The document discusses Java virtual machines and how Java code is executed. It defines a virtual machine as an abstraction layer that allows programs to interact with different operating systems. It then describes how the Java virtual machine works, executing Java bytecode and allowing code to run on any platform. It outlines the components of the JVM, including the class loader, execution engine, and garbage collector.

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document of baabtra –Mentoring Partner
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4. Virtual Machine
“A virtual machine is a layer of abstraction that gives a
program, one simplified interface for interacting with a
variety of physical computers and their operating systems.”

5. Java Virtual Machine
• JVM is not a real hardware machine but a software layer which resembles
an hardware platform.
• JVM converts Java byte code into machine language and executes it.
• The byte code can be executed on any platform where there exist JVM.
• JVM‘s are available for many hardware and software platforms.
• The use of the same byte code for all JVM’s on all platforms allows Java to
be described as a "write once, run anywhere" programming language.
• Thus, the JVM is a crucial component of the Java platform.

6. Diagram Of JVM
Java
Source
Code(.java)
Java
Object
Code(.class)
Java Compiler
(javac)
Computer
Operating System
Java Interpreter
Java Virtual Machine
Object Code is loaded into
Java Virtual Machine

7. Components Of JVM
Class Loader Execution Engine
Run-time data areas
Java Virtual Machine
.class file

8. Components Of JVM
1. Byte code verifier
2. Class loader
3. Execution engine
4. Garbage collector
5. Security Manager

9. 1. Byte code Verifier:
• Byte code verifier is used to verify the byte code.
• Also checks for unusual code.
• So it is a crucial component for security.
2. Class loader:
• Class loader loads java classes into java virtual machine and creates
the instance of java.lang.class.
• The main feature is that JVM doesn’t need to have any knowledge
about the classes that will be loaded at runtime.
• It performs three main functions of JVM, namely: loading, linking
and initialization

10. 3. Execution Engine:
• The execution engine helps JVM to convert bytecode into machine code.
• It has two parts:
a) Interpreter
b) Just-in-time-interpreter
• Execution engine is responsible for executing the instructions contained in
the methods of loaded classes.
4. Garbage Collector:
• Garbage collection is the process of automatically freeing objects that are
no longer referenced by the program.
• It periodically check for the object on heap , whose link is broken so it can
collect garbage from heap.
• Garbage collection relieves java programmer from memory management.

11. 5. Security Manager:
• Security manager constantly monitors the code.
• It is a special java object that is responsible for guarding security policies
for java applications.
• It is always consulted before any potentially dangerous operation is
requested by a java application.

12. JDK ‘n’ JRE
Diagram to show the
relations between JVM,
JRE & JDK

13. Java Runtime Environment(JRE):
• Provides the libraries, the Java Virtual Machine, and other components to
run applets and applications written in the Java programming language.
• The JRE does not contain tools and utilities such as compilers or
debuggers for developing applets and applications.
Java Development Kit(JDK):
• JDK is a bundle of software that is used to develop Java based
applications.
• The JDK is a superset of the JRE, and contains everything that is in the
JRE, plus set of API classes, Java compiler and debugger, and additional
files needed to write Java applets and applications.
• Needs more disk space than JRE

14. Just In Time(JIT):
• JIT are advanced part of Java Virtual machine
• It optimize byte code to machine instruction conversion by compiling
similar byte codes at same time
• Thus reducing overall execution time.
• JIT performs several optimizations such as in-lining function.
• JIT compiler improves performance

15. In Short....
• Java Virtual Machine (JVM) is platform dependent.
• Java is platform independent.
• JVM does not really exists but is created within another environment.
• The JVM stands on top of the operating system (OS).

16. Is
Java a
Compiled or Interpreted
Language

17. Java is both compiled and interpreted
language.
Java compiles using javac and JIT, and
interprets using Java virtual machine.

18. “From Writingto
Execution”

19. 1
2
“Programmer write source code and store that
into a .java file.”
Note: Name of your Java source file must be same
with the public class declared inside that file, for
example if there is public class called Order inside
Java file, then it name must be Order.java
“javac compiles .java fie and creates
class files, which contains byte code”

20. 3
4
“JVM executes these class files and gathers
statistics of execution run.”
“When JVM has enough data to make decision, JIT
compiles frequently used byte codes into native
code, which is then directly executed by platform.
This is how JIT provides performance boost to
Java application.”

21. Any Doubts...???

22. Thanks For
Listening...
The End

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