CODEWITHSUNDEEP
javabackgroundjvm
daemonThread
daemonThread

Reputation: 243

How hello world app works in background in java

I want to know what happens in background when I run a simple java hello world application without any extra initial parameters.

For example in background how java call windows functions like drawing a simple window:

public  class example
{  
        public static void main(String args[])
        {
           System.out.println("Hello World!");
        }
}

or

public  class example2 extends Jframe
{  
        public static void main(String args[])
        {
           new example2().setvisible(true);
        }
}

Upvotes: 2

Views: 222

Answers (4)

x4nd3r
x4nd3r

Reputation: 900

If you are asking how the JVM interacts with the underlying operating system, this article gives a general, platform-independent overview of its architecture.

I think you may be particularly interested in the section headed "The Byte Code Instruction Set", which states that when executing the line:

System.out.println("Hello world!");

At compile time, the Java compiler converts the single-line print statement to the following byte code:

0 getstatic #6 <Field java.lang.System.out Ljava/io/PrintStream;>
3 ldc #1 <String "Hello world!"> 
5 invokevirtual #7 <Method java.io.PrintStream.println(Ljava/lang/String;)V> 
8 return

It may also be worth noting that JDK includes a tool that you can use to examine byte code, called the class file disassembler. You can make use of this tool using the javap command in your terminal of choice.

Upvotes: 1

The_One_And_Only
The_One_And_Only

Reputation: 81

First of all your first example of code only prints "Hello World!" into a terminal. As for calling a window, the OS handles it. In case you haven't noticed there is a difference between a window on OSX and one on Windows.

Upvotes: 0

Ivan Nevostruev
Ivan Nevostruev

Reputation: 28743

How java call windows functions like drawing a simple window?

I guess you are asking about Java Native Interface.

Upvotes: 1

Dodd10x
Dodd10x

Reputation: 3349

http://docs.oracle.com/javase/tutorial/ui/overview/intro.html

Java is intended to be platform independent. It's not using native windows calls, at least not directly.

Upvotes: 1

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