Question

Our application is using initialization code that depends on the order static code is executed and I’m wondering if this order will be consistent across all JVMs.

Here is a sample of what I mean:

public class Main {

    static String staticVar = "init_value";

    public static void main(String[] args) {

        System.out.println(A.staticVar);
        staticVar = "mainValue";
        System.out.println(A.staticVar);
    }
}

public class A {
    static String staticVar = Main.staticVar;
}

will give:

init_value
init_value

and

public class Main {

    static String staticVar = "init_value";

    public static void main(String[] args) {

        // System.out.println(A.staticVar);
        staticVar = "mainValue";
        System.out.println(A.staticVar);
    }
}

public class A {
    static String staticVar = Main.staticVar;
}

will give (on my environment):

mainValue

To summarize, across all JVMs, is static code always executed when we use a class for the first time?

Answer 1

EDIT: Despite all the reassurances below, if you’re thinking of relying on this sort of thing, I would try hard to refactor your code so that it doesn’t crop up. While it is guaranteed to work, it’s also likely to make your code very brittle. The fact that static initializers get called “invisibly” makes them relatively hard to reason about and debug.

---

Yes, this is guaranteed by the language specification. From section 8.7 of the spec:

Any static initializers declared in a class are executed when the class is initialized and, together with any field initializers (§8.3.2) for class variables, may be used to initialize the class variables of the class (§12.4).

StaticInitializer: static Block

It is a compile-time error for a static initializer to be able to complete abruptly (§14.1, §15.6) with a checked exception (§11.2). It is a compile-time error if a static initializer cannot complete normally (§14.21).

The static initializers and class variable initializers are executed in textual order.

And from section 12.4:

Initialization of a class consists of
executing its static initializers and
the initializers for static fields
declared in the class. Initialization
of an interface consists of executing
the initializers for fields declared
in the interface.

Before a class is
initialized, its direct superclass
must be initialized, but interfaces
implemented by the class need not be
initialized. Similarly, the
superinterfaces of an interface need
not be initialized before the
interface is initialized.

A class or interface type T will be
initialized immediately before the
first occurrence of any one of the
following:

• T is a class and an instance of T is
  created.
• T is a class and a static
  method declared by T is invoked.
• A
  static field declared by T is
  assigned.
• A static field declared by T
  is used and the field is not a
  constant variable (§4.12.4).
• T is a
  top-level class, and an assert
  statement (§14.10) lexically nested