The document provides an overview of the Java Virtual Machine (JVM), detailing its architecture and key components, such as the bytecode verifier, class loader, execution engine, garbage collector, and security manager. It explains how the JVM handles bytecode, optimizes execution with Just-In-Time (JIT) compilation, and manages memory through various types of garbage collectors. Additionally, it discusses common out-of-memory errors and their potential causes, as well as the importance of security policies within the JVM.

1. JAVA VIRTUAL MACHINE
THE REAL THING: JAVA VIRTUAL MACHINE

2. http://frontech.com.tr
HIZIR SEFA İRKEN
SEFA.IRKEN@FRONTECH.COM.TR

3. THE REAL THING: JAVA VIRTUAL MACHINE
WHAT?
▸ VM is a layer of abstraction
▸ once only Java, but not anymore (after JDK 5)
▸ write once, run anywhere
▸ memory managed
▸ concurrent
▸ secure

4. THE REAL THING: JAVA VIRTUAL MACHINE
OUTSIDE

5. THE REAL THING: JAVA VIRTUAL MACHINE
INSIDE .CLASS FILE
JAVA VIRTUAL MACHINE
CLASS LOADER EXECUTION ENGINE
RUNTIME DATA AREAS

6. THE REAL THING: JAVA VIRTUAL MACHINE
INSIDE - KEY COMPONENTS

7. INSIDE
COMPONENTS OF JVM
1. bytecode verifier
2. class loader
3. execution engine
4. garbage collector
5. security manager

8. BYTECODE VERIFIER
BYTECODE VERIFIER
‣ first step in JVM is inspection
‣ checks for unusual code
‣ crucial for security

9. BYTECODE VERIFIER
WHAT BYTECODE CHECKED FOR
‣ variables are initialized before they are used
‣ method calls match the types of object references
‣ rules for accessing private data and methods are not
violated.
‣ local variable accesses fall within the runtime stack
‣ the runtime stack does not overflow

10. BYTECODE VERIFIER
HOW BYTECODE INSTRUCTIONS LOOK LIKE

11. CLASS LOADER
CLASS LOADER
‣ JVM doesn't need to know anything about runtime classes
‣ performs three main functions of JVM:
‣ loading
‣ linking
‣ initialization
‣ loads from anywhere
‣ every runtime has at least three class loaders

12. CLASS LOADER
THE LOADERS
‣ bootstrap class loader:
loads the system classes (typically from rt.jar)
‣ extension class loader:
loads “standart extensions” from the jre/lib/ext directory
jre/lib/endorsed will force override standard APIs
‣ system class loader:
loads the application classes from classpath.

13. EXECUTION ENGINE
EXECUTION ENGINE
‣ convert bytecode to machine code
‣ responsible for executing instructions contained in the
methods of loaded classes
‣ It has two parts:
‣ interpreter (ahead-of-time)
‣ JIT compiler aka just-in-time interpreter

14. JIT
JUST IN TIME - JIT
‣ advanced part of JVM
‣ starts after the program has started and compiles the code on the fly
‣ JIT has access to dynamic runtime information whereas a standard compiler
doesn't
‣ optimizes by compiling similar bytecodes at same time
‣ reduces overall execution time, improves performance

15. JIT
OPTIMIZATIONS
‣ inlining
‣ local optimizations
‣ control flow optimizations
‣ global optimizations
‣ native code generation
‣ reference: http://www.ibm.com/support/knowledgecenter/SSYKE2_7.0.0/
com.ibm.java.lnx.70.doc/diag/understanding/jit_optimize.html

16. JIT
IN ACTION

17. GARBAGE COLLECTOR
GARBAGE COLLECTOR
‣ garbage collecting: freeing objects that are no longer referenced by the program
‣ in short checks periodically for not-needed objects
‣ no memory management needed from programmer, but be careful
‣ types:
‣ serial collector
‣ parallel collector
‣ parallel compacting collector
‣ concurrent mark-sweep (CMS) collector
‣ garbage-first (G1) garbage collector

18. GARBAGE COLLECTOR
G1 IS DESIGNED FOR
‣ can operate concurrently with applications threads like the CMS collector
‣ compact free space without lengthy GC induced pause times
‣ need more predictable GC pause durations
‣ do not want to sacrifice a lot of throughput performance
‣ do not want to require a much larger Java heap

19. HEAP
HEAP STRUCTURE

20. HEAP
AFTER YOUNG GENERATION COLLECTION

21. SECURITY MANAGER
SECURITY MANAGER
‣ not the security you think
‣ an object responsible for guarding security policies
‣ constantly monitors the code
‣ always consulted before any potentially dangerous operation

22. SECURITY MANAGER
EXAMPLE

23. SECURITY MANAGER
OUTPUT

24. OUT OF MEMORY!
BONUS TOPIC

25. OUT OF MEMORY
TYPES
‣ java heap space
‣ permgen space
‣ gc overhead limit exceeded
‣ unable to create new native thread
‣ and more..

26. OUT OF MEMORY
PERMGEN
‣ occurs when JVM wants to load new class definitions, but there is not enough
space in PermGen space
‣ you are trying to use more than you have - increase PermGen space
‣ possible class loader leak in your application or app server - needs code fix

27. OUT OF MEMORY
GC OVERHEAD LIMIT EXCEEDED
‣ overhead limit is the policy to allow jam to detect potential OOM conditions
beforehand
‣ caused by:
‣ too many full GC iterations
‣ too much time spent in GC
‣ fine tune jvm params and your GC, use GC logging

28. OUT OF MEMORY
UNABLE TO CREATE NEW NATIVE THREAD
‣ JVM ask for new thread from OS, OS can not allocate a new thread anymore
‣ check ulimit, this is a OS config
‣ use thread-pools if possible
‣ reduce heap space
‣ perform vertical scaling

29. OUT OF MEMORY
JAVA HEAP SPACE
‣ you ran out of allocated memory for your application
‣ memory can be increased by JVM params
‣ check your code for memory leaks and unused datasets
‣ diagnose problem with heap dump -jmap

30. REFERENCES
REFERENCES
‣ https://en.wikipedia.org/wiki/Memory_management#DYNAMIC
‣ https://en.wikipedia.org/wiki/Java_virtual_machine
‣ http://www.oracle.com/technetwork/java/javase/tech/memorymanagement-whitepaper-1-150020.pdf
‣ https://books.google.com.tr/books?id=K7i6AgAAQBAJ
‣ https://docs.oracle.com/javase/8/docs/technotes/guides/#javavm
‣ https://blog.frankel.ch/java-security-manager/#gsc.tab=0
‣ https://docs.oracle.com/javase/tutorial/essential/environment/security.html
‣ http://www.slideshare.net/surbhiiiii/javajava-virtual-machine
‣ http://www.slideshare.net/BaabtraMentoringPartner/java-virtual-machine-43141334
‣ http://www.slideshare.net/pritybhudolia/jvm-20513648
‣ http://www.slideshare.net/aparnachaudhary/understanding-jvm
‣ https://docs.oracle.com/javase/specs/jvms/se8/html/index.html
‣ https://docs.oracle.com/javase/specs/jvms/se7/jvms7.pdf
‣ http://www.oracle.com/technetwork/tutorials/tutorials-1876574.html
‣ https://docs.oracle.com/javase/8/docs/technotes/guides/vm/
‣ https://docs.oracle.com/javase/8/docs/technotes/guides/vm/G1.html