Java Virtual Machine Bytecodes Internals

1. JVM Bytecodes
~ Anvay Patil

2. We spend 90% of our time in developing/maintaining
10% of code.

3. From the book:
Screenshot from Chapter 5

4. JVM=J+VM
Let’s get little curious here

5. What is VM?
A Virtual Machine is a high level abstraction on top of native operating system
that emulates a physical machine.
Two types of Virtual Machines:
1. Process Virtual Machine.
2. System Virtual Machine.

6. JVM -> Process Virtual Machine
A Process Virtual machine enables the same platform/environment for execution on
multiple operating systems and different hardware architectures.
They could be of two type:
1. Stack Based Process Virtual machine (HotSpot Virtual machine/ Primary JVM)
2. Register Based Process Virtual machine (Android’s Dalvik Virtual machine)

7. Stack Based
vs
Register based
Virtual Machine
Types

8. Now few questions?
1. Stack based vs register based Virtual machine, does it affect bytecodes which is
generated by Java Compiler ?
2. What makes Java achieve platform independency ?
3. Is platform independency, for only Java language or it is true for other JVM languages
too ?

9. Bytecodes
So what is the Magic behind the bytecodes?
What exactly are bytecodes?
Why should i be aware about it?

10. Bytecode?
JVM
➔ one byte instruction set
➔ 256 possible opcodes (unsigned bytecode)
➔ 200 in use on current
Instruction sets designed for efficient
execution by software interpreter which can
do suitable further compilation into
machine code may be at runtime.
Microsoft CLR
➔ 2 bytes instruction sets
➔ Stack based VM

11. Why know JVM Bytecodes?
1. Can give deep understanding about the
platform
2. Can read the byte code
3. Can modify the bytecode
4. Can build own language?
5. Use libraries which can generate bytecodes.

12. Java -VM
Android’s
Dalvik -VM

13. Jython VM
CPython
x=7,y=8
print x+y
Jython
Compiler
CPython
Compiler
Cpython
Interpreter
Jython
Interpreter15
CPython VM Java VM
CPython Bytecode Java Bytecode
Python source code

14. JVM Languages: < OMG :-O >
Jython
Mirah
JRuby
MicroFocus Visual COBOL
JScheme
Javascal/Pascal4J
Javascal
JTclJawk
JLua
Erjang
Jaskell
JPerl
JPerl Groovy
Clojure
Xtend Ceylon Kotlin
More than 800 implementations of JVM and more than 250 JVM languages
Scala
Fhatom
Gosu
Processing
Drools
Frink Open-XION
Redline SmallTalk
Ioke Nashorn JS
JudoScript
LolCode
NestVM JPC
PizzaNoop
jvm.go

15. JVM=Polyglot-VM

16. Simple Program
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world");
}
}
> javac HelloWorld.java
> javap -c HelloWorld.class
> javap -verbose HelloWorld.class

17. javap
● Java Class File disassembler
● Basic operations and class structure
○ javap HelloWorld.class
● -c flag include bytecodes
○ javap -c HelloWorld.class
● -public -private -protected option
○ javap -private HelloWorld.class
○ javap -public HelloWorld.class
○ javap -protected HelloWorld.class
● -verbose for stack size, locals, args etc.
○ javap -verbose HelloWorld.class

18. Instruction set:
● Stack instruction, since stack based machine
● Basic operations
● Flow control
● Class structures
● Exception Handling

19. Stack Manipulation:

20. Typed Opcodes:
b byte
s short
c char
i int
l long
f float
d double
a reference
<type> <operation>
Constant Values
Local vars (load, store)
Array Operations (aload, astore)
Math ops (add, sub, mul, div)
Boolean bitwise
Comparisons
Conversions
Type
Operations
There is no instruction for boolean, and can play with 0 or 1

21. Constant Values:

22. Local variables:

23. Arrays:

24. Math Operations:

25. Boolean and Bitwise Operations:

26. Conversions:

27. Compare operations:

28. Flow Control:

29. More Flow Control:

30. Instructions for Classes

31. Instructions for Classes

32. Let’s execute HelloWorld
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String Hello Java User Group
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return

33. Q & A :)

34. References
● Java Virtual Machine Specification
● Inside JVM
● JVM Programming Languages

35. Thank you :)