JVM Internals Demystified

༼ つ ◕_◕ ༽つ JVM Internals ༼ つ ◕_◕ ༽つ

Demystified! ( °͡ ʖ͜ °͡)

༼ 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!

Hi! I’m Hanneli

São Paulo, Brazil

No advanced stuffSimplified way

Started learning Java for Web AppsNever had looked at JVM

It was difficult to learn the internals(at least for me)

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

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 methodsClazz.method();

object.method();

static

instance

From Assembly classes:

op code + operandoperation + operand

operation + operand

add r1, r2

public int add(int r1, int r2)

operation operators == parameters

Method calls in JVMstaticinstance

operation+

operand

operation +

operandinvokevirtual 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 not 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 uniqueSo 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

JVMClass stuff Class stuff Class stuff

Non-HeapCode Cache

Permanent G

eneration

Done?object

༼ つ ◕_◕ ༽つ object ༼ つ ◕_◕ ༽つ

Method Area

JVM

Class stuff Class stuff Class stuff

Non-Heap

Permanent G

eneration

HEAPPerm

anent Generation

objectobject

object

Young gen

JVM - The Heap

HEAP

Lol lol = new LOL();

lol

object object

object

JVM - The Heap

HEAP

Permanent G

eneration

object

object

objectobject

object

object

objectobject

object

object

objectobject

object

object

objectobject

Hey, Im’m heavy

JVM - The Heap

HEAP

Permanent G

eneration

object

object

objectobject

object

object

objectobject

object

object

objectobject

object

object

objectobject

Checks all the objects on the heap

Garbage collector

Old gen

Young generation

JVM - The Heap

HEAPobject object

Eden

lolSurvivor

object

Will be taken for GC

Last stepWhere 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 ValuePointer to Method AreaPointer to Heap Area

OperandOperandOperand

Operand Stack

JVM - Sum upProgram Counter Stack

Frame

Frame

Frame

Frame

Heap Non-Heap

Object instances Class Stuff/ Method Area

Similar to Computer

Architecture

What are those weird parameters?

-XX -Xms

HEAP

HEAP

JVM Optimization Techniques

Why should I study JVM?

New languagesOptimizationsArchitecture ideasHardware and AI

༼ つ ◕_◕ ༽つ JVM ༼ つ ◕_◕ ༽つ

( °͡ ʖ͜ °͡) JVM ( °͡ ʖ͜ °͡)

Thanks!!

Questions?

@[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

• http://www.amazon.com.br/Java-Virtual-Machine-Definition-Verification-ebook/dp/B000V9G6QW/ref=sr_1_1?ie=UTF8&qid=1435028967&sr=8-1&keywords=java+virtual+machine