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 empowers you to build your own languages. 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 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
0: new #2

11. [POSITION]: [OPERATION] [OPERAND]
A mnemonic that represents one of
the 256 possible opcodes
0: new #2

12. [POSITION]: [OPERATION] [OPERAND]
Arguments to the operation.
The number and type of these
depends on the operation in use.
0: new #2

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. Why Read Bytecode?
• Diagnose performance problems
• Reverse engineering
• Security audit
• Debug legacy code
• FindBugs plugins that work across languages

42. Writing
JVM Bytecode

43. 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

44. ASM
http://asm.ow2.org/

46. 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

47. Jitescript
https://github.com/qmx/jitescript

48. An ECMAScript runtime for the JVM

49. 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
}

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

51. 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();
}});
}};

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();
}});
}};

53. 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

54. JiteClass .class
.toBytes()

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

56. 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

57. This is our machine:
An empty stack

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

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

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

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

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

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

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

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

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

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

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

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

70. 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. 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)) {
}
}

73. 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));
}

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

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

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

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

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

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

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. 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});
}

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

85. 14: dup
15: ifeq 39
...
36: goto 14
39: pop

86. $ java Compiler “< < + + +”
Generated Main.class
$ java Main
42

87. Byteman
http://byteman.jboss.org/

88. Javassist
http://www.javassist.org/

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

90. 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/

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

92. Improve performance

93. Improve performance

94. Obfuscate code

95. Obfuscate code

96. Make a DSL…

97. No, make a new
JVM Language

98. (do 10 (print (* 7 5)))

99. pragprog.com

100. The Dragon Book

101. 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

102. ASM Bytecode Outline
ProGuard
https://plugins.jetbrains.com/plugin/5918
http://proguard.sourceforge.net/
Dr. Garbage
http://www.drgarbage.com/

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

104. Thank You
Joe Kutner
@codefinger
http://www.slideshare.net/jkutner/programming-jvm-bytecode