Creation date: 15 March, 2011

Static and instance initilization blocks

Static initialization blocks run when the class is first loaded. An instance initialization blocks run when the instance is created. An initializastion blocks run in the order in which they appear in the class file (from the top down). Instance initializer block runs after the call to super() in a constructor. Based on these rules we can demonstrate it in the following code:

class A {
  static {
    System.out.println("static block of A");
  }
  {
    System.out.println("instance block of A");
  }
  public A() {
    System.out.println("constructor of A");
  }
}

class B extends A {
  static {
    System.out.println("static block 1 of B");
  }
  {
    System.out.println("instance block 1 of B");
  }
  static {
    System.out.println("static block 2 of B");
  }
  {
    System.out.println("instance block 2 of B");
  }
  public B() {
    System.out.println("constructor of B");
  }
  public static void main(String[] args) {
    new B();
  }
}

If we run this code we'll get the following output:

static block of A
static block 1 of B
static block 2 of B
instance block of A
constructor of A
instance block 1 of B
instance block 2 of B
constructor of B

As we said the static blocks run once, when the class is first loaded. But when we run new B(), Java first calls the constructor of the super class(class A). Before running the constructor of class A, Java calls the constructor of Object class. Then it calls the instance initializing block of A and prints instance block of A. Then it runs constructor method of A. After this call, the instance initializer blocks are called in their creation order. Although one of static initializer block is between two instance initializer blocks, JVM ignores it during objet instantiaation as well as it ignores object initializer blocks during class loading time.

Author: Jafar N.Aliyev (Jsoft)

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