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

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

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

Bonjour 

Hi! I’m Hanneli São Paulo, Brazil 

Hi! I’m Hanneli Meanwhile here in Canada 

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No advanced stuff Simplified way 

If you understand computer architecture classes You understand JVM 

Why do I Need to Understand Internals? 

We live in a simplified era 

All externalised 

All externalised (Almost)Normal dressed developers (external) 

Do I need internal details? 

Developer showing internal details 

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

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

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

How do I get started? 

JVM - Low level Assembly 

I know how to write Java code 

༼ つ ◕_◕ ༽つ classes ༼ つ ◕_◕ ༽つ 

What happens when I invoke a method in Java 

WAIT How many kinds of methods do you know? 

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

From Assembly classes: op code + operand operation + operand 

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

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

Where are the Operands? Special place 

Where are the Operands? Real place 

Symbolic place called pool 

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

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

Each fake reference for each method is unique 

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

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

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

Done? object 

༼ つ ◕_◕ ༽つ object ༼ つ ◕_◕ ༽つ 

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

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

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 

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 

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

Last step Where does my application run? 

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

༼ つ ◕_◕ ༽つ thread ༼ つ ◕_◕ ༽つ 

JVM - Thread Thread Heap Non-Heap 

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 

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

༼ つ ◕_◕ ༽つ JVM ༼ つ ◕_◕ ༽つ 

( ͡° ͜ ʖ ͡°) JVM ( ͡° ͜ ʖ ͡°) 

Thanks!! Questions? @hannelita [email protected] 

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