Java: What is the difference between <init> and <clinit>?

I am unable to understand the following text... Does it mean that <clinit> is for empty constructors? Why is important to have two different versions?

https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html

2.9. Special Methods

At the level of the Java virtual machine, every constructor (§2.12) appears as an instance initialization method that has the special name <init>. This name is supplied by a compiler. Because the name <init> is not a valid identifier, it cannot be used directly in a program written in the Java programming language. Instance initialization methods may be invoked only within the Java virtual machine by the invokespecial instruction, and they may be invoked only on uninitialized class instances. An instance initialization method takes on the access permissions (§2.7.4) of the constructor from which it was derived.

A class or interface has at most one class or interface initialization method and is initialized (§2.17.4) by invoking that method. The initialization method of a class or interface is static and takes no arguments. It has the special name <clinit>. This name is supplied by a compiler. Because the name <clinit> is not a valid identifier, it cannot be used directly in a program written in the Java programming language. Class and interface initialization methods are invoked implicitly by the Java virtual machine; they are never invoked directly from any Java virtual machine inw2struction, but are invoked only indirectly as part of the class initialization process.

Solution 1:

<init> is the (or one of the) constructor(s) for the instance, and non-static field initialization.

<clinit> are the static initialization blocks for the class, and static field initialization.

class X {

   static Log log = LogFactory.getLog(); // <clinit>

   private int x = 1;   // <init>

   X(){
      // <init>
   }

   static {
      // <clinit>
   }

}

Solution 2:

<init> denotes a constructor, <clinit> denotes a static initializer: "Static Initialization Blocks" in the Java Tutorial, Static initializer in Java.

Solution 3:

The difference between <init> and <clinit> is that <init> is used for constructor methods that initialise an object instance, whereas <clinit> is used to initialise the class object itself. For instance initialisation of any static class level fields is done in <clinit> when the class is loaded and initalised.

Solution 4:

Just to add If you use Class.forName method, it only intializes the class. So from within this method, it makes a call only to clinit and when you use newInstance on the object returned from forName, it will call init for the instance initialization. You can use below code to see it in debug.

public class ByteCodeParent
{
 public static String name="ByteCode";
 public ByteCodeParent()
{
    System.out.println("In Constructor");
}

 static
 {
     System.out.println("In Static");
 }

 {
     System.out.println("In Instance");
 }

To test, use

   Class<ByteCodeParent> bcp2 =(Class<ByteCodeParent>) Class.forName("ByteCodeParent");
ByteCodeParent bcp4= bcp2.newInstance();