An introduction to the Java Virtual Machine

1. Java Virtual Machine
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 1

2. Java Virtual Machine - start
Java Source Program
Java Source Program
Program
Program
Running
Running
Java ‘Compiler’
Java ‘Compiler’
Program
Program
Running
Running
JVM on LINUX PC
JVM on LINUX PC
Byte
Byte
Code
Code
JVM on Windows PC
JVM on Windows PC
JVM on iMAC
JVM on iMAC
JVM on UNIX
JVM on UNIX
© Katrin Becker
All rights reserved.
Program
Program
Running
Running
Java Virtual Machine
Program
Program
Running
Running
Jan 18, 2014 2

3. Java Virtual Machine - source
Java Source Program
Java Source Program
© Katrin Becker
All rights reserved.
-Written in standard Java
-Target machine irrelevant
-Doesn’t matter which machine
it’s created on.
Java Virtual Machine
Jan 18, 2014 3

4. Java Virtual Machine - compile
Java Source Program
Java Source Program
>> javac MyProg.java
Java ‘Compiler’
Java ‘Compiler’
Java ‘compiler’ does syntactic analysis
and parsing.
Translates source into Byte Code
MyProg.class
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 4

5. Java Virtual Machine - Byte Code
Java Source Program
Java Source Program
Bytecode is
the same for ALL machines
Java ‘Compiler’
Java ‘Compiler’
and ALL versions of Java
Up to this point everything looks
and behaves identically
on ALL machines
Bytecode is assembler-like, but there
Byte
Byte
is no really real machine that
‘understands’ bytecode.
Code
Code
Called Bytecode because most
instructions are one or two
bytes long
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 5

6. Java Virtual Machine – ready-set-..
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
•Bytecode can be transported to
any machine with a bytecode
interpreter [called the “Java
Virtual Machine”]
•Assemblers run on real
machines, so…..
•Java Bytecode runs on a
“Virtual Machine”
Java Virtual Machine
Jan 18, 2014 6

7. Java Virtual Machine – go
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
•The Virtual Machine is like a
simulator – it simulates a real
machine, but…
•It also ‘tells’ the real machine
on which it is currently running
to do the stuff the virtual
machine would be doing
•This requires it to be ‘tailormade’ for the machine it is
running on…
Java Virtual Machine
Jan 18, 2014 7

8. Java Virtual Machine - Linux
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
JVM on LINUX PC
JVM on LINUX PC
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 8

9. Java Virtual Machine – Linux-b
Java Source Program
Java Source Program
Program
Program
Running
Running
Java ‘Compiler’
Java ‘Compiler’
JVM on LINUX PC
JVM on LINUX PC
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 9

10. Java Virtual Machine - Windows
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
JVM on Windows PC
JVM on Windows PC
Java Virtual Machine
Jan 18, 2014 10

11. Java Virtual Machine – Windows-b
Java Source Program
Java Source Program
Program
Program
Running
Running
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
JVM on Windows PC
JVM on Windows PC
Java Virtual Machine
Jan 18, 2014 11

12. Java Virtual Machine - iMAC
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
JVM on iMAC
JVM on iMAC
Java Virtual Machine
Jan 18, 2014 12

13. Java Virtual Machine – iMAC-b
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
© Katrin Becker
All rights reserved.
Program
Program
Running
Running
JVM on iMAC
JVM on iMAC
Java Virtual Machine
Jan 18, 2014 13

14. Java Virtual Machine - UNIX
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
JVM on UNIX
JVM on UNIX
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 14

15. Java Virtual Machine – UNIX-b
Java Source Program
Java Source Program
Java ‘Compiler’
Java ‘Compiler’
Byte
Byte
Code
Code
JVM on UNIX
JVM on UNIX
© Katrin Becker
All rights reserved.
Java Virtual Machine
Program
Program
Running
Running
Jan 18, 2014 15

