What does "class loading deadlock" mean here?

Question

I have this classes:

public class User {

    public static final NonRegisteredUser NON_REG_USER = new NonRegisteredUser();

    //...

    public static class NonRegisteredUser extends User {
        //...
    }

}

And code inspector is detecting this warning:

Referencing subclass NonRegisteredUser from superclass User initializer might lead to class loading deadlock

What does it mean exactly?

I think it is pretty clear what it says. You initialize a NonRegisteredUser in the User class. Every time you load the User class, since the NON_REG_USER is static it will lead to an eternal loop cause the NonRegisteredUser extends the User and so on.. — giannis christofakis
– giannis christofakis, Commented Dec 28, 2017 at 14:44
And so on and so on, and depending on implementation of JVM it may not initialize at all. — Antoniossss
– Antoniossss, Commented Dec 28, 2017 at 14:45

Kayaman · Accepted Answer · 2017-12-28 14:53:21Z

14

The deadlock can only occur if you have 2 threads and one starts to load User and one starts to load NonRegisteredUser. There are synchronizations in place that will cause a deadlock, but then it requires separate threads. If the loading happens in a single thread, there is no deadlock as the thread owns both locks.

Hence the might in the message. However deadlocks usually do tend to require a specific environment, so there's nothing weird about that.

answered Dec 28, 2017 at 14:53

Kayaman

73.9k6 gold badges91 silver badges128 bronze badges

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

Héctor Valls Over a year ago

Thank you. I'm working on Android, so better I'm gonna solve the warning, just in case.

Vaiden · Accepted Answer · 2022-02-22 07:12:28Z

3

Class loader begins loading User.

Static members are init first, in order of appearance. So the class loader sees the NonRegisteredUser class, and tries loading the User class for its initialization.

Next, class loader begins loading User.

Static members are init first, in order of appearance. So the class loader sees the NonRegisteredUser class, and tries loading the User class for its initialization.

Next, class loader begins loading User.

Static members are init first, in order of appearance. So the class loader sees the NonRegisteredUser class, and tries loading the User class for its initialization...

edited Feb 22, 2022 at 7:12

answered Dec 28, 2017 at 14:46

Vaiden

16.2k8 gold badges65 silver badges97 bronze badges

Héctor Valls Over a year ago

Thank you. So, why is that code working? Does it make sense?

Vaiden Over a year ago

It's probably working because what I just said is not 100% correct. The compiler build a single init process for the class, so the JVM won't blindely iterates over it over and over during runtime. There is probably some optimization in play here, that fixes this kind of issues.

giannis christofakis Over a year ago

@Héctor That's a nice question, what happens when you try to use the User.NON_REG_USER attribute or initialize the User.NonRegisteredUser class ?

Héctor Valls Over a year ago

@giannischristofakis It works fine. But I'm working on android with several threads, the error may occur so I'm gonna decouple the classes

Collectives™ on Stack Overflow

What does "class loading deadlock" mean here?

2 Answers 2

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related