Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembler helps the C or C++ programmer. Java bytecode is the form of instructions that Java virtual machine executes. This knowledge is crucial when debugging and doing performance and memory usage tuning. The presenter will share his knowledge on what bytecode means for your platform and how to create compiler while using some awesome tools.

1. Mastering byte code

2. The master plan
• JVM
• javap
• opcode
• ClassLoader
• Toolkits
• Puzzles

3. Stack
1+2
PUSH 1 1
12+ PUSH 2 2

4. Stack
1+2
PUSH 1 3
12+ PUSH 2
ADD

5. Stack
1+2
ICONST_1 3
12+ ICONST_2
IADD

6. Frame

8. Local Variables & Stack

9. Java Stack Machine
• JVM is a stack-based machine
• Each thread has a stack
• Stack stores frames
• Frame is created on method invocation
• Frame:
o
Operand stack
o
Array of local variables
o
Reference to the runtime constant pool

10. Compiling to binary
C/C++ (*.h, *.cpp) Java (*.java)
Assembler Bytecode
Intel opcode Sun opcode

11. javap
• Java class file disassembler
• Used with no options shows class
structure only
• Methods, superclass, interfaces, etc
• -c – shows the bytecode
• -private – shows all classes and
members
• -s – prints internal types signatures
• -verbose – prints stack size, number of
locals and args for methods

12. javap -c -verbose Clazz

13. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}

14. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}
javap SimpleProgram -c

15. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}
javap SimpleProgram -c
Compiled from "SimpleProgram.java"
public class SimpleProgram extends java.lang.Object{
public SimpleProgram();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3; //String Hello Java Fusion!
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}

16. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}
javap SimpleProgram -c
Compiled from "SimpleProgram.java"
public class SimpleProgram extends java.lang.Object{
public SimpleProgram();// The default constructor
Code:
0: aload_0 //Push this sto stack
1: invokespecial #1; //Method java/lang/Object."":()V //Invoke <init> on this
4: return
public static void main(java.lang.String[]);
Code:
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3; //String Hello Java Fusion!
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}

17. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}
javap SimpleProgram -c
Compiled from "SimpleProgram.java"
public class SimpleProgram extends java.lang.Object{
public SimpleProgram();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; //get static field
3: ldc #3; //String Hello Java Fusion! //Load String to the stack
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V //invoke method with parameter
8: return
}

18. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}
javap SimpleProgram -c
Compiled from "SimpleProgram.java"
public class SimpleProgram extends java.lang.Object{
public SimpleProgram();
Code:
0: aload_0
1: invokespecial #1; //Method java/lang/Object."":()V
4: return
public static void main(java.lang.String[]);
Code:
0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3; //String Hello Java Fusion!
5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
}

19. public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}
javap SimpleProgram -c -verbose
Compiled from "SimpleProgram.java“
public class SimpleProgram extends java.lang.Object
SourceFile: "SimpleProgram.java"
minor version: 0
major version: 50
Constant pool:
const #1 = Method #6.#20; // java/lang/Object."<init>":()V
const #2 = Field #21.#22; // java/lang/System.out:Ljava/io/PrintStream;
const #3 = String #23; // Hello Java Fusion!
const #4 = Method #24.#25; // java/io/PrintStream.println:(Ljava/lang/String;)V
const #5 = class #26; // SimpleProgram
const #6 = class #27; // java/lang/Object
сonst #7 = Asciz <init>;
...

20. Bytecode
One-byte instructions
256 possible opcodes
207 in use
49 currently unassigned for opcodes and are reserved
for future use

21. TYPE OPERATION
Instructions fall into a number of broad groups:
•Local variables and stack interaction (e.g. aload_0,istore)
•Array operations (aload, astore)
•Arithmetic and logic (e.g. ladd,fcmpl)
•Type conversion (e.g. i2b,d2i)
•Object creation and manipulation (new, putfield)
•Operand stack management (e.g. swap,dup2)
•Control transfer (e.g. ifeq, goto)
•Method invocation and return (e.g. invokespecial,areturn)
•Operations with constant values (ldc, iconst_1)
•Math (add, sub, mul, div)
•Boolean/bitwise operations (iand, ixor)
•Comparisons (cmpg, cmpl, ifne)
•...

