack-Based Model of
Computation

17. Exception in thread "main" java.lang.NullPointerException
at Printer.printString(Printer.java:13)
at Printer.print(Printer.java:9)
at Printer.main(Printer.java:19)
Stack Frame
Operand
Stack
Local
Variable
Array
Constant Pool
#1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...

18. Stack Frame
Operand StackLocal Vars Constant Pool
#1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

19. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

20. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<ArrayList>

21. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<ArrayList>

22. invokespecial
Used to invoke an initializer, private method, or superclass method
invokeinterface
Used to invoke an interface method
invokestatic
Used to invoke a class-level method (i.e. static methods)
invokevirtual
Used to invoke an instance method
invokedynamic
Used to invoke methods dynamically (new shiny)

23. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<ArrayList>

24. ...
Constant pool:
#1 = Methodref #7.#16 // java/lang/Object...
#2 = Class #17 // java/util/ArrayList
#3 = Methodref #2.#16 // java/util/ArrayList.”<init>”
#4 = String #18 // Hello
#5 = Methodref #2.#19 // java/util/ArrayList.add:(L...
#6 = Class #20 // Main
#7 = Class #21 // java/lang/Object
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 main
#13 = Utf8 ([Ljava/lang/String;)V
#14 = Utf8 SourceFile
#15 = Utf8 Main.java
#16 = NameAndType #8:#9 // "<init>":()V
#17 = Utf8 java/util/ArrayList
$ javap -v -c Main

25. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<ArrayList>
Stack Frame
Operand StackLocal Vars Constant Pool
#1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...

26. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

27. ...
Constant pool:
#1 = Methodref #7.#16 // java/lang/Object...
#2 = Class #17 // java/util/ArrayList
#3 = Methodref #2.#16 // java/util/ArrayList.”<init>”
#4 = String #18 // Hello
#5 = Methodref #2.#19 // java/util/ArrayList.add:(L...
#6 = Class #20 // Main
#7 = Class #21 // java/lang/Object
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 main
#13 = Utf8 ([Ljava/lang/String;)V
#14 = Utf8 SourceFile
#15 = Utf8 Main.java
#16 = NameAndType #8:#9 // "<init>":()V
#17 = Utf8 java/util/ArrayList
#18 = Utf8 Hello
#19 = NameAndType #22:#23 // add:(Ljava/lang/Object;)Z
$ javap -v -c Main

28. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<“Hello”>

29. Stack Frame
Operand StackLocal Vars Constant Pool
<ArrayList>
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<“Hello”>

30. ...
Constant pool:
#1 = Methodref #7.#16 // java/lang/Object...
#2 = Class #17 // java/util/ArrayList
#3 = Methodref #2.#16 // java/util/ArrayList.”<init>”
#4 = String #18 // Hello
#5 = Methodref #2.#19 // java/util/ArrayList.add:(L...
#6 = Class #20 // Main
#7 = Class #21 // java/lang/Object
#8 = Utf8 <init>
#9 = Utf8 ()V
#10 = Utf8 Code
#11 = Utf8 LineNumberTable
#12 = Utf8 main
#13 = Utf8 ([Ljava/lang/String;)V
#14 = Utf8 SourceFile
#15 = Utf8 Main.java
#16 = NameAndType #8:#9 // "<init>":()V
#17 = Utf8 java/util/ArrayList
#18 = Utf8 Hello
#19 = NameAndType #22:#23 // add:(Ljava/lang/Object;)Z
$ javap -v -c Main

31. java/util/ArrayList.add:(Ljava/lang/Object;)Z
B - byte
C - char
S - short
I - int
J - long
F - float
D - double
Z - boolean
V - void
L - object

32. Stack Frame
Operand StackLocal Vars Constant Pool
ArrayList
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
“Hello”

33. Object
“Hello”
...
Methodref
Target
Arguments
Operand Stack

34. Stack Frame
Operand StackLocal Vars Constant Pool
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
<ArrayList>
<“Hello”>
Stack Frame
Operand StackLocal Vars Constant Pool
#1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
true

35. Stack Frame
Operand StackLocal Vars Constant Pool
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
true

36. Stack Frame
Operand StackLocal Vars Constant Pool
ArrayList #1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return

37. swap

38. iadd

39. getstatic [CLASS] [FIELD]

