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Programming JVM Bytecode with Jitescript

Joe Kutner, profile picture
Joe Kutner

The document discusses Java Virtual Machine (JVM) bytecode, detailing the structure, compilation process, and examples of Java classes. It includes instructions on reading bytecode, diagnosing performance issues, and offers tools for reverse engineering and debugging. Additionally, it mentions various JVM opcodes and their significance in method invocation and operand stack manipulation.

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Programming JVM Bytecode
Joe Kutner @codefinger JVM Langs Owner @Heroku
.class.java
$ 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?
javap
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
[POSITION]: [OPERATION] [OPERAND] Location of the instruction in the raw binary data 0: new #2
[POSITION]: [OPERATION] [OPERAND] A mnemonic that represents one of the 256 possible opcodes 0: new #2
[POSITION]: [OPERATION] [OPERAND] Arguments to the operation. The number and type of these depends on the operation in use. 0: new #2
0: new #2 3: dup 4: invokespecial #3 7: ldc #4 9: invokevirtual #5 12: pop 13: return
... 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
0: new #2 3: dup 4: invokespecial #3 7: ldc #4 9: invokevirtual #5 12: pop 13: return
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 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
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 <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>
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 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>
... 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 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 ...
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
... 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 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”>
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”>
... 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
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 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”
Object “Hello” ... Methodref Target Arguments Operand Stack
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
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
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
swap
iadd
getstatic [CLASS] [FIELD]
iconst_1
Why Read Bytecode? • Diagnose performance problems • Reverse engineering • Security audit • Debug legacy code • FindBugs plugins that work across languages
Writing JVM Bytecode
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
ASM http://asm.ow2.org/
Programming JVM Bytecode with Jitescript
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
Jitescript https://github.com/qmx/jitescript
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 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(); }}); }};
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(); }}); }};
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
JiteClass .class .toBytes()
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") {{ defineMethod("main", ACC_PUBLIC | ACC_STATIC, sig(void.class, String[].class), 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)) { } }
Programming JVM Bytecode with Jitescript
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)) { } }
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)); }
if ("p".equals(cmd)) { 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}); }
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}); }
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}); }
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 “< < + + +” Generated Main.class $ java Main 42
Byteman http://byteman.jboss.org/
Javassist http://www.javassist.org/
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
Improve performance
Improve performance
Obfuscate code
Obfuscate code
Make a DSL…
No, make a new JVM Language
(do 10 (print (* 7 5)))
pragprog.com
The Dragon Book
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
ASM Bytecode Outline ProGuard https://plugins.jetbrains.com/plugin/5918 http://proguard.sourceforge.net/ Dr. Garbage http://www.drgarbage.com/
The next time you compile… Run javap against the class file.
Thank You Joe Kutner @codefinger http://www.slideshare.net/jkutner/programming-jvm-bytecode

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Programming JVM Bytecode with Jitescript

  • 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?
  • 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
  • 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
  • 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”
  • 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
  • 41. Why Read Bytecode? • Diagnose performance problems • Reverse engineering • Security audit • Debug legacy code • FindBugs plugins that work across languages
  • 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
  • 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
  • 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
  • 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)); }
  • 75. else if ("+".equals(cmd)) { iconst_1(); iadd(); }
  • 76. else if (“-".equals(cmd)) { iconst_1(); isub(); }
  • 77. else if (“>".equals(cmd)) { iconst_0(); }
  • 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
  • 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
  • 97. No, make a new JVM Language
  • 98. (do 10 (print (* 7 5)))
  • 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
  • 103. The next time you compile… Run javap against the class file.