Active vs Passive usage of classes/Interfaces

[Hrishikesh Yeshwant Alshi]

This is referring to the class/interface loading by the class loader: What is active and passive usage of classes and interfaces? Could anyone please elaborate this concept a bit? Or at least refer me to some good material for the same.

Many thanks in advance.

[Devaka Cooray]

This sounds not related to SCJP. Moving to JiG

[Hrishikesh Yeshwant Alshi]

Hi Devaka,

I think there is some confusion. This is related with SCJP.

[Devaka Cooray]

Hrishikesh Yeshwant Alshi wrote:This is related with SCJP.

What did you mean by active/passive usage? That's something I never heard before.
Since the underneath methodologies of ClassLoader are not bound to the scope of SCJP, I guess JiG is the best place to discuss this. If another moderator feels this is related to SCJP, this could be moved back to SCJP forum.

[Rob Spoor]

I agree with JiG being a better location.

[Jesper de Jong]

I've also never heard of "active" or "passive" usage of classes and interfaces. Where did you read this?

[Hrishikesh Yeshwant Alshi]

Did I use the incorrect technical term - "Active/Passive usage of Classes/Interfaces"?

I found this in JLS - here

Apologies if that created confusion.

Thanks,
-Hrishikesh

[Jesper de Jong]

That page explains in a few places that classes or interfaces that are not actively used might not be initialized at all. There's this example:

class Super {
	static { System.out.print("Super "); }
}

class One {
	static { System.out.print("One "); }
}

class Two extends Super {
	static { System.out.print("Two "); }
}

class Test {
	public static void main(String[] args) {
		One o = null;
		Two t = new Two();
		System.out.println((Object)o == (Object)t);
	}
}

And the text then says: "The class One is never initialized, because it not used actively and therefore is never linked to."

That means that when you run this program, the static initializer of class One will not be run, so you do not see the output "One". This is because there is no thread that needs class One, so the JVM will not load and initialize it. Classes are only loaded and initialized when they are actually needed.

There's no such thing as "passively" using a class.

[Sid War]

Page number 239
Java 2 Virtual Machine
by Bill Venners

[Campbell Ritchie]

Welcome to the Ranch

Please tell us briefly what page 239 says; I don't have that book.

[Sid War]

following six situations qualify as active uses of a type
1. execution of 'new' instruction
2. Invocation of static method of a class.
3. Use or assignment of a static field declared by a class or interface, except for static final fields.
4. Invocation of certain reflective methods in Java API
5. Initialization of a subclass of a class
6. Designation of a class as the initial class(with main() method) when JVM starts

Example of passive use:
A use of a non-final field is an active use of only class that declares the field.
A static field declared in a class might be referred to via a subclass. This is a passive use of the subclass and active use of the super class.
This use will not trigger the initialization of the subclass.

[Jesper de Jong]

I think we shouldn't think about this in a too complicated way. I guess that with "passive use", the book just means "the opposite of active use".

Note that the Java Language Specification (in the chapter that Hrishikesh mentioned) doesn't use the words "passive use" anywhere, so it's not official terminology from the specification.

[Campbell Ritchie]

I think creating a new terminology which nobody else uses is potentially a cause of confusion.

[Hasitha Subhashana]

As far as I know active use is

If you create new keyword to create an object from a class its considered as active use of that class.

If a class has a static method and if you call this static method this is active use of that class.

Also if you assign a value to a static variable that is also considered as active use of that class.

There's few more .

https://www.youtube.com/watch?v=CQQzYCjNdIw&list=PLD-mYtebG3X-rF1hU16AC3Rf9E-mAAkXJ&index=4

this video 15.00 you will find what I'm telling you. I hope this is what you were looking for at the time. Hope this will be useful for anyone else looking for the same.

[Campbell Ritchie]

Welcome to the Ranch

Did you produce that video yourself?

[Hasitha Subhashana]

Campbell Ritchie wrote:Welcome to the Ranch

Did you produce that video yourself?

Hey no. This is one of my mentor. Its his video