java - Is instantiating non final static variables Thread-Safe? -

is 2 class same?

public class singletone {      // yes bug if change class way     //  private static singletone instance = new singletone();     // , empty constructor?     private static singletone instance;      private singletone() {         instance = new singletone();     }      public static singletone getinstance(){         return instance;     } }  

and

public class singletone {      private final static singletone instance = new singletone();      private singletone() {     }      public static singletone getinstance(){         return instance;     } }  

is there thread-safety issue in none final variable instantiated constructor?

question 2:

what differences between

    private final static singletone instance = new singletone();      private singletone() {     } 

and this:

    private final static singletone instance;      private singletone() {         instance = new singletone();     } 

queston 1

your first example doesnt work.

as if make call singletone.getinstance() before object of type has been created return null

second example works fine

question 2

same situation, youre instatiating static field in constructor. doesnt make sense cant guarantee constructor has been called before accessing static field.

you can instead:

private final static singletone instance;  static {     instance = new singletone(); } 

this instantiate static field when class first loaded.

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answering comment.

if this:

private static final singletone instance = new singletone(); 

that thread safe if first thread hasnt finished initializing class , thread tries access other thread block.

see question more info: thread safety of static blocks in java