Re: SingletonFactory and safe publication

=?windows-1252?Q?Arne_Vajh=F8j?= <>
Tue, 02 Dec 2014 18:52:26 -0500
On 12/2/2014 6:20 PM, Eric Sosman wrote:

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

     Brian Goetz agrees that this code is incorrect, calling it
"a commonly suggested nonfix." He explains that although the
accesses to `instance' will be consistent because `volatile'
ensures it, any accesses to the member variables of the new
Singleton are *not* consistent (unless they are `volatile', too).
You could get a sequence like this:

     Thread T1 finds `instance' null, obtains the lock, finds
     that `instance' is still null, and calls the constructor.

     The constructor (running in T1) stores initial values in
     the member variables of the new Singleton. We presume
     that at least some of these variables are not `volatile'.

     The constructor finishes, and now T1 stores the new
     reference to `instance'. Because `instance' is `volatile',
     T1 ensures that the new value is actually flushed from
     store buffers and write caches and so on, and appears in
     stable memory.

     Thread T2 now finds `instance' non-null, and starts using
     it to refer to the Singleton's member variables (either
     directly or by calling the Singleton's methods).

     Unfortunately, the values stored by Singleton's constructor
     may still be sitting in caches and what-not, and may not yet
     have been flushed to stable memory. Even if the constructor
     running in T1 stored 42 in some member variable, T2 may
     read the value as zero.

     ... because there is no "happens-before" between T1's storing
     of the value and T2's reading of it.

     In short, making sure that `instance' is safe is not sufficient;
you also need to worry about everything `instance' refers to, directly
or indirectly.

But that is with the pre-5 memory model (the article is from 2001).

They changed the treatment of volatile in the memory model
in Java 5.

Or have I misunderstood something?


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