Static initializer not invoked for a derived class

Question

The following Java code does not invoke the static initializer of class B. Why?

Code:

class A 
{
    static 
    {
        System.out.println("A static init");
    }

    public static void f() 
    {
        System.out.println("f() called");
    }
}

class B extends A 
{
    static 
    {
        System.out.println("B static init");
    }
}

public class App
{
    public static void main( String[] args)
    {
        B.f(); //invokestatic  #16                 // Method com/db/test/B.f:()V
    }
}

Program output:

A static init
f() called

Tested on JDK 1.8.0_25

Answer 1

There is no such thing as "static constructor". It's a static initialization block, and it is only executed when the class is initialized. Since you are calling a static method of class A (even though you are referring to it via class B), there is no need to initialize class B. Calling B.f(); is the same as calling A.f();.

The static initialization block of class B will be executed if you create an instance of class B or access a static member/method of class B.

Here are the only conditions that trigger class initialization (JLS 12.4.1):

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 (§7.6), and an assert statement (§14.10) lexically nested within T (§8.1.3) is executed.

Answer 2

Since only class A defines the method f(), class B is loaded but not initialized.

You could use java -verbose:class MyClassName to check this.

On a jdk6 / jdk 8 machine, this will be printed.

[Loaded App from file:/C:/XXXX/]
[Loaded A from file:/C:/XXXXXX]
[Loaded B from file://C:/XXXXXXX]
A static init
f() called

Class B will be initialized lazily but loaded greedily (since it is being referred).

Change your code to A.f(). Then you will see that B is not loaded.

[Loaded App from file:/C:/XXXX/]
[Loaded A from file:/C:/XXXXX]
A static init
f() called

Note : Class loading and initialization are 2 different things. Check documentation for Class.forName() for details.