Class loading, initialization order

Question

public class A{ static int a = 1111; static { a = a-- - --a; } { a = a++ + ++a; } public static void main(String[] args) { System.out.println(a); } }

Ask what the program will display. It is clear to me that if we have not created an instance of the class, then the class-level variables (static) will be initialized, and the static block will also be executed, even if an instance of the class is not created. And the answer is here 2. Because at the time of the call, only the static initialization block was executed. But my question is different. I thought there would be an NPE exception, since the variable does not exist yet. I thought so because I believed that the public static void main(String[] args) entry point is executed first, and then in order of initialization passes.

So at what point the static field and the stat were initialized. class А block?

First, the entire class is loaded, including all static initialization, and only then can any methods be called
At first all classes are loaded and all static is initialized in the established order, i.e.
Otherwise, I thought that the entry point is a method, which means it is first created.
And what if there are two classes A and B and each has two stats.
First class A and then B, as well as located in the folder .. But if the first variable in class A is assigned the value of a variable from class B, then the order is a little disturbed.
First, class A, reached the first variable, got into class B, initialized its fields completely (or just the same static field?) And then returned and initialized the second variable of class A. It seems this is the subtleties of classLoader that you shouldn’t climb yet
@Turalllb Static class variables are initialized when the class is loaded.
It is loaded when someone calls to it (creates an instance or refers to a static member).
The class that contains the main method is accessed by the virtual machine when we start the program, specifying the class as an argument to it.
If a class is not used by anyone, then it is not initialized.

Accepted Answer · 2018-06-27T17:49:49

Class initialization

Before initialization, loading the necessary classes (ClassLoader, java.lang ... requires separate consideration)

Initialization order:

The class initialization block is initialized first.
Then initialization blocks are initialized.
Then the public static void main (String [] args) method is called, which is the entry point to the application. It should be written according to the standard.
And at the very end, the class constructor is called.

Code example:

  /* Инициализация класса Порядок инициализации: 1. Первым инициализируется статический блок инициализации класса 2. Затем инициализируются блоки инициализации 3. Затем вызывается метод public static void main(String[] args), который является точкой входа в приложение и в самом конце вызывается конструктор класса *Примечание.* 1. Метод, имя которого совпадает с конструктором не вызывается при инициализации. 2. Метод, имя которого main() в моем коде, но который не является точкой входа, не вызывается при инициализации. Не является он точкой входа, потому как возвращает void и не имеет аргументов. 3. Обычные статические методы не вызываются при инициализации. 4. Обычные нестатические методы не вызываются при инициализации. */ package myInitClass; /** * * @author vvm */ public class InitClass { InitClass(){ // конструктор класса System.out.print(" Конструктор "); } { // блок инициализации System.out.print(" 3 "); } void InitClass(){// имя метода совпадает с именем конструктора но возвращает void // метод не вызывается при инициализации System.out.print(" метод InitClass "); } public static void main(String[] args) { System.out.print(" 2 "); new InitClass(); } public void main(){ //- это обычный метод с именем main // имя метода совпадает с именем точки входа, но не имеет аргументов // метод не вызывается при инициализации System.out.print(" метод main - не точка входа "); } public static void method(){ // статический метод // метод не вызывается при инициализации System.out.print("статичные метод с именем method()"); } static { // статический блок инициализации System.out.print(" 1 "); } } /* run: 1 2 3 Конструктор BUILD SUCCESSFUL (total time: 0 seconds) */

I am familiar with the order of initialization, just for some reason, the public static void main(String[] args) method attached particular importance, he thought the very first one and initialized, since it is an entry point.
The public static void method main (String [] args) is really an entry point.
If you write a public method void main () {} - then it will be a normal method and it will not be called during initialization.
But before the entry point, initialization blocks are called.

Class loading, initialization order

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