JVM Internals Demystified

Confoo 2015

Transcript

1. ༼ つ ◕_◕ ༽つ JVM Internals ༼ つ ◕_◕ ༽つ
   Demystified! ( ͡° ͜
   ʖ ͡°)
   ༼ つ ◕_◕ ༽つ ༼ つ ◕_◕ ༽つ
   

   View Slide

2. What we will NOT see here
   • Pure JVM boring theory
   • Assembly fear stuff
   • 0xFF
   • 0100111
   

   View Slide

3. What we will see here
   • Why internals?
   • Method Area
   • Heap
   • Threads
   • Further reading
   • Funny Gifs and images!
   

   View Slide

4. Bonjour
   

   View Slide

5. Hi! I’m Hanneli
   São Paulo, Brazil
   

   View Slide

6. Hi! I’m Hanneli
   Meanwhile here in Canada
   

   View Slide

7. View Slide

8. No advanced stuff
   Simplified way
   

   View Slide

9. If you understand computer architecture
   classes
   You understand JVM
   

   View Slide

10. Why do I Need to
    Understand Internals?
    

    View Slide

11. We live in a simplified era
    

    View Slide

12. All externalised
    

    View Slide

13. All externalised
    (Almost)Normal dressed
    developers (external)
    

    View Slide

14. Do I need internal details?
    

    View Slide

15. Developer showing internal
    details
    

    View Slide

16. Memory analyse
    • Help! Perm gem problems!
    • Search for memory leaks
    • Memory analyser tools
    • Understand how JVM works
    

    View Slide

17. What’s the relation between:
    JVM
    Memory
    Bytecode
    Assembly
    Low Level
    OpCode
    Operator
    Operation
    Java
    

    View Slide

18. View Slide

19. How do I get started?
    • http://www.artima.com/insidejvm/ed2/jvmP.html
    

    View Slide

20. How do I get started?
    

    View Slide

21. JVM - Low level
    Assembly
    

    View Slide

22. I know how to write Java
    code
    

    View Slide

23. ༼ つ ◕_◕ ༽つ classes ༼ つ ◕_◕ ༽つ
    

    View Slide

24. What happens when I
    invoke a method in Java
    

    View Slide

25. WAIT
    How many kinds of
    methods do you know?
    

    View Slide

26. 2 types of methods
    Clazz.method();
    object.method();
    static
    instance
    

    View Slide

27. From Assembly classes:
    op code + operand
    operation + operand
    

    View Slide

28. operation + operand
    add r1, r2
    public int add(int r1, int r2)
    operation operators == parameters
    

    View Slide

29. Method calls in JVM
    static
    instance
    operation
    +
    operand
    operation
    +
    operand
    invokevirtual invokestatic
    

    View Slide

30. Where are the Operands?
    Special
    place
    

    View Slide

31. Where are the Operands?
    Real place
    

    View Slide

32. Symbolic place called pool
    

    View Slide

33. JVM keeps this fake
    references
    It’s a strategy to deal with
    dynamic linking
    

    View Slide

34. While JVM does need the
    content of this fake box
    (pool), it remais
    unresolved
    

    View Slide

35. Each fake reference for
    each method is unique
    

    View Slide

36. Each fake reference for
    each method is unique
    So JVM can replace the
    fake calls with true
    invokes.
    

    View Slide

37. Back to methods
    Clazz.method(int a);
    object.method(parameter);
    class Lol {
    public int myMethod(int a){
    //
    }
    }
    Class stuff
    

    View Slide

38. Method Area
    JVM
    Class stuff Class stuff Class stuff
    Non-Heap
    Code Cache
    Permanent Generation
    

    View Slide

39. Done?
    object
    

    View Slide

40. ༼ つ ◕_◕ ༽つ object ༼ つ ◕_◕ ༽つ
    

    View Slide

41. Method Area
    JVM
    Class stuff Class stuff Class stuff
    Non-Heap
    Permanent Generation
    HEAP
    Permanent Generation
    object
    object
    object
    

    View Slide

42. Young gen
    JVM - The Heap
    HEAP
    Lol lol = new LOL();
    lol
    object object
    object
    

    View Slide

43. JVM - The Heap
    HEAP
    Permanent Generation
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    Hey, Im’m
    heavy
    

    View Slide

44. JVM - The Heap
    HEAP
    Permanent Generation
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    object
    Checks all the objects
    on the heap
    Garbage collector
    

    View Slide

45. Old gen
    Young generation
    JVM - The Heap
    HEAP
    object object
    Eden
    lol
    Survivor
    object
    Will be taken for GC
    

    View Slide

46. Last step
    Where does my application
    run?
    

    View Slide

47. Last step
    public static void
    main(String[] args){ …}
    

    View Slide

48. ༼ つ ◕_◕ ༽つ thread ༼ つ ◕_◕ ༽つ
    

    View Slide

49. JVM - Thread
    Thread
    Heap Non-Heap
    

    View Slide

50. JVM - Thread
    Thread
    Program Counter Stack
    Frame
    Frame
    Frame
    Frame
    Frame
    Local variables []
    Return Value
    Pointer to Method Area
    Pointer to Heap Area
    Operand
    Operand
    Operand
    Operand Stack
    

    View Slide

51. JVM - Sum up
    Program Counter Stack
    Frame
    Frame
    Frame
    Frame
    Heap Non-Heap
    Object instances
    Class Stuff/
    Method Area
    Similar to
    Computer
    Architecture
    

    View Slide

52. ༼ つ ◕_◕ ༽つ JVM ༼ つ ◕_◕ ༽つ
    

    View Slide

53. ( ͡° ͜
    ʖ ͡°) JVM ( ͡° ͜
    ʖ ͡°)
    

    View Slide

54. Thanks!!
    Questions?
    @hannelita
    hannelita@gmail.com
    

    View Slide

55. References
    • http://www.artima.com/insidejvm/ed2/jvmP.html
    • http://www.cubrid.org/blog/dev-platform/understanding-jvm-internals/
    • http://architects.dzone.com/articles/understanding-jvm-internals
    • http://www.ourdailycodes.com/2013/09/inside-java-jvm-memory-
    structure-2.html
    • http://javarevisited.blogspot.ca/2011/04/garbage-collection-in-java.html
    • http://www.slideshare.net/BalamuruganSoundararajan/invoke-dynamics
    • http://www.javaworld.com/article/2076949/learn-java/how-the-java-virtual-
    machine-handles-method-invocation-and-return.html
    • http://blog.jamesdbloom.com/JVMInternals.html#threads
    

    View Slide