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JVM

Disclaimer: This presentation is prepared by trainees of baabtra as a part of mentoring program. This is not official document of baabtra –Mentoring PartnerBaabtra-Mentoring Partner is the mentoring division of baabte System Technologies Pvt . Ltd

JAVA

• Java programming language was originally developed by Sun

Microsystems.

• Object Oriented : In Java, everything is an Object based.

• Platform independent.

Virtual Machine

• A virtual machine (VM) is a software implementation of a

machine (for example, a computer) that executes programs

like a physical machine.

•  Virtual machines are separated into two major

classifications:

system virtual machine and process virtual machine.

JVM

• A Java virtual machine (JVM) is a process virtual

machine that can execute java byte code.

• JVM converts Java byte code into machine language and

executes it.

• JVM is platform dependent : JVMs are available for many

hardware and software platforms.

• JVM gives Java the flexibility of platform independence.

Diagram of JVM

Java source code Byte codeJava compiler

Java Virtual Machine

Java Interpreter

Computer Operating System

Byte code loaded into JVM

Block DiagramClass loader sub

system

Execution engine

Class loader

• All Java virtual machines include one class loader that is

embedded in the virtual machine. 

• Class loader loads java classes into java virtual machine.

• Class loader reads bytecode and creates the instance of

java.lang.class. 

Execution engine

• The execution engine helps JVM to convert bytecode into

machine code.

• A mechanism responsible for executing the instructions

contained in the methods of loaded classes.

Stack

• Stack stores various method arguments and local variables of

any method .

• There are three registers that help in stack manipulation.

– Vars register

– Frame register

– Optop register

Sections in stack

• Local variable

– This section contain all local variable being used by current method.

• Execution environment

– Execution section is used to maintain the operation of stack itself.

• Operand stack

– The operand stack is used as a work space by bytecode instruction.

Method Area

• This is the area where byte code is placed.

• The program counter points to the next instruction in the

method area.

• After the execution of an instruction , JVM set pc to next

instruction.

Garbage Collected Heap

• The object in java program are stored in Garbage collected

heap.

• Arrays in java program are objects, hence they also stored

in heap.

• Java does not have a free operator to free any previously

allocated memory.

• Java done this automatically by Garbage collection

mechanism.

• JRE is an acronym for Java Runtime Environment.

• Java Runtime Environment contains JVM, class libraries, and

other supporting files.

• It does not contain any development tools such as compiler,

debugger, etc

JRE

JDK

• JDK is an acronym for Java Development Kit . It physically

exists . It contains JRE + development tools.

JIT

• Just-in-time Compiler -is the part of the Java Virtual Machine

(JVM) that is used to speed up the execution time.

• JIT compiles parts of the byte code that have similar

functionality at the same time, and hence reduces the amount

of time needed for compilation. 

Is java a compiled or interpreted language ?

• Java is a compiled language because the source code is

converted into byte code.

• Java is an interpreted language because the byte code is

interpreted by a virtual machine. 

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