22. Opcode type
Prefix/Suffix Operand Type
i integer
l long
s short
b byte
c char
f float
d double
a reference

23. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

24. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

25. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

26. public java.lang.String getName();
Code:
Stack=1, Locals=1, Args_size=1
0: aload_0
1: getfield #2; //Field name:Ljava/lang/String;
4: areturn
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LGet;

27. ClassLoader
- Works with bytecode
- Has native methods
- Bytecode Verification via SecurityManager.class

28. Toolkits
JVM:
•ASM
•Javassist
•BCEL (Byte Code Engineering Library)
Jrebel
Terracotta

29. Common sample
public class SimpleProgram
{
public static void main(String[] args)
{
System.out.println("Hello Java Fusion!");
}
}

30. ASM sample
public class ASMSample extends ClassLoader implements Opcodes {
{
public static void main(String[] args)
{
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
cw.visit(V1_1, ACC_PUBLIC, "SimpleProgram", null, "java/lang/Object", null);
}
}

31. ASM sample
public class ASMSample extends ClassLoader implements Opcodes {
{
public static void main(String[] args)
{
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
cw.visit(V1_1, ACC_PUBLIC, "SimpleProgram", null, "java/lang/Object", null); // creates a GeneratorAdapter
for the (implicit) constructor
Method m = Method.getMethod("void <init> ()");
GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC, m, null, null, cw);
mg.loadThis();
mg.invokeConstructor(Type.getType(Object.class), m);
mg.returnValue();
mg.endMethod();
}
}

32. ASM sample
public class ASMSample extends ClassLoader implements Opcodes {
{
public static void main(String[] args)
{
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
cw.visit(V1_1, ACC_PUBLIC, "SimpleProgram", null, "java/lang/Object", null);
// creates a GeneratorAdapter for the (implicit) constructor
Method m = Method.getMethod("void <init> ()");
GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC, m, null, null, cw);
mg.loadThis();
mg.invokeConstructor(Type.getType(Object.class), m);
mg.returnValue();
mg.endMethod(); // creates a GeneratorAdapter for the 'main' method
m = Method.getMethod("void main (String[])");
mg = new GeneratorAdapter(ACC_PUBLIC + ACC_STATIC, m, null, null, cw);
mg.getStatic(Type.getType(System.class), "out",Type.getType(PrintStream.class));
mg.push("Hello Java Fusion!");
mg.invokeVirtual(Type.getType(PrintStream.class),
Method.getMethod("void println (String)"));
mg.returnValue();
mg.endMethod();
cw.visitEnd();
}
}

33. ASM sample
public class ASMSample extends ClassLoader implements Opcodes {
{
public static void main(String[] args)
{
ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_MAXS);
cw.visit(V1_1, ACC_PUBLIC, "SimpleProgram", null, "java/lang/Object", null);
// creates a GeneratorAdapter for the (implicit) constructor
Method m = Method.getMethod("void <init> ()");
GeneratorAdapter mg = new GeneratorAdapter(ACC_PUBLIC, m, null, null, cw);
mg.loadThis();
mg.invokeConstructor(Type.getType(Object.class), m);
mg.returnValue();
mg.endMethod();
// creates a GeneratorAdapter for the 'main' method
m = Method.getMethod("void main (String[])");
mg = new GeneratorAdapter(ACC_PUBLIC + ACC_STATIC, m, null, null, cw);
mg.getStatic(Type.getType(System.class), "out",Type.getType(PrintStream.class));
mg.push("Hello Java Fusion!");
mg.invokeVirtual(Type.getType(PrintStream.class),
Method.getMethod("void println (String)"));
mg.returnValue();
mg.endMethod();
cw.visitEnd(); byte[] code = cw.toByteArray();
ASMSample loader = new ASMSample ();
Class<?> exampleClass = loader.defineClass("SimpleProgram", code, 0, code.length);
// uses the dynamically generated class to print 'Hello Java Fusion'
exampleClass.getMethods()[0].invoke(null, new Object[] { null } );
}
}

34. ASM vs. Javassist
• Javassist source level API is much easier to use than the
actual bytecode manipulation in ASM
• Javassist provides a higher level abstraction layer over complex
bytecode level operations. Javassist source level API requires very less or
no knowledge of actual bytecodes, so much easier & faster to
implement.
• Javassist uses reflection mechanism which makes it slower
compared to ASM which uses Classworking techniques at runtime.
• Overall ASM is much faster & gives better performance than
Javassist. Javassist uses a simplified version of Java source code, which
it then compiles into bytecode. That makes Javassist very easy to use, but
it also limits the use of bytecode to what can be expressed within the limits
of the Javassist source code.
• In conclusion, if someone needs easier way of dynamically
manipulating or creating Java classes Javassist API should be used &
where the performance is the key issue ASM library should be used.

35. JRebel
Redeploying sucks, so JRebel
eliminates it. How?
JRebel maps your project workspace directly to the
application under development. When you change any
class or resource in your IDE, the change is immediately
reflected in the application, skipping the build and
redeploy phases.