16. Java Virtual Machine ..TA-Da
Java Source Program
Java Source Program
Program
Program
Running
Running
Java ‘Compiler’
Java ‘Compiler’
Program
Program
Running
Running
JVM on LINUX PC
JVM on LINUX PC
Byte
Byte
Code
Code
JVM on Windows PC
JVM on Windows PC
JVM on iMAC
JVM on iMAC
JVM on UNIX
JVM on UNIX
© Katrin Becker
All rights reserved.
Program
Program
Running
Running
Java Virtual Machine
Program
Program
Running
Running
Jan 18, 2014 16

17. Once more from the top
A slightly different
perspective….
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 17

18. Java Source :: JVM -1
JVM source written in C
JVM source written in C
Somebody writes the “Java Virtual Machine”;
Jim Gosling probably wrote the first one.
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 18

19. Java Source :: JVM -2
JVM source written in C
JVM source written in C
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
JVM.c gets compiled to produce….
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 19

20. Java Source :: JVM -3
JVM source written in C
JVM source written in C
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
There has to be a specially tailored
one of these for the specific
machine we are currently using.
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 20

21. Java Source :: JVM –3b
JVM source written in C
JVM source written in C
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
The ‘Java Virtual Machine’ is now
ready to be used (invoked).
It can sit around on the system
indefinitely.
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 21

22. Java Source :: JVM -4
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
Along comes My
Program,
written in Java
© Katrin Becker
All rights reserved.
JVM source written in C
JVM source written in C
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
Java Virtual Machine
Jan 18, 2014 22

23. Java Source :: JVM -5
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
>> javac MyProg.java
Results in…..
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
The Java compiler gets invoked –
It takes standard Java source as input
And produces standard Java ByteCode as output
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 23

24. Java Source :: JVM -6
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
Byte
Byte
Code
Code
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
Now I am free to take my MyProg.class to
any computer I choose,
Any brand; any flavour; anywhere in the world
PROVIDED…
It has a working JVM.exe
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 24

25. Java Source :: JVM –6b
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
Byte
Byte
Code
Code
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
When I say:
>> java MyProg.class
© Katrin Becker
All rights reserved.
The target machine loads, and then starts to
run the
Java Virtual Machine….
Java Virtual Machine
Jan 18, 2014 25

26. Java Source :: JVM -7
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
Byte
Byte
Code
Code
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
Machine-dependent
Machine-dependent
Linker/Loader
Linker/Loader
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 26

27. Java Source :: JVM -8
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
Byte
Byte
Code
Code
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
Machine-dependent
Machine-dependent
Linker/Loader
Linker/Loader
The JVM expects
a file containing
ByteCode as input
© Katrin Becker
All rights reserved.
‘Java’ Program
‘Java’ Program
Running bytecode
Running bytecode
as input
as input
Java Virtual Machine
Jan 18, 2014 27

28. Java Source :: JVM -9
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
Byte
Byte
Code
Code
MyProg.class
MyProg.class
Program
Program
Running
Running
© Katrin Becker
All rights reserved.
The output
JVM.exe
JVM.exe
from the JVM
(executable version of
(executable version of
is the output
JVM)
JVM)
that your
program
Machine-dependent
Machine-dependent
produces
Linker/Loader
Linker/Loader
(unless of
course the
‘Java’ Program
‘Java’ Program JVM detects a
Running bytecode
Running bytecode problem)
as input
as input
Java Virtual Machine
Jan 18, 2014 28

29. Java Source :: JVM -10
Java Source Program:
Java Source Program:
MyProg.java
MyProg.java
JVM source written in C
JVM source written in C
Java ‘Compiler’
Java ‘Compiler’
C compiler compiles ‘JVM.c’
C compiler compiles ‘JVM.c’
Byte
Byte
Code
Code
JVM.exe
JVM.exe
(executable version of
(executable version of
JVM)
JVM)
Machine-dependent
Machine-dependent
Linker/Loader
Linker/Loader
MyProg.class
MyProg.class
Program
Program
Running
Running
© Katrin Becker
All rights reserved.
‘Java’ Program
‘Java’ Program
Running bytecode
Running bytecode
as input
as input
Java Virtual Machine
MyProg
MyProg
output
output
Jan 18, 2014 29

30. The end.
© Katrin Becker
All rights reserved.
Java Virtual Machine
Jan 18, 2014 30