HelloWorld program to a JVM code

Question

Let's consider the following piece of code:

package p;
public class Simple {

public static void main(String[] args){
    System.out.println("HELLO!\n");
   }
}

and JVM code for that ( javap -c Simple.class):

Compiled from "Simple.java"

public class p.Simple {
  public p.Simple();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
       3: ldc           #3                  // String HELLO!\n
       5: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
       8: return
}

And I have a questions:

Probably, the entry point is the main function. But, before that, it is neccesary to invoke a constructor for Simple class. So, how does it work? I mean, who calls firstly the construactor and then pass a control to main? If I am wrong please explain me how the control flow works.
Instructions like getstatic take arguments like #2 . What does it mean #2?
Comment looks like: // Method java/lang/Object."<init>":()V What does it mean V?
How does the control flow work?

"But, before that, it is neccesary to invoke a constructor for Simple class." No, it is not. main is a static method. — tkausl
– tkausl, Commented Oct 12, 2016 at 10:31
I see no constructor being invoked. Yes one is created by default for any class without an explicit constructor, but I don't see one being invoked anywhere in the code above. — Hovercraft Full Of Eels
– Hovercraft Full Of Eels, Commented Oct 12, 2016 at 10:33

Mike Thomsen · Accepted Answer · 2016-10-12 11:12:14Z

2

The Java compiler automatically generates a constructor when none is defined explicitly. In your example, it's not called at all. It's just that autogeneration policy.
It is a reference to the PrintStream property out on System. The Java compiler automatically converts import statements into fully qualified object declarations.
V means void
Probably something like this:
1. Load the reference.
2. Create an anonymous String object with that value.
3. Push a stack frame into place that jumps to the println method's entry point with reference to #2 as the parameter.

What #2 means:

import java.util.List;

List x;

becomes simply this:

java.util.List x = new java.util.ArrayList();

when the compiler generates bytecode. java.lang is automatically imported.

answered Oct 12, 2016 at 11:12

Mike Thomsen

37.7k11 gold badges63 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Gilgamesz Over a year ago

So, what is #X? Does the compiler "scans" for constants/object declarations and put it in array indexed by #X? "The Java compiler automatically generates a constructor when none is defined explicitly. In your example, it's not called at all. It's just that autogeneration policy." Ok. If I defined my own constructor for Simple class, when it will be called?

apangin Over a year ago

@Gilgamesz #X is the index into the constant pool. See "The class File Format"

Holger Over a year ago

The example is misleading. The sole declaration List x; does not suddenly include the instantiation of java.util.ArrayList after compiling.

Evgeniy Dorofeev · Accepted Answer · 2016-10-12 10:45:19Z

0

Constructor is not called in this case because main() is static.
getstatic gets the value of System.out static field
V means void, that is main() return type is void

answered Oct 12, 2016 at 10:45

Evgeniy Dorofeev

137k31 gold badges209 silver badges288 bronze badges

1 Comment

Gilgamesz Over a year ago

"getstatic gets the value of System.out static field" Ok, But, where is it? Is the value a reference? "Constructor is not called in this case because main() is static." Ok, that is true. main() is static so it cannot call constructor. Does it mean that constructor is not called at whole?

Collectives™ on Stack Overflow

HelloWorld program to a JVM code

2 Answers 2

3 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related