40. iconst_1

41. Writing
JVM Bytecode

42. 000000B0: 73 01 00 0A 4C 43 6F 6D 70 69 6C 65 72 3B 01 00
000000C0: 07 63 6F 6D 70 69 6C 65 01 00 32 28 5B 4C 6A 61
000000D0: 76 61 2F 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 29
000000E0: 4C 6D 65 2F 71 6D 78 2F 6A 69 74 65 73 63 72 69
000000F0: 70 74 2F 4A 69 74 65 43 6C 61 73 73 3B 01 00 06
00000100: 74 6F 6B 65 6E 73 01 00 13 5B 4C 6A 61 76 61 2F
00000110: 6C 61 6E 67 2F 53 74 72 69 6E 67 3B 01 00 04 6D
00000120: 61 69 6E 01 00 16 28 5B 4C 6A 61 76 61 2F 6C 61
00000130: 6E 67 2F 53 74 72 69 6E 67 3B 29 56 01 00 04 61
00000140: 72 67 73 01 00 09 6A 69 74 65 43 6C 61 73 73 01
00000150: 00 1D 4C 6D 65 2F 71 6D 78 2F 6A 69 74 65 73 63
00000160: 72 69 70 74 2F 4A 69 74 65 43 6C 61 73 73 3B 01
00000170: 00 06 6F 75 74 70 75 74 01 00 1A 4C 6A 61 76 61
00000180: 2F 69 6F 2F 46 69 6C 65 4F 75 74 70 75 74 53 74
00000190: 72 65 61 6D 3B 01 00 0A 45 78 63 65 70 74 69 6F
000001A0: 6E 73 07 00 3A 01 00 0A 53 6F 75 72 63 65 46 69
000001B0: 6C 65 01 00 0A 43 6F 6D 70 69 6C 65 72 2E 6A 61
000001C0: 76 61 0C 00 14 00 15 01 00 0A 43 6F 6D 70 69 6C
000001D0: 65 72 24 31 01 00 04 4D 61 69 6E 0C 00 14 00 3B
000001E0: 01 00 08 43 6F 6D 70 69 6C 65 72 0C 00 1B 00 1C

43. ASM

45. MethodVisitor mv = cv.visitMethod(
Opcodes.ACC_PRIVATE,
"setupLogger",
"(Lorg/apache/maven/cli/MavenCli$CliRequest;)Lorg/codehaus/
plexus/logging/Logger;", null, null);
mv.visitCode();
mv.visitTypeInsn(
Opcodes.NEW,
"com/github/jcgay/maven/color/logger/AnsiColorLogger");
https://github.com/jcgay/maven-color

46. Jitescript

47. An ECMAScript runtime for the JVM

48. JiteClass
➤ CodeBlock
➤ CodeBlock
➤ CodeBlock
Compiled from "Main.java"
public class Main {
public Main();
Code:
0: aload_0
1: invokespecial #1
4: return
public static void main...
Code:
0: new #2
3: dup
4: invokespecial #3
7: ldc #4
9: invokevirtual #5
12: pop
13: return
}

49. new JiteClass("Main") {{
defineMethod("main", ACC_PUBLIC | ACC_STATIC,
sig(void.class, String[].class),
new CodeBlock() {{
// ...
}});
}};

50. new JiteClass("Main") {{
defineMethod("main", ACC_PUBLIC | ACC_STATIC,
sig(void.class, String[].class),
new CodeBlock() {{
new(p(ArrayList.class));
dup();
invokestatic(
p(ArrayList.class), ”<init>”, sig(void.class));
ldc(“Hello”)
invokevirtual(
p(ArrayList.class), “add”,
sig(boolean.class, Object.class));
pop();
voidreturn();
}});
}};

51. new JiteClass("Main") {{
defineMethod("main", ACC_PUBLIC | ACC_STATIC,
sig(void.class, String[].class),
new CodeBlock() {{
getstatic(p(System.class), "out", ci(PrintStream.class));
ldc(“42");
invokevirtual(
p(PrintStream.class),
"println",
sig(void.class, Object.class));
voidreturn();
}});
}};

52. new JiteClass("Main") {{
defineMethod("main", ACC_PUBLIC | ACC_STATIC,
sig(void.class, String[].class),
new CodeBlock() {{
getstatic(p(System.class), "out", ci(PrintStream.class));
ldc(“42");
invokevirtual(
p(PrintStream.class),
"println",
sig(void.class, Object.class));
voidreturn();
}});
}};
0: getstatic #13
3: ldc #15
5: invokevirtual #21
8: return

53. JiteClass .class
.toBytes()

54. Brainfu++
> > + + + + + + [ - s + + + + + + + s ] < p

55. p
>
<
: print a variable
: push a 0 onto the stack
: pop the top value off of the stack
+
-
[
: increment a variable
: decrement a variable
: start a loop
] : end a loop
s : swap the top two values of the stack

56. This is our machine:
An empty stack

57. > > + + + + + + [ - s + + + + + + + s ] < p

58. > > + + + + + + [ - s + + + + + + + s ] < p
Push two zeros
0
0

59. > > + + + + + + [ - s + + + + + + + s ] < p
Inc to 6
0
6

60. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
0
6

61. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
0
5
Decrement

62. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
5
0
Swap

63. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
5
7
Increment

64. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
7
5
Swap

65. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
7
5
End of loop

66. > > + + + + + + [ - s + + + + + + + s ] < p
Loop
42
0
Repeat until 0

67. > > + + + + + + [ - s + + + + + + + s ] < p
42
Pop and Print

68. public JiteClass compile(final String program) {
return new JiteClass("Main") {{
defineMethod("main", ACC_PUBLIC | ACC_STATIC,
sig(void.class, String[].class),
new CodeBlock() {{
// ...
}});
}};
}

