Programming JVM Bytecode

Joe Kutner
@codeﬁnger
JVM Langs Owner
@Heroku

$ 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?

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

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

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

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

Location of the instruction in
the raw binary data

A mnemonic that represents one of
the 256 possible opcodes

Arguments to the operation.
The number and type of these
depends on the operation in use.

...
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

Stack-Based Model of
Computation

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
...

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

Stack Frame
<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

Stack Frame
<ArrayList>
#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>

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)

Stack Frame
<ArrayList>
#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
#1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...

...
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

Stack Frame
<ArrayList>
#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”>

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

Stack Frame
ArrayList
#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”

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

Stack Frame
#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
#1 = Methodref
#2 = Class
#3 = Methodref
#4 = String
#5 = Methodref
...
true

Stack Frame
#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

Stack Frame
#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

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

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

An ECMAScript runtime for the JVM

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
}

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

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();
}});
}};

new CodeBlock() {{
getstatic(p(System.class), "out", ci(PrintStream.class));
ldc(“42");
invokevirtual(
p(PrintStream.class),
"println",
sig(void.class, Object.class));
voidreturn();
}});
}};

new CodeBlock() {{
ldc(“42");
invokevirtual(
"println",
voidreturn();
}});
}};
0: getstatic #13
3: ldc #15
5: invokevirtual #21
8: return

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

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

This is our machine:
An empty stack

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

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

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

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

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

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

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

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

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

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

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

public JiteClass compile(final String program) {
return new JiteClass("Main") {{
new CodeBlock() {{
// ...
}});
}};
}

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)) {
}
}

dup();
invokestatic(
p(String.class),
"valueOf",
sig(String.class, int.class));
swap();
invokevirtual(
"print",
}

brainPrint();
}

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

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

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

else if (“<".equals(cmd)) {
pop();
}

else if (“s".equals(cmd)) {
swap();
}

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});
}

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

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

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

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

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

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

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/

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

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

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

Thank You
Joe Kutner
@codeﬁnger

Programming JVM Bytecode

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