什么时候初始化静态变量？

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

The order of initialization is:

1. Static initialization blocks
2. Instance initialization blocks
3. Constructors

The details of the process are explained in the JVM specification document.

Answer 5

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

Answer 6

The static variable can be intialize in the following three ways as follow choose any one you like

1. you can intialize it at the time of declaration

2. or you can do by making static block eg:

    static { // whatever code is needed for initialization goes here }

3. 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 } }

Answer 7

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.