Java static block refer to static variable in another class

Question

public class A {
  public static String HOST;

  static {
    HOST = ...;
  }
}

public class B {
    public static String URL;

    static{
         URL = A.HOST + ...;
    }
}

My question is if A.HOST would be correctly initialised before B use it? Is this behaviour is defined in spec?

if you make any reference to A first, A will be fully initialized, and whenever you call B for the first time, it will work fine. If you call B first, it will cascade to A when encountering A.HOST. — njzk2, Jul 16 '15 at 18:18
the funny part would be if A had something like `HOST = B.FOO`. Not sure that one would even compile, though. — njzk2, Jul 16 '15 at 18:19

xp500 · Accepted Answer · 2015-07-16T18:15:54.077

Yes that behavior is well defined here.

In short, citing from that link

Initialization of a class or interface consists of executing the class or interface initialization method <clinit>

...

A class or interface may be initialized only as a result of:

The execution of any one of the Java Virtual Machine instructions new, getstatic, putstatic, or invokestatic that references the class or interface (§new, §getstatic, §putstatic, §invokestatic). All of these instructions reference a class directly or indirectly through either a field reference or a method reference.

Upon execution of a new instruction, the referenced class or interface is initialized if it has not been initialized already.

Upon execution of a getstatic, putstatic, or invokestatic instruction, the class or interface that declared the resolved field or method is initialized if it has not been initialized already.

The first invocation of a java.lang.invoke.MethodHandle instance which was the result of resolution of a method handle by the Java Virtual Machine (§5.4.3.5) and which has a kind of 2 (REF_getStatic), 4 (REF_putStatic), or 6 (REF_invokeStatic).

Invocation of certain reflective methods in the class library (§2.12), for example, in class Class or in package java.lang.reflect.

The initialization of one of its subclasses.

Its designation as the initial class at Java Virtual Machine start-up (§5.2).

The <clinit> method is the method (created by the compiler) that initializes static variables and has the code that you put in the static block

In your case, when the static block of class B runs (which is what <clinit> will do), it will have a getStatic opcode, requesting A.HOST. So the initialization of A will be triggered, and A.HOST initialized. So you will read the proper value.

And your conclusion is yes or no, A will be initialized before B uses it ? — John, Jul 15 '15 at 11:03
@user3360241 Just edited, let me know if it's still not clear. — xp500, Jul 15 '15 at 11:05

score 0 · Answer 2 · answered Jul 15 '15 at 10:58

The static block for a class gets executed when the class is accessed, either to create an instance, or to access a static method or field.

This depends on the code that we are executing.

In your case, when we do A.HOST, it calls static block of class A also.

Refer this

score 0 · Answer 3 · edited May 23 '17 at 11:51

0

The static initializer for a class gets run when the class is first accessed, either to create an instance, or to access a static method or field.

Source: In what order do static initializer blocks in Java run?

edited May 23 '17 at 11:51

Community

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answered Jul 15 '15 at 10:58

CodezMe

81
1
2
9

score 0 · Answer 4 · answered Jul 17 '23 at 21:45

I came across your post since I have similar doubt.

When there is a cycle dependency, it does looks like it is problematic in some cases. Here is my test code.

The output from https://www.tutorialspoint.com/online_java_compiler.php for below is ABC_null_TT2_EFG

 class A {
    public static String HOST;

    static {
        HOST = "ABC_" + B.TT + "_" + B.TT2;
    }
    
}

public class B {
    
    public static String URL;

    static {
        URL = A.HOST + "_EFG";
    }
    public static void main(String[] args) {
        System.out.println(URL);
    }
    public static String TT = "TT";
    public final static String TT2 = "TT2";
}

That's not caused by a cyclic dependency, but because non-final static intialisers are execute in order -- try moving the assignment to `TT` to before the initialiser block. — tgdavies, Jul 20 '23 at 04:54

score -3 · Answer 5 · answered Jul 15 '15 at 11:16

-3

A static block will be executed,before the main() method executes.

answered Jul 15 '15 at 11:16

Gopinath

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2

could you explain how your answer is related to the question asked? – Blip Jul 15 '15 at 11:24
I mean code in the static block executes prior to main method, class B is written next to class A,probably string get initialized and there would be no problem when it is accessed by class B. – Gopinath Jul 15 '15 at 11:34
I think you did not understand the question. `B.URL` and `A.HOST` are both static. But `B.URL` initialisation requires `A.HOST`. So the problem is as `A` has not yet been initialised when `B.URL` is being initialised, will it cause any problem or not? – Blip Jul 15 '15 at 11:39
I just confused with program flow,yes it is a problem.Thanks for your comment. – Gopinath Jul 15 '15 at 11:42

Java static block refer to static variable in another class

5 Answers5