JVM INTERNALSIntroduction to Virtual Machines and the JVM

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Luiz Testonwww.fracta.cc

www.corkjug.com

1st meetup in14/10

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VIRTUAL MACHINES

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Mimics a Real Machine

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Loads and execute code

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CLASSLOADERS

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Java classes are loaded on demand

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How they are loaded is up to the classloader

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Classloaders are hierarchical

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boot jars

LibraryA.jar

LibraryB.jar

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boot jars

LibraryA.jar

LibraryB.jar

Load class “Main”

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boot jars

LibraryA.jar

LibraryB.jar

Load class “Main”

Not Here

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boot jars

LibraryA.jar

LibraryB.jar

Load class “Main”

Not Here

Not Here

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boot jars

LibraryA.jar

LibraryB.jar

Load class “Main”

Not Here

Not Here

Wait…

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boot jars

LibraryA.jar

LibraryB.jar

Load class “Main”

Not Here

Not Here

Not Here

run time generated code

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boot jars

LibraryA.jar

LibraryB.jar

Load class “Main”

Not Here

Not Here

Not Here

run time generated code Here!

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HOW TO EXECUTE CODE?

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INTERPRETING

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Line of text AST

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Simple example: sum of two numbers

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int variable: j

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int variable: jint variable: i

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int variable: jint variable: i

sum

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int variable: jint variable: i

sum

return

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Errors usually are caught at runtime

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COMPILING

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Code compiled to binary prior to the execution.

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Some errors can be caught at compile time

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Binary can be: native code, VM bytecode and so on…

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Bytecode: Byte sized OPCODE

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REGISTER BASED VM

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Works like your processor

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Simple example: summing two numbers

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R1 R2 RN…

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R1 R2 RN…1

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R1 R2 RN…1

+2

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R1 R2 RN…3

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STACK BASED VM

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Works like you HP48G calculator

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1

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1

2

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1

2+

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3

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JVM IS STACK BASED

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Eventually byte code is compiled to native code on the fly

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DYNAMIC MEMORY

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C/C++ Approach: memory as a big array

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index size variable

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index size variable

1 1 i

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index size variable

1 1 i

2 2 l

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index size variable

1 1 i

2 2 l

3 4 c

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index size variable

1 1 i

3 4 c

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Few caveats for this approach

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Possible memory fragmentation

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Possible memory fragmentation

4 sized var doesn’t fit

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Possible memory leak

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Possible memory leak

used vars unused vars

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Possible invalid pointer

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Possible invalid pointer

variable pointing here

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JVM based approach: Garbage Collector

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SIMPLE GC ALGORITHMS

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MARK AND SWEEP

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VISIBLE REF

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COPY

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VISIBLE REF

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IN PRACTICE

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Eden

Survivor 1

Survivor 2

GC

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Eden

Survivor 1

Survivor 2

GC

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Eden

Survivor 1

Survivor 2

Wait…GC

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Eden

Survivor 1

Survivor 2

off you goGC

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Eden

Survivor 1

Survivor 2

GC

t

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Eden

Survivor 1

Survivor 2

GC

t ?

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Eden

Survivor 1

Survivor 2

GC

t ?

Wait…

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Eden

Survivor 1

Survivor 2

GC

? t

Wait…

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Eden

Survivor 1

Survivor 2

GC

? t

off you go

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Eden

Survivor 1

Survivor 2

GC

? t

t2

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Eden

Survivor 1

Survivor 2

GC

? t

t2

t is still used within t2

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Eden

Survivor 1

Survivor 2

GC

? t

t2

? is not used…

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Eden

Survivor 1

Survivor 2

GC

? t

t2

Whatever, I have plenty of memory

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Few caveats for this approach

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GC Wait…

GC needs to be properly configured

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GC …

GC can be unpredictable

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GC

No control over memory layout

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GC

No control over memory layout

Let me do this job!

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GCManaging memory

is hard!

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GC Having a GC doing the hard work is good.

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WHAT ABOUT PERM GEN?

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@Deprecated

HANDS ON

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• Create a .java file

• Compile it into a .class file

• Analyse its binary content

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magic number

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Source code info (can be removed by

compilation args)

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Optimisations?

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Yes, we appended to a string, but strings are immutable in Java.

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the addTimes method

Time to take a look on the JVM Assembly code

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main method

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arg to int into a variable

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new SumArg1Arg2Times

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StringBuilder optimisation

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Constructor

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can you see the loop?

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can you see the loop?

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add method changing a field

Going further, let’s look the native assembly code

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• Google for java dissablembler plugin. Install it (hdis-i386 or hdis-amd64)

• See the native assembly output

• Enjoy analysing it

large iteration count, so it can be JIT compiled

native code for java.lang.Object constructor

native code for addTimes

related to the loop within addTimes

native assembly

related bytecode

native code for add method

bytecode for add method

QUESTIONS?

WHO AM I

Luiz Teston

• 15 years on the field mostly as a consultant, working on non trivial projects using Java, C++ and functional programming

• lteston@fracta.cc• http://fracta.cc• http://linkedin.com/in/teston• http://twitter.com/FeuTeston

KEEP IN TOUCH

Feedback about the presentation appreciated.

lteston@fracta.cc

www.fracta.cc

REFERENCES

• Garbage Collection: Algorithms for Automatic Dynamic Memory Management, by Richard Jones and Rafael Lins.

• Virtual Machines: Versatile Platforms for Systems and Processes, by Jim Smith and Ravi Nair.

• https://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoader.html

• http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode/

• http://docs.oracle.com/javase/specs/jvms/se8/html/index.html

• http://mechanical-sympathy.blogspot.ie/2013/06/printing-generated-assembly-code-from.html

• http://www.slideshare.net/CharlesNutter/redev-2011-jvm-jit-for-dummies-what-the-jvm-does-with-your-bytecode-when-youre-not-looking

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