Java virtual machine

By: Nikhil
Sharma.
JAVA VIRTUAL
MACHINE(JVM)

Java Platform.
Editions of Java.
Big Picture of Java.
Java “Fanboi”.
What is JVM.
What JVM does.
Internal Architecture of
JVM.
Data Types of JVM.
Garbage Collection.
What is JDK & JRE.
Relation among JVM,
JDK & JRE.
Security Promises of
JVM.
INDEX

Java Platform is a suite of programs that
facilitate developing & running programs written
in java programming language.
The platform is not specific to any processor or
Operating System.
There are multiple platforms each targeting a
different class of devices.
JAVA PLATFORM

Java SE: It is the Java Standard Edition that
contains basic core java classes.
Java EE: It is the Java Enterprise Edition & it
contain classes that are beyond Java SE.
Java ME: It stands for Java Micro Edition for
developing codes for portable devices.
EDITIONS OF JAVA

 Speaking of Java as a
language as opposed to the
JVM platform, James
Gosling, the Father of Java,
said "Most people talk about
Java the language, and this
may sound odd coming from
me, but I could hardly care
less."
 He went on to explain, "What
I really care about is the Java
Virtual Machine as a concept,
because that is the thing that
ties it all together."
JVM “FANBOI”

Java Virtual Machine is the heart of entire Java
program execution process.
It’s an abstract machine. It is a specification that
provides runtime environment in which java
bytecode can be executed.
JVM is platform dependent.
It is responsible for taking .class file &
converting each byte code instructions into the
machine language instruction that can be
executed by the microprocessor.
WHAT IS JVM

The JVM performs following operation—
Loads code.
Verifies code.
Executes code.
Provides Runtime Environment.
WHAT JVM DOES

First of all, it loads .class file into memory.
Then it verifies whether all byte code
instructions are proper or not. If it finds any
instruction suspicious, the execution is rejected
immediately.
If the byte instructions are proper, then it
allocates necessary memory to execute the
program.
STEPS UNDERTAKEN

Classloader: Classloader is a subsystem of JVM
that is used to load class files.
Method Area: Method area is the memory block,
which stores the class code, code of the
variables, and code of the method in java
programs.
Heap: It is the runtime data area in which
objects are allocated.
CONTD.

Java Stacks: Java stacks are memory area where
the java methods are executed. While executing
methods, a separate frame will be created in the
java stack, where the method is executed. JVM
uses separate threads(or process) to execute each
methods.
PC Registers: These are memory areas which
contains memory address of instructions of the
methods.
CONTD.

Native Method Stacks: It contains all the native
methods used in the applications. Native
methods are executed in these stacks.
Native Method Interface/Libraries: To execute
the native methods, generally native method
libraries are required. These header files are
located & connected to JVM by a program,
called Native method interface.
CONTD.

Execution Engine: Execution engine contains—
i. Interpreter.
ii. JIT(Just In Time) complier.
these are responsible for converting the byte
code instructions into machine code so that the
processor will execute them.
CONTD.

STORAGE & COMPUTATION
TYPE INSIDE THE JVM

Each object consumes some memory, of which
all there is a limited area. Eventually, the
memory allocated to these objects must be
reclaimed when they aren’t used. The JVM
reclaims these objects automatically through a
process called Garbage collection.
An object is ready to be garbage collected when
it is no longer alive.
Garbage collector uses many algorithm but the
most commonly used algorithm is mark & sweep.
GARBAGE COLLECTION.

JRE is an acronym for Java Runtime
Environment. It is used to provide runtime
environment. It is the implementation of JVM.
WHAT IS JRE

JDK is an acronym for Java Development Kit. It
physically exists. It contains JRE + development
tools.
WHAT IS JDK

RELATION AMONG JVM, JDK
& JRE.

 Every object is constructed exactly once before it is
used.
 Every local variable & field is initialized before it is
used.
 Final methods cannot be overridden, & final classes
cannot be sub classed.
 Many More..
The java platform security architecture depends on all the
promises & many more..
SECURITY PROMISES OF
JVM

Java virtual machine

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Java virtual machine