69. for (String cmd : program.split(“ “)) {
if ("p".equals(cmd)) {
} else if ("+".equals(cmd)) {
} else if ("-".equals(cmd)) {
} else if (">".equals(cmd)) {
} else if ("<".equals(cmd)) {
} else if ("s".equals(cmd)) {
} else if ("[".equals(cmd)) {
} else if ("]".equals(cmd)) {
}
}

71. for (String cmd : program.split(“ “)) {
if ("p".equals(cmd)) {
} else if ("+".equals(cmd)) {
} else if ("-".equals(cmd)) {
} else if (">".equals(cmd)) {
} else if ("<".equals(cmd)) {
} else if ("s".equals(cmd)) {
} else if ("[".equals(cmd)) {
} else if ("]".equals(cmd)) {
}
}

72. if ("p".equals(cmd)) {
dup();
invokestatic(
p(String.class),
"valueOf",
sig(String.class, int.class));
getstatic(p(System.class), "out", ci(PrintStream.class));
swap();
invokevirtual(
p(PrintStream.class),
"print",
sig(void.class, Object.class));
}

73. if ("p".equals(cmd)) {
brainPrint();
}

74. else if ("+".equals(cmd)) {
iconst_1();
iadd();
}

75. else if (“-".equals(cmd)) {
iconst_1();
isub();
}

76. else if (“>".equals(cmd)) {
iconst_0();
}

77. else if (“<".equals(cmd)) {
pop();
}

78. else if (“s".equals(cmd)) {
swap();
}

79. Stack<LabelNode[]> loopStack = new Stack<LabelNode[]>();
else if (“[“.equals(cmd)) {
LabelNode begin = new LabelNode();
LabelNode end = new LabelNode();
label(begin);
dup();
ifeq(end);
loopStack.push(new LabelNode[] {begin, end});
}

80. Stack<LabelNode[]> loopStack = new Stack<LabelNode[]>();
else if (“[“.equals(cmd)) {
LabelNode begin = new LabelNode();
LabelNode end = new LabelNode();
label(begin);
dup();
ifeq(end);
loopStack.push(new LabelNode[] {begin, end});
}

81. Stack<LabelNode[]> loopStack = new Stack<LabelNode[]>();
else if (“[“.equals(cmd)) {
LabelNode begin = new LabelNode();
LabelNode end = new LabelNode();
label(begin);
dup();
ifeq(end);
loopStack.push(new LabelNode[] {begin, end});
}

82. Stack<LabelNode[]> loopStack = new Stack<LabelNode[]>();
else if (“[“.equals(cmd)) {
LabelNode begin = new LabelNode();
LabelNode end = new LabelNode();
label(begin);
dup();
ifeq(end);
loopStack.push(new LabelNode[] {begin, end});
}

83. else if (“]“.equals(cmd)) {
LabelNode[] labelNodes = loopStack.pop();
LabelNode begin = labelNodes[0];
LabelNode end = labelNodes[1];
go_to(begin);
label(end);
}

84. 14: dup
15: ifeq 39
...
36: goto 14
39: pop

85. $ java Compiler “< < + + +”
$ java Main
42

86. public class BrainCodeBlock extends CodeBlock {
public void brainPrint(String s) { /* ... */ }
public void brainPush(BrainObject o) { /* ... */ }
public BrainObject brainPop() { /* ... */ }
public void brainLoop(Predicate<CodeBlock> b) {
// ...
}
}

87. Javassist

88. CtClass

89. ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get("test.Rectangle");
cc.setSuperclass(pool.get("test.Point"));
cc.writeFile();

90. class ThirdParty {
public void run() {
fetchData();
processData();
invokeService();
resolveConflicts();
finalize();
}
}

91. ClassPool cp = ClassPool.getDefault();
CtClass cc = cp.get(“ThirdParty");
CtMethod m = cc.getDeclaredMethod(“fetchData");
m.addLocalVariable("t", CtClass.longType);
m.insertBefore(
"t = System.currentTimeMillis();");
m.insertAfter(
“{“ +
“ t = System.currentTimeMillis() - t;” +
“ System.out.println("Time (ms): " + t);” +
”}");
byte[] byteCode = cc.toBytecode();
cc.detach();
http://www.tomsquest.com/blog/2014/01/intro-java-agent-and-bytecode-manipulation/

92. calling ThirdParty
fetching data...
Time (ms): 55
processing data...
Time (ms): 6032

93. Make a DSL…

94. No, make a new
JVM Language

95. (print (* 7 5))

96. pragprog.com

97. The Dragon Book

98. Brainfu** (Jipsy)
List of JVM Bytecode Operations
http://github.com/jkutner/jipsy
http://docs.oracle.com/javase/specs/jvms/se8/html/
Jitescript
https://github.com/qmx/jitescript

99. The next time you compile…
Run javap against the class file.

100. Thank You
Joe Kutner
@codefinger