Execution of Java static blocks in subclasses

问题

I am preparing myself for Java certification test and I have found an interesting question related to the execution of Java static blocks. I have spent a lot of time reading about this topic, but I didn't find the answer I was looking for.

I know that static blocks are executed when the class is loaded into JVM or when the main method is invoked, but...

package oneClassTasks;

class Parent {
    static int age;
}

class Child extends Parent {
    static {
        age = 5;
        System.out.println("child's static block");
    }
}

public class XXX {

    public static void main(String args[]) {
        System.out.println("Child age is : "+ Child.age);
    }

}

The output is:

Child age is : 0

If I include verbose output with -verbose:class, then the output is:

...
[Loaded java.security.BasicPermissionCollection from C:\Program Files\Java\jre1.8.0_161\lib\rt.jar]
[Loaded oneClassTasks.XXX from file:/D:/temp/bin/]
[Loaded sun.launcher.LauncherHelper$FXHelper from C:\Program Files\Java\jre1.8.0_161\lib\rt.jar]
[Loaded java.lang.Class$MethodArray from C:\Program Files\Java\jre1.8.0_161\lib\rt.jar]
[Loaded java.lang.Void from C:\Program Files\Java\jre1.8.0_161\lib\rt.jar]
[Loaded oneClassTasks.Parent from file:/D:/temp/bin/]
[Loaded oneClassTasks.Child from file:/D:/temp/bin/]
Child age is : 0
[Loaded java.lang.Shutdown from C:\Program Files\Java\jre1.8.0_161\lib\rt.jar]
[Loaded java.lang.Shutdown$Lock from C:\Program Files\Java\jre1.8.0_161\lib\rt.jar]

We can see here that Child class is loaded into JVM.

Can someone explain why the static block from Child class is not executed?

回答1:

You saw that Child class was loaded, but it wasn't initialized.

Accessing Child.age doesn't cause the initialization of the Child class, since age is a member of Parent class. Therefore only Parent class is initialized, and age remains 0.

12.4.1. When Initialization Occurs

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.

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.

In your case you accessed a static field declared by Parent, so only Parent is initialized.

回答2:

In this case, Child.age is equivalent to Parent.age. JRE will treat it as Parent.age, that's why only Parent is statically initialized, and Child's static initializer is skipped.

来源：https://stackoverflow.com/questions/48989134/execution-of-java-static-blocks-in-subclasses

标签

java

static

jvm

execution