36. How JRebel works
1) Classes
• JRebel integrates with the JVM and rewrites each class to be
updateable
• JRebel versions each class individually, instead of an application or
module at a time
• It does not use classloaders!
• Changes to classes are always visible in the Reflection API

37. How JRebel works
2) Workspace mapping
petclinic.war
• JRebel integrates with application
servers, frameworks and your IDE
• When a class or resource is being
looked up, JRebel redirects straight
to the workspace
• When an HTTP resource needs to
be served, JRebel serves it from the
workspace

38. Size and speed issues

39. Size and speed issues
top1 and top2 are functionally identical

40. Size and speed issues
Method int top1()
0 aload_0 //Push the object reference(this) at index
//0 of the local variable table.
1 getfield #6 <Field int intArr[]>
//Pop the object reference(this) and push
//the object reference for intArr accessed
//from the constant pool.
4 iconst_0 //Push 0.
5 iaload //Pop the top two values and push the
//value at index 0 of intArr.
6 ireturn //Pop top value and push it on the operand
//stack of the invoking method. Exit.

41. Size and speed issues
Method int top2()
0 aload_0
1 astore_2
2 aload_2
3 monitorenter
4 aload_0
5 getfield #6 <Field int intArr[]>
8 iconst_0 //Push 0.
9 iaload
10 istore_1
11 jsr 19
14 iload_1 .
15 ireturn
16 aload_2
17 monitorexit . Exception table: //If any exception occurs between
18 athrow . from to target type //location 4 (inclusive) and location
19 astore_3 4 16 16 any //16 (exclusive) jump to location 16
20 aload_2
21 monitorexit
22 ret 3
.

42. Size and speed issues
top1 and top2 are functionally identical
top1 is 13% faster than top2 as well as much smaller.

43. Puzzle Cutting Class
public class Strange2 {
public class Strange1 {
public static void main(String[] args) {
public static void main(String[] args) {
Missing m; try {
try {
m = new Missing(); } catch
Missing m = new Missing(); } catch
(java.lang.NoClassDefFoundError ex) {
(java.lang.NoClassDefFoundError ex) {
System.out.println("Got it!");
System.out.println("Got it!");
}
}
}
}
}
}
class Missing {
Missing() { }
}

44. Puzzle Cutting Class
public class Strange2 {
public class Strange1 {
public static void main(String[] args) {
public static void main(String[] args) {
Missing m; try {
try {
m = new Missing(); } catch
Missing m = new Missing(); } catch
(java.lang.NoClassDefFoundError ex) {
(java.lang.NoClassDefFoundError ex) {
System.out.println("Got it!");
System.out.println("Got it!");
}
}
}
}
}
}
class Missing {
Missing() { }
}

45. Puzzle Cutting Class
public class Strange2 {
public class Strange1 {
public static void main(String[] args) {
public static void main(String[] args) {
Missing m; try {
try {
m = new Missing(); } catch
Missing m = new Missing(); } catch
(java.lang.NoClassDefFoundError ex) {
(java.lang.NoClassDefFoundError ex) {
System.out.println("Got it!");
System.out.println("Got it!");
}
}
}
}
}
}
class Missing {
Missing() { }
}
???
System.out.println("Got it!"); throws an uncaught
NoClassDefFoundError

46. Puzzle Cutting Class
public class Strange2 {
public class Strange1 {
public static void main(String[] args) {
public static void main(String[] args) {
Missing m; try {
try {
m = new Missing(); } catch
Missing m = new Missing(); } catch
(java.lang.NoClassDefFoundError ex) {
(java.lang.NoClassDefFoundError ex) {
System.out.println("Got it!");
System.out.println("Got it!");
}
}
}
}
class Missing {
Missing() { }
}
System.out.println("Got it!"); Expected throws an uncaught
NoClassDefFoundError
throws an uncaught Result System.out.println("Got it!");
NoClassDefFoundError

47. Puzzle Cutting Class
0: new #2; // class Missing
3: dup
4: invokespecial #3; // Method Missing."<init>":()V
7: astore_1
8: goto 20
11: astore_1
12: getstatic #5; // Field System.out:Ljava/io/PrintStream;
15: ldc #6; // String "Got it!"
17: invokevirtual #7; // Method PrintStream.println:(String;)V
20: return
Exception table:
from to target type
0 8 11 Class java/lang/NoClassDefFoundError
The corresponding code for Strange2.main differs in only one instruction:
11: astore_2 //Store ex catch parameter
javap -c Strange1

48. Using reflection
public class Strange {
public static void main(String[] args) throws Exception {
try {
Object m = Class.forName("Missing").newInstance();
} catch (ClassNotFoundException ex) {
System.err.println("Got it!");
}
}
}

49. Benefits
• Improve speed and size of your applications
• Critical when debugging and doing performance and
memory usage tuning
• Realtime injection
• Build ORM project
• Clustering with AOP
• Know your platform!
• Create your own compile/decompiler?