The document discusses the execution flow of the main() method inside the Java Virtual Machine (JVM). It explains that the main() method acts as the initial entry point and starting point for program execution. When a Java class is run, the JVM loads the class and executes the main() method first. Any other methods in the class can only be executed if explicitly called from main(). It then provides answers to some common questions about the main() method, such as that it is user-defined but has a predefined syntax, and that the String array parameter allows passing runtime arguments to the program. Finally, it outlines the step-by-step process the JVM follows to load and execute a Java class containing a main() method

1. EXECUTION FLOW OF MAIN()
METHOD INSIDE JVM
in JAVA Saurabh Uniyal
MCA VI
HNBGU Srinagar
Garhwal
27-May-2016

2. CONTENTS
 Why main(-) method
 Some main() method FAQ’s
 User defined or Predefined ?
 In which class main(-) method is define by sun ?
 Why main() method has String parameter array ?
 How JVM execute a java class ?

3. WHY MAIN(-) METHOD
 main(-) is a initial point
of a class Execution
JVM
Internal code
Example.java
Class Example {
public static void main(String[]
args)
{
System.out.println(“main
Block”);
}
Output :
main Block
If we create some
other method in a
class
static void m1()
{
System.out.println(“method 1
body”);
}
static void m2()
{
System.out.println(“method 2
body ”);
}
Then JVM only
execute main(-)
method’s logic

4. WHY MAIN(-) METHOD(CONT’D)
JVM
Internal code
Example.java
Class Example {
public static void main(String[]
args)
{
System.out.println(“main Block”);
}
Output :
main Block
method 2 body
static void m1()
{
System.out.println(“method 1
body”);
}
If we want to execute
those method’s logics
then we must call these
methods from main(-)
method
m2();
static void m2()
{
System.out.println(“method 2
body ”);
}

5.  so that we can say main method is a mediator between JVM and Java
Programmer.
JVM
class
main()

6. FAQ’S ABOUT METHOD(-)

7. QUES: IS MAIN METHOD IS USER DEFINED OR PREDEFINED
Ans:
main(-) method is User defined method with Predefined Syntax
Public static void main(String[] args)
{
}
given by : sun Public static void main(String[] args)
{
…………….
…………….
}
Complete logic is Programmer’s logic

8. QUES: IN WHICH CLASS MAIN(-) METHOD IS
DEFINE BY SUN ?
Ans:
main(-) is not physically defined in any class.
It’s syntax is logically specified by sun.

9. WHY MAIN() METHOD HAS STRING PARAMETER ARRAY ?
For reading runtime values from keyboard()
C:Windowssystem32cmd.exeC:_
Class Example {
public static void main(String[ ] args)
{
String name=arg[0];
String addr =arg[1];
System.out.println(“name: ”+name);
System.out.println(“addr : ”+addr);
}
}
Compile
Microsoft Windows [Version 10.0.10240]
(c) 2015 Microsoft Corporation. All rights reserved.
C:Userssaurav>C:Userssaurav> javac Example.java
X[]-
C:Userssaurav>
Compiler
generate
Example.class
file in same
directoryC:Userssaurav> java Example sherlock NYsherlock NY
name :sherlock
addr : NY

10. HOW JVM EXECUTE THIS “EXAMPLE1” JAVA CLASS
Class Example1 {
static int x=10;
static int y=20;
int a=30;
int b=40;
public static void main(String[ ] args)
{
int p=50;
int q=60;
Eample1 e1=Example();
}
}
Example1.java

11. J V M
Virtual
Machine
Java
JVM System based
virtual machine
 VirtualBox
 VMWare
Process based
virtual machine
 JVM
 PVM
Runtime instance

12. C:> java Example1
Method Area
JVM
Heap Area JAVA Stack Area
Programm Counter
Register Area
Native method Area
Class
data
Class
data
Class
data
Class
data Class
data
Class
data
Class
data
object
object
object
object
object
object
object
object
object
Class
data
Class
data
Class
data
Class
data
Class
data
Class
data
Class
data
Class
data
Class
data
object
object
object
object
object
Runtime
area
One
program
counter
per
Thread
Responsible
For native
method
execution

13. C:> java Example1
Method Area
JVM
Heap Area JAVA Stack Area
Programm Counter
Register Area
Native method Area
Class Loader

14. Jdk/jre/lib/ex/*.jar
Class Loader
Application Class Loader
Extension Class Loader
Bootstrap Class Loader
Searching in Jdk/jre/lib/rt.jar
If found
load
class to
method
area
Searching in
If found
load
class to
method
area
If not found
If not found
Class path systemSearching in
If found
load
class to
method
area
If not found
returns
ClassNotFoundException
Using “Parent Delegation Algorithm”

15. Method Area Heap Area JAVA Stack Area
Programm Counter
Register Area
Native Area
Class Loader
If found, then load this class in method area by
creating java.lang.Class Object
x 0
y 0
a
b
main(s[]){ }
x = 10
y = 20
10
20p s v main(S args[])
args1010
p50
q60
length 0
1010
e12020
2020
a 30
b 40
Static variable
Linking
&
Execute
native
methods
uisng JNI

16. REFERENCES
“Inside the Java Virtual Machine”, Bill Venners
“JVM Architecture with SCJP”, Harikrishna sir
www.nataraj.in

17. THANK YOU
sauravuniyal@gmail.com