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Kapil Thakkar
Kapil Thakkar

Reputation: 39

JVM (Java Virtual Machine) is an abstract machine.

I am a java beginner. i just read a line " JVM (Java Virtual Machine) is an abstract machine." can anyone please help me to understand term abstract machine. what is abstract machine.

Upvotes: 1

Views: 4362

Answers (5)

Tadele Ayelegn
Tadele Ayelegn

Reputation: 4746

It means not real machine, however, it acts like a machine. Not any machine but as a machine called Central Processing Unit(CPU).

You could understand this like, whenever you write java program, you are writing instructions for a machine, but this machine is abstract, which is called Java Virtual Machine(JVM).

For each Operating system(OS) there is a Virtual Machine so that you do not have to write different versions of your java program to different OS.The JVM that you have in your JDK will handle the translation to each OS.

Your java program passes these steps to be understood by the underlying OS.

.java (Source file) gets compiled by compiler and becomes .class(class file) and this will be passed to JVM to be interpreted to machine language.

Class file is the one that has instructions for JVM

Upvotes: 0

miiiii
miiiii

Reputation: 1620

JVM = JAVA VIRTUAL MACHINE:- The word virtual itself indicates that it doesn't exists PHYSICALLY.

Elaborated here:

Abstract means HIDDEN.

  1. When assembly programs runs on a computer, it is executed in the system runtime environment:

    Properties

    • Platform dependent (if compiled for windows, a program will run only in windows not in Linux/UNIX etc.)
    • Not portable (same as above)
    • Systems runtime (in user's PC mainly under OS's control)

  2. When Java Program runs into a computer, it is executed in another virtual machine (JVM) for which, the runtime environment is provided by JRE (JAVA Runtime Environment), it is installed automatically when you install JDK (Java Development Kit).

Without JRE, it is impossible to run Java Programs (Update: You can bundle your custom JRE with your code, in that case no need to install JDK or JRE separately but only in JDK9 & above)

This JVM itself runs in the system runtime (in user's PC/OS) but when Java program runs it is loaded into this running JVM.

For more practical AND visual experience:(for Windows only)

  1. Open Task Manager
  2. Go to Processes Tab
  3. Find Java(TM) Platform SE Binary (This is JVM) - Java Instance

And now run a java program and write some code to delay the execution, like multi-threading with wait and notify (you can find such programs on google), due to this running program your Java Instance will show a little high memory and disk usage (not much high but slightly higher than that when no program was running in JRE/JVM). These processes you see in Task Manager, are running in System Runtime, and your Java program will not be listed there.

Instead it will be running inside this already running JRE.

This is the reason why JVM is ABSTRACT.

Now, do a little cross check and prove it..

Run 2-3 java programs, either keep them in longer waiting or just write Input Scanner and don't provide input, program will continue to run in blocking state so that we can see them later.

Once gain, confirm that only one instance of JVM/Java is running in task manager (Depends on how many JREs you have installed, sometimes IDEs can also create one instance, so better close it first for clear observation)

Now, where are those 2-3 Java Programs running?? Open VisualVM (it's under the same package/folder where your java executable resides)..

In this VisualVM, you can clearly see that, all your RUNNING Java Programs are listed.

Now, open side by side windows... Task Manager, VisualVM and one of your Running Code's Console.

Verify,

  • in Task manager - 1 instance of Java Binary.
  • in VisualVM - 3 different instances of your programs (if you run 3 java porgram)

Now, provide input in one of your code so that blocking state goes away and program terminates successfully.

Now verify,

  • in Task Manager - still 1 instance of Java Binary.
  • in VisualVM - 2 instances, because 1 code terminated/finished.

So, all your Java programs run under a Virtual Machine, a machine that is hidden, physically not available, abstract.

Upvotes: 3

Stephen C
Stephen C

Reputation: 719616

An abstract machine is a machine that does not have a physical existence.

Abstractness

  • A bicycle has a physical existence. I can touch it. It is not abstract.

  • A computer chip has a physical existence. I can touch it. It is not abstract.

  • A JVM is an executing program ... and has no physical existence. I cannot touch it. It is abstract.

Machine-ness:

  • A bicycle is a machine for transporting people.

  • A computer chip is a machine for executing programs that are coded in the instruction set of the chip; e.g. Intel x86 machine code.

  • A JVM is also a machine for executing programs that are coded in the instruction set of all JVMs; i.e. Java bytecodes.

Upvotes: 3

v_haruty
v_haruty

Reputation: 86

It is a bit philosophical question. When java application is compiled, the compiler produces its own "assembly" code that can not run directly on a hardware, it needs JVM. So, JVM creates an environment for java code similar to that of a machine. In other words from java perspective it is a machine, but in reality is a program that sits on top of a computer's operating system.

Upvotes: 0

ThiepLV
ThiepLV

Reputation: 1279

Abstract machine means all of java application run in JVM and JVM runs in different platform as Window, Linux, Mac ...

JVM is interface to java application can run all platform

Upvotes: 0

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