JVM Internals Demystified

Transcript

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   ༼ つ ◕_◕ ༽つ JVM Internals ༼ つ ◕_◕ ༽つ

   Demystified! ( ͡° ͜ ʖ ͡°) ༼ つ ◕_◕ ༽つ ༼ つ ◕_◕ ༽つ
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   What we will NOT see here • Pure JVM boring

   theory • Assembly fear stuff • 0xFF • 0100111
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   What we will see here • Why internals? • Method

   Area • Heap • Threads • Further reading • Funny Gifs and images!
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   Bonjour

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   Hi! I’m Hanneli São Paulo, Brazil

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   Hi! I’m Hanneli Meanwhile here in Canada

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   None
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   No advanced stuff Simplified way

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   If you understand computer architecture classes You understand JVM

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    Why do I Need to Understand Internals?

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    We live in a simplified era

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    All externalised

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    All externalised (Almost)Normal dressed developers (external)

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    Do I need internal details?

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    Developer showing internal details

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    Memory analyse • Help! Perm gem problems! • Search for

    memory leaks • Memory analyser tools • Understand how JVM works
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    What’s the relation between: JVM Memory Bytecode Assembly Low Level

    OpCode Operator Operation Java
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    None
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    How do I get started? • http://www.artima.com/insidejvm/ed2/jvmP.html

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    How do I get started?

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    JVM - Low level Assembly

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    I know how to write Java code

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    ༼ つ ◕_◕ ༽つ classes ༼ つ ◕_◕ ༽つ

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    What happens when I invoke a method in Java

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    WAIT How many kinds of methods do you know?

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

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    From Assembly classes: op code + operand operation + operand

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    operation + operand add r1, r2 public int add(int r1,

    int r2) operation operators == parameters
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    Method calls in JVM static instance operation + operand operation

    + operand invokevirtual invokestatic
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    Where are the Operands? Special place

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    Where are the Operands? Real place

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    Symbolic place called pool

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    JVM keeps this fake references It’s a strategy to deal

    with dynamic linking
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    While JVM does need the content of this fake box

    (pool), it remais unresolved
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    Each fake reference for each method is unique

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    Each fake reference for each method is unique So JVM

    can replace the fake calls with true invokes.
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    Back to methods Clazz.method(int a); object.method(parameter); class Lol { public

    int myMethod(int a){ // } } Class stuff
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    Method Area JVM Class stuff Class stuff Class stuff Non-Heap

    Code Cache Permanent Generation
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    Done? object

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    ༼ つ ◕_◕ ༽つ object ༼ つ ◕_◕ ༽つ

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    Method Area JVM Class stuff Class stuff Class stuff Non-Heap

    Permanent Generation HEAP Permanent Generation object object object
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    Young gen JVM - The Heap HEAP Lol lol =

    new LOL(); lol object object object
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    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
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    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
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    Old gen Young generation JVM - The Heap HEAP object

    object Eden lol Survivor object Will be taken for GC
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    Last step Where does my application run?

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

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    ༼ つ ◕_◕ ༽つ thread ༼ つ ◕_◕ ༽つ

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    JVM - Thread Thread Heap Non-Heap

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    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
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    JVM - Sum up Program Counter Stack Frame Frame Frame

    Frame Heap Non-Heap Object instances Class Stuff/ Method Area Similar to Computer Architecture
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    ༼ つ ◕_◕ ༽つ JVM ༼ つ ◕_◕ ༽つ

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    ( ͡° ͜ ʖ ͡°) JVM ( ͡° ͜ ʖ

    ͡°)
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    Thanks!! Questions? @hannelita hannelita@gmail.com

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    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