Re: SingletonFactory and safe publication

=?ISO-8859-1?Q?Arne_Vajh=F8j?= <>
Mon, 01 Dec 2014 21:34:08 -0500
On 12/1/2014 9:16 PM, John wrote:


I am reading this article( It says the following code is GOOD:

public class SafeDCLFactory {
   private volatile Singleton instance;

   public Singleton get() {
     if (instance == null) { // check 1
       synchronized(this) {
         if (instance == null) { // check 2
           instance = new Singleton();
     return instance;

I feel disagree, by learning from this article(

If without 'volatile', the code above has two problems, not one problem: 1)other threads may not see the content of 'instance' which is initiated by constructing thread; 2)It is possible thread A is in the middle of constructing the object, so the reference 'instance' is not null, but what it references maybe an partially initiated object, so thread B may get this partially initiated object, causing crash later on.

Adding 'volatile' only solves the first problem, not the second.


The constructor should complete before instance get assigned.

                                                                  According to the second article above, the CORRECT way is:
class Foo {
     private volatile Helper helper;
     public Helper getHelper() {
         Helper result = helper;
         if (result == null) { //1st check
             synchronized(this) {
                 result = helper;
                 if (result == null) { //2nd check
                     helper = result = new Helper();
         return result;

     // other functions and members...

This is the same concept just with some irrelevant but confusing extras.

I would like to ask help to understand why the following code is GOOD. The code uses 'final', instead of 'volatile':
public class FinalWrapper<T> {
     public final T value;
     public FinalWrapper(T value) {
         this.value = value;

public class Foo {
    private FinalWrapper<Helper> helperWrapper;

    public Helper getHelper() {
       FinalWrapper<Helper> wrapper = helperWrapper;

       if (wrapper == null) { //1st check
           synchronized(this) {
               if (helperWrapper == null) { //2nd check
                   helperWrapper = new FinalWrapper<Helper>(new Helper());
               wrapper = helperWrapper;
       return wrapper.value;

The Wikipedia link actually explains that:

"Semantics of final field in Java 5 can be employed to safely publish
the helper object without using volatile"


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