java：在类初始化期间初始化同一个类的静态字段

Question

It looks like deadlock happened during starting the application. I have jstack file in which i found that one thread call TariffModelManager.<clinit> method and there are others multiple threads calling TariffModelManager.getInstance() method. The code of TariffModelManager following:

public class TariffModelManager{
    ...
    private static final TariffModelManager tariffModelManager = TariffModelManager.getInstance();
    ...
    private static volatile TariffModelManager _instance;

    private TariffModelManager(){}

    public static TariffModelManager getInstance() {
        if(_instance == null) {
            synchronized(TariffModelManager.class) {
                if(_instance == null) _instance = new TariffModelManager();
            }
        }
        return _instance;
    }
    ... 
}

I think, this is an apparent mistake to initialize static field by calling getInstance() method in which checked another field (_instance), that described bellow in the code, but what is really happened?

1. During class initialization, static fields are initialized from up to down;
2. we trying to init "tariffModelManager" field...
3. calling method getInstance(), that checks _instance field (but it is not initialized!);
4. then we must execute constructor of this class;
5. if _instance field has been initialized in constructor, can it be set to null in static fields initialization sequence?

My question is not how to solve this, but describe what is really happened when this code executed!

UPDATE: Here i found detailed class initialization procedure, but it is still not clear, in which order "tariffModelManager" and "_instance" fields are initialized.

Thanks!

Answer 1

You can verify what's happening by putting field breakpoints on the two static fields.

I've modified your sample a bit:

class TariffModelManager {
    private static final TariffModelManager tariffModelManager = TariffModelManager.getInstance();
    private static volatile TariffModelManager _instance = new TariffModelManager();

    private TariffModelManager() {
        System.out.println("From Constructor: " + this);
    }

    public static TariffModelManager getInstance() {
        if (_instance == null) {
            synchronized (TariffModelManager.class) {
                if (_instance == null) {
                    _instance = new TariffModelManager();
                    System.out.println("Creating new instance: " + _instance);
                }
            }
        }
        return _instance;
    }

    public static void main(String[] args) {
        TariffModelManager.getInstance();

        System.out.println("tariffModelManager: " + TariffModelManager.tariffModelManager);
        System.out.println("_instance: " + TariffModelManager._instance);
    }
}

And here is the output:

From Constructor: TariffModelManager@ea4a92b
Creating new instance: TariffModelManager@ea4a92b
From Constructor: TariffModelManager@3c5a99da
tariffModelManager: TariffModelManager@ea4a92b
_instance: TariffModelManager@3c5a99da

The static field tariffModelManager is initialized first. When the getInstance method is called _instance is null - ie the default value and not initialized yet. Then it is initialized inside that method and the value is assigned to tariffModelManager as well. Then it is reinitialized again according to the static initialization order: Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

This is funny, because

class TariffModelManager {
    private static final TariffModelManager tariffModelManager = _instance;
    private static volatile TariffModelManager _instance = new TariffModelManager();

    private TariffModelManager() {
        System.out.println("From Constructor: " + this);
    }

    public static TariffModelManager getInstance() {
       return _instance;
    }
}

is invalid java, but the static methods gives you a way to "work around" it (well tariffModelManager will be null, not really what's intended, but still compiles):

class TariffModelManager {
    private static final TariffModelManager tariffModelManager = getInstance();
    private static volatile TariffModelManager _instance = new TariffModelManager();

    private TariffModelManager() {
        System.out.println("From Constructor: " + this);
    }

    public static TariffModelManager getInstance() {
       return _instance;
    }
}