Infinite recursion in object/static creation/Why is this code throwing NullPointerException?

Question

I do not understand why the following code isn't this a causing a Stack overflow equivalent for Heap (OutOfMemoryError or something) since it is some kind of infinite recursion (is it not?). Does the static initialization provide a guard against something like this and throws NullPointerException instead?

Edit: I do not believe I was understood: public static void main(String[] args) calls a static method from StaticClass. Before this method, doSmth(), is called StaticClass needs to be loaded first. When a class is loaded, all the static code is ran; in the StaticClass class, objClass is a static field attributed with the value of a new ObjClass() and thus shouldn't be null (to throw NullPointerException). Obviously new ObjClass() can not be instantiated because in order to do this you need to load the StaticClass and this already happens when StaticClass is loading (thus the analogy with infinite recursion, or probably deadlock analogy). The problem is the JVM says the objClass is null instead of saying it couldn't initialize new ObjClass() because some kind of recurrent call.

In a normal case, NullPointerException is thrown because of the caller. But in this case, you can change only the callee (remove the line with the comment at the end, in the ObjClass constructor) and then you won't receive NullPointerException.

package pack;

public class ObjClass
{
    public ObjClass() {
        StaticClass.doSmth();//if removed, no NullPointerException
    }

    public String getSomething() {
        return "get";
    }

    public static void main(String[] args) {
        StaticClass.loadStaticClass();
    }
}

class StaticClass {
    private static ObjClass objClass = new ObjClass();

    static void loadStaticClass() {
    }

    static void doSmth()             {
        System.out.println(objClass.getSomething());
    }
}

which gives:

Exception in thread "main" java.lang.ExceptionInInitializerError
    at pack.ObjClass.main(ObjClass.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.lang.NullPointerException
    at pack.ObjClass$StaticClass.doSmth(ObjClass.java:39)
    at pack.ObjClass.<init>(ObjClass.java:20)
    at pack.ObjClass$StaticClass.<clinit>(ObjClass.java:34)
    ... 6 more

Brian Agnew · Accepted Answer · 2012-12-18 15:24:26Z

4

Referencing StaticClass instantiates objClass, which calls the ObjClass constructor, which calls doSmth(). This uses the reference objClass, but that's not been assigned yet, since it needs a fully constructed objClass to refer to.

Briefly,

this.x = doMethod();

requires doMethod() to complete before the reference x is assigned to the result.

What are you trying to do ? Or is this some sort of exercise ?

answered Dec 18, 2012 at 15:24

Brian Agnew

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3 Comments

Random42 Over a year ago

I know the Java evaluation strategy pretty well; that's not the problem. The problem is why isn't objClass assigned? Why doesn't it enter an infinite recursion and the JVM gives up after the first try; since there is static ObjClass objClass = new ObjClass(), objClass should be instantiated when StaticClass is loaded?

Brian Agnew Over a year ago

And how does it instantiate that reference ? By calling the ObjClass constructor. The reference isn't assigned 'til that completes (otherwise another thread could use that reference and it would point to an incomplete object)

Random42 Over a year ago

What I do not understand is how is it possible to go to the content of doSmth() static method before loading the class StaticClass. If calling the ObjClass constructor doesn't finish, why isn't an exception thrown here?

Peter Lawrey · Accepted Answer · 2012-12-19 09:45:50Z

4

No matter how determined you are they there must be a complicated answer to this problem, is very simple. You get a NullPointerException because you dereferenced a null value. Nothing more.

public class Main {
    public static void main(String... args) {
        new A();
    }

    static class A {
        static {
            System.out.println("Start of class A initialisation, A.b is " + A.b);
        }

        static final B b = new B();

        static void method(String from) {
            System.out.println("Called a method in A from " + from + ", A.b is " + A.b);
            B.method();
        }

        static {
            System.out.println("End of class A initialisation, A.b is " + A.b);
        }
    }

    static class B {
        static {
            System.out.println("Start of class B initialisation, A.b is " + A.b);

            A.method("B static block");

            System.out.println("End of class B initialisation, A.b is " + A.b);
        }

        B() {
            A.method("B() constructor");
            System.out.println("Only after this should A.b be set.");
        }

        static void method() {
            System.out.println("Called a method in B, A.b is " + A.b);
        }
    }
}

prints

Start of class A initialisation, A.b is null
Start of class B initialisation, A.b is null
Called a method in A from B static block, A.b is null
Called a method in B, A.b is null
End of class B initialisation, A.b is null
Called a method in A from B() constructor, A.b is null
Called a method in B, A.b is null
Only after this should A.b be set.
End of class A initialisation, A.b is Main$B@33b7b32c

As you can see, one class can call another before it has finished initialising and a reference before it has been initialised can be examined and it will be null, even a final one.

It's throwing a NullPointerException because objClass hasn't been set yet.

It won't be set until you return from the constructor, but you are in the constructor, so it will have it's default value of null.

BTW, This is nothing like http://en.wiktionary.org/wiki/infinite_recursion

I do not understand why the following code isn't this a causing a Stack overflow equivalent for Heap (OutOfMemoryError or something)

They are all different errors with specific meanings. The error thrown when a reference is used without being initialised is NullPointerException.

Does the static initialization provide a guard against something like this and throws NullPointerException instead?

The fields doesn't have to be static for this to happen. The same happens to non-static fields if you access them before they have been initialised.

objClass should be instantiated when StaticClass is loaded?

It hasn't loaded, in fact it never loads because it throws an Exception before it completes. If you where to try to use this class after this, you would get a NoClassDefError as the class failed to load.

edited Dec 19, 2012 at 9:45

answered Dec 18, 2012 at 15:23

Peter Lawrey

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9 Comments

Random42 Over a year ago

I know the reason why a NullPointerException is thrown. The problem is that objClass should be instantiated when the StaticClass is loaded (and that is before actually calling doSmth()) and NullPointerException shouldn't be there. But when StaticClass is loaded objClass cannot be instantiated because it needs StaticClass to be loaded and thus some kind of infinite recursion (or a deadlock).

Peter Lawrey Over a year ago

Or a logic bug. You have created code which cannot possibly work as each needs the other to finish first.

Random42 Over a year ago

Yes that is true, it obviously is a logic bug. But why NullPointerException?

Peter Lawrey Over a year ago

Either the constructor finishes nor does the class load, an Exception is thrown before that happens.

Peter Lawrey Over a year ago

It's a NullPointerException as you try to access objClass before it has been set and is null which is the default value.

|

kosa · Accepted Answer · 2012-12-18 15:23:25Z

1

  private static ObjClass objClass = new ObjClass();

While class loading jvm tries to create instance for ObjClass, reference objClass points to null by the time ObjClass constructor called (instance creation not done yet).\

You are trying to call objClass.getSomething(), you are calling getSomething() on null reference which is causing NullPointerException.

answered Dec 18, 2012 at 15:23

kosa

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2 Comments

Random42 Over a year ago

That's what I do not understand; how can the method doSmth() be evaluated before the class StaticClass finishes loading? And by the time the class is done loading the object objClass should point to an object not to null; if ObjClass cannot be instantiated, it should throw an exception mentioning this.

Random42 Over a year ago

Actually it says that there was a problem in the static initializer and the cause of it is NullPointerException.

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