Final fields initialization order

java jls

2022-09-01 13:18:24

Here is some code that calls static method A.f() on class that is not initialized yet. Can someone explain behavior of this code in terms of JLS?

class A {
    final static Object b = new B();
    final static int S1 = 1;
    final static Integer S2 = 2;

    static void f() {
        System.out.println(S1);
        System.out.println(S2);
    }
}

class B {
    static {
        A.f();
    }
}

public class App
{
    public static void main( String[] args )
    {
        A.f();
    }
}

Output:

1
null
1
2

答案 1

A.f() in triggers initialization of class .App.main()A

All constant variables are initialized. The only constant variable is , which now is .S11

Then, the other static fields are initialized in textual order. is the first field, which triggers initialization of class , which in turn calls . is simply because it is not initialized yet. Initialization of is now complete. Last but not least, is initialized to the object .bBA.f()S2nullbS2Integer2

S2 is not a constant variable because it is not of the primitive type but of the reference type . is an auto-boxing shorthand for .intIntegerS2 = 2;S2 = Integer.valueOf(2);

If a declarator in a field declaration has a variable initializer, then the declarator has the semantics of an assignment (§15.26) to the declared variable.

[…]

Note that fields that are constant variables (§4.12.4) are initialized before other fields (§12.4.2). This also applies in interfaces (§9.3.1). Such fields will never be observed to have their default initial values (§4.12.5), even by devious programs.staticstatic

8.3.2. Field Initialization

A constant variable is a variable of primitive type or type that is initialized with a constant expression (§15.28). Whether a variable is a constant variable or not may have implications with respect to class initialization (§12.4.1), binary compatibility (§13.1, §13.4.9), and definite assignment (§16 (Definite Assignment)).finalString

4.12.4. final Variables

A constant expression is an expression denoting a value of primitive type or a that does not complete abruptly and is composed using only the following:String

Literals of primitive type and literals of type String

[…]

15.28. Constant Expressions

For each class or interface C, there is a unique initialization lock . The mapping from C to is left to the discretion of the Java Virtual Machine implementation. The procedure for initializing C is then as follows:LCLC

[…]

Otherwise, record the fact that initialization of the object for C is in progress by the current thread, and release .ClassLC

Then, initialize the fields of C which are constant variables (§4.12.4, §8.3.2, §9.3.1).static

[…]

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.

12.4.2. Detailed Initialization Procedure

Every variable in a program must have a value before its value is used:

Each class variable, instance variable, or array component is initialized with a default value when it is created (§15.9, §15.10.2):

[…]

For all reference types (§4.3), the default value is .null

4.12.5. Initial Values of Variables

答案 2

It seems that this issue doesn't belong to , but we have deal with .JLSJVMS

On stage before process there is Preparation substage, which involves:Linkingresolution

creating the static fields for a class or interface and initializing such fields to their default values

and more:

explicit initializers for static fields are executed as part of initialization (§5.5), not preparation

while initialization includes:

The execution of any one of the Java Virtual Machine instructions new

Initializing of primitive types includes writing their initial value. For reference type fields their default value is because before Resolution substage jvm doesn't "know" which class is associated to apropriate symbolic reference name of a class. null