The document discusses the Java Virtual Machine (JVM) and its key components. It explains that the JVM is a virtual computing environment that executes Java bytecode. The bytecode is executed by the JVM using an operand stack. The document also discusses how objects and classes are represented in memory, including details on object headers, class file formats, and the classloading process. It concludes by mentioning the jdb debugging tool for debugging Java programs without an IDE.

1. JVM Internals
FTD Group

2. acknowledgment
● God
● iMasters
● FTD Group

3. who we are?
FTD Group
http://friendstechday.wordpress.com/about/

4. JVM? WORA? What?

5. Java Virtual Machine
● The Java Virtual Machine Specification
● Written in C/C++
● Stack based VM
● Different implementations
● Symbolic reference
● Garbage Collected
● Java Bytecode
● JIT (Just-In-Time)

7. Stack based VM

8. 1 + 2 = ?

9. (+ 1 2)
Lisp? RPN?

10. operand stack
PUSH 1 1
1
PUSH 2 2
ADD 3

11. operand stack - bytecode
iconst_1 1
1
iconst_2 2
iadd 3

12. Java bytecode is the instruction set
of the Java virtual machine
opcode = 1 or 2 bytes
256 possible opcodes
198 are currently in use
51 are reserved for future use
3 are set aside as permanently unimplemented
list of all opcodes: http://docs.oracle.com/javase/specs/jvms/se8/html/jvms-6.html

13. variable declaration
int i;
0: iconst_0 // push 0 to top of the operand stack
1: istore_1 // Pop value from top of operand stack and
store as local variable 1

14. object instantiation
Object ftd = new Object();
0: new #2 // Class java/lang/Object
1: dup // duplicate the value on top of the stack
2: invokespecial #3 // Method java/lang/Object “<init>()V”

15. #1, #4, #100? #Wat?

16. Runtime Constant Pool
The constant pool contains the following
types:
● Integer, Long, Float, Double
● String, Uft8
● Class
● NameAndType
● Fieldref, Methodref, InterfaceMethodref

17. Java Bytecode Type Description
B byte signed byte
C char Unicode character
D double double-precision floating-point value
F float single-precision floating-point value
I int integer
J long long integer
L<classname> reference an instance of class <classname>
S short signed short
Z boolean true or false
[ reference one array dimension

18. Java Code Java Bytecode Expression
double d[][][]; [[[D
Object mymethod(int I, double d, Thread t) (IDLjava/lang/Thread;)Ljava/lang/Object;

19. javap -v SimpleClass

20. Not only Java Stack lives,
but also Heap.

21. Java Heap memory

22. Code Cache
used for compilation and storage of methods that
have been compiled to native code by the JIT
compiler

23. if >= java1.8
no PermGen
yes Metaspace

24. everything is wonderful
in Java?.... it depends bro!

25. which the size of a Java
String with "Java"
content?
String java = “Java”;

26. answer: 64 bytes
Ahh… how.. WATTT???

27. Java memory consumption
8 bytes Object Header
8 bytes Object reference
(x86 may use XX:+UseCompressedOops)
8 bytes alignment (WAT?)

28. data alignment
5 bytes used 3 bytes paddind
8 bytes alignment

29. String explanation
Field Type Size (bytes)
HEADER 16
value “Java” char[] (16 + 8) + (4 * 2) = 32
offset int 4
count int 4
hash int 4
PADDING = 60 + 4
TOTAL 64 bytes

30. which the size of a java.
lang.Boolean?
Boolean b = false;

31. answer: 24 bytes
Ahh… how.. WATTT???

32. Ok .. But what about
classes? OOP?

33. Class File Format
ClassFile {
u4 magic;
u2 minor_version;
u2 major_version;
u2 constant_pool_count;
cp_info contant_pool[constant_pool_count – 1];
u2 access_flags;
u2 this_class;
u2 super_class;
u2 interfaces_count;
u2 interfaces[interfaces_count];
u2 fields_count;
field_info fields[fields_count];
u2 methods_count;
method_info methods[methods_count];
u2 attributes_count;
attribute_info attributes[attributes_count];
}

34. javap -v SimpleClass

35. And what about the
Classloader?

36. JVM initialization order
1. Create a initial class using Bootstrap
classloader;
2. links the initial class;
3. initializes it;
4. and invokes the public class method void
main(String[])

37. find, check headers class file, super loading, binary created
3 steps, verifying and preparing the type and its direct supers
structurally correct, and obeys the semantic requirements
allocation of memory foe data structures
(optional) checking corrected symbolic references
executing initialization method <clinit>

39. Java debug without ide?
Is it possible?

40. jdb
(included in the JDK 1.2
package)

41. THANK YOU!
FTD Group
JOIN US!
“And be not conformed to this world: but be ye transformed by
the renewing of your mind, that ye may prove what {is} that good,
and acceptable, and perfect, will of God.” (Romans 12:2)