CS Lesson: Introduction to the Java virtual Machine

Apr 11, 2023 1Java Virtual Machine© Katrin BeckerAll rights reserved.

Java Virtual Machine


Java Virtual Machine - start

JVM on Windows PCJVM on Windows PC

Java ‘Compiler’Java ‘Compiler’

ByteCodeByteCode

Java Source ProgramJava Source Program

JVM on iMACJVM on iMAC

JVM on UNIXJVM on UNIX

JVM on LINUX PCJVM on LINUX PC

ProgramRunningProgramRunning





Java Virtual Machine - source

Java Source ProgramJava Source Program -Written in standard Java-Target machine irrelevant-Doesn’t matter which machine

it’s created on.


Java Virtual Machine - compile



>> javac MyProg.java

Java ‘compiler’ does syntactic analysisand parsing.

Translates source into Byte CodeMyProg.class


Java Virtual Machine - Byte Code


ByteCodeByteCode


Bytecode is the same for ALL machines and ALL versions of Java

Up to this point everything looks and behaves identicallyon ALL machines

Bytecode is assembler-like, but thereis no really real machine that ‘understands’ bytecode.

Called Bytecode because most instructions are one or twobytes long


Java Virtual Machine – ready-set-..


ByteCodeByteCode


•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 – go


ByteCodeByteCode


•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 ‘tailor-made’ for the machine it is running on…


Java Virtual Machine - Linux


ByteCodeByteCode




Java Virtual Machine – Linux-b


ByteCodeByteCode





Java Virtual Machine - Windows



ByteCodeByteCode



Java Virtual Machine – Windows-b



ByteCodeByteCode




Java Virtual Machine - iMAC


ByteCodeByteCode




Java Virtual Machine – iMAC-b


ByteCodeByteCode





Java Virtual Machine - UNIX


ByteCodeByteCode




Java Virtual Machine – UNIX-b


ByteCodeByteCode


JVM on UNIXJVM on UNIX ProgramRunningProgramRunning


Java Virtual Machine ..TA-Da



ByteCodeByteCode










Once more from the topA slightly different

perspective….


Java Source :: JVM -1JVM source written in CJVM source written in C

Somebody writes the “Java Virtual Machine”;Jim Gosling probably wrote the first one.



C compiler compiles ‘JVM.c’C compiler compiles ‘JVM.c’

JVM.c gets compiled to produce….




JVM.exe(executable version of

JVM)


JVM)

There has to be a specially tailored one of these for the specific

machine we are currently using.


Java Source :: JVM –3bJVM source written in CJVM source written in C



JVM)


JVM)

The ‘Java Virtual Machine’ is now ready to be used (invoked).

It can sit around on the system indefinitely.





JVM)


JVM)

Java Source Program:MyProg.java


Along comes My Program,written in Java





JVM)


JVM)




>> javac MyProg.javaResults in…..

The Java compiler gets invoked –

It takes standard Java source as input

And produces standard Java ByteCode as output





JVM)


JVM)




ByteCodeByteCode

Now I am free to take my MyProg.class toany computer I choose, Any brand; any flavour; anywhere in the worldPROVIDED…It has a working JVM.exe


Java Source :: JVM –6bJVM source written in CJVM source written in C



JVM)


JVM)




ByteCodeByteCode

When I say:>> java MyProg.class

The target machine loads, and then starts to run theJava Virtual Machine….





JVM)


JVM)




ByteCodeByteCode

Machine-dependent Linker/Loader






JVM)


JVM)




ByteCodeByteCode



‘Java’ ProgramRunning bytecode

as input


as input

The JVM expectsa file containingByteCode as input





JVM)


JVM)




ByteCodeByteCode



MyProg.class ProgramRunning



as input


as input

The output from the JVM is the output that your program produces (unless of course the JVM detects a problem)


Java Source :: JVM -10


ByteCodeByteCode





JVM)


JVM)






as input


as input

MyProg output

MyProg output

JVM source written in CJVM source written in C


The end.

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CS Lesson: Introduction to the Java virtual Machine