When are static variables initialized

Question

I am wondering when static variables are initialized to their default values  Is it correct that when a class is loaded  static vars are created  allocated   then static initializers and initializations in declarations are executed  At what point are the default values are given   This leads to the problem of forward reference   Also please if you can explain this in reference to the question asked on Why static fields are not initialized in time  and especially the answer given by Kevin Brock on the same site  I can t understand the 3rd point

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See   JLS 8 7  Static Initializers JLS 12 2  Loading of Classes and Interfaces JLS 12 4  Initialization of Classes and Interfaces  The last in particular provides detailed initialization steps that spell out when static variables are initialized  and in what order  with the caveat that final class variables and interface fields that are compile-time constants are initialized first   I m not sure what your specific question about point 3  assuming you mean the nested one   is  The detailed sequence states this would be a recursive initialization request so it will continue initialization

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From See Java Static Variable Methods         It is a variable which belongs to the class and not to    object instance    Static variables are initialized only once   at the start of the    execution  These variables will be initialized first  before the initialization of any instance    variables   A single copy to be shared by all instances of the class   A static variable can be accessed directly by the class name and    doesn   t need any object       Instance and class  static  variables are automatically initialized to standard default values if you fail to purposely initialize them   Although local variables are not automatically initialized  you cannot compile a program that fails to either initialize a local variable or assign a value to that local variable before it is used   What the compiler actually does is to internally produce a single class initialization routine that combines all the static variable initializers and all of the static initializer blocks of code  in the order that they appear in the class declaration   This single initialization procedure is run automatically  one time only  when the class is first loaded   In case of inner classes  they can not have static fields     An inner class is a nested class that is not explicitly or implicitly   declared static                Inner classes may not declare static initializers    8 7  or member interfaces         Inner classes may not declare static members  unless they are constant variables       See JLS 8 1 3 Inner Classes and Enclosing Instances  final fields in Java can be initialized separately from their declaration place this is however can not be applicable to static final fields  See the example below   final class Demo       private final int x      private static final int z     must be initialized here       static                z   10     It can be initialized here             public Demo int x                this x x     This is possible            z 15  compiler-error - can not assign a value to a final variable z           This is because there is just one copy of the static variables associated with the type  rather than one associated with each instance of the type as with instance variables and if we try to initialize z of type static final within the constructor  it will attempt to reinitialize the static final type field z because the constructor is run on each instantiation of the class that must not occur to static final fields

User · Answer

Static fields are initialized when the class is loaded by the class loader   Default values are assigned at this time   This is done in the order than they appear in the source code

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The static variable can be intialize in the following three ways as follow choose any one you like   you can intialize it at the time of declaration or you can do by making static block eg    static              whatever code is needed for initialization goes here        There is an alternative to static blocks     you can write a private static method  class name       public static varType myVar   initializeVar         private static varType initializeVar                initialization code goes here

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Starting with the code from the other question   class MyClass     private static MyClass myClass   new MyClass      private static final Object obj   new Object      public MyClass         System out println obj      will print null once         A reference to this class will start initialization   First  the class will be marked as initialized   Then the first static field will be initialized with a new instance of MyClass     Note that myClass is immediately given a reference to a blank MyClass instance   The space is there  but all values are null   The constructor is now executed and prints obj  which is null   Now back to initializing the class   obj is made a reference to a new real object  and we re done   If this was set off by a statement like   MyClass  mc   new MyClass    space for a new MyClass instance is again allocated  and the reference placed in mc    The constructor is again executed and again prints obj  which now is not null   The real trick here is that when you use new  as in WhatEverItIs  weii   new WhatEverItIs  p1  p2    weii is immediately given a reference to a bit of nulled memory   The JVM will then go on to initialize values and run the constructor   But if you somehow reference weii before it does so--by referencing it from another thread or or by referencing from the class initialization  for instance--you are looking at a class instance filled with null values

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The order of initialization is    Static initialization blocks Instance initialization blocks Constructors   The details of the process are explained in the JVM specification document

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static variable   It is a variable which belongs to the class and not to object instance  Static variables are initialized only once   at the start of the execution when the Classloader load the class for the first time     These variables will be initialized first  before the initialization of any instance variables A single copy to be shared by all instances of the class A static variable can be accessed directly by the class name and doesn   t need any object

[java] When are static variables initialized?

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