What makes initializing an object reference from a static initializer to publish the object safely?

Question

Consider the following code :

   // This code safely publishes the Publishable object
   public static Publishable publishable= new Publishable();

I have seen such a way of publishing the custom Publishable object and I read that this is safe. My question is :

Is this a really a way of safe publication ?
If the answer to 1 is no then why ? If yes, then also please explain why ?

score 7 · Accepted Answer · answered Jul 03 '12 at 10:22

7

According to JCiP:

static initializers are executed by the JVM at class initialization time [...] this mechanism is guaranteed to safely publish any objects initialized in this way.

Reference: JLS 12.4.2 - item 9:

Next, execute either the class variable initializers and static initializers of the class, or the field initializers of the interface, in textual order, as though they were a single block.

answered Jul 03 '12 at 10:22

assylias

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+1 Worth making `publishable` final to be sure its not changed. – Peter Lawrey Jul 03 '12 at 10:24
@PeterLawrey That is not necessary for safe publication per se, although I get your point. – assylias Jul 03 '12 at 10:26
@assylias by "class initialization" do you mean "class loading" ? – Inquisitive Jul 03 '12 at 10:28
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@Subhra class loading can be performed without initialization, but often the two are assumed to go together. – Peter Lawrey Jul 03 '12 at 10:29

score 3 · Answer 2 · answered Jul 03 '12 at 10:24

3

All code which is run from a static block or initialised staticly is thread safe.

I would make the field final if you can.

 public static final Publishable publishable= new Publishable();

or even make the Publishable an enum

 public enum Publishable {
      INSTANCE;

 }

answered Jul 03 '12 at 10:24

Peter Lawrey

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You said "All code run from a static block or initialized statically is thread safe". But my question was why it is thread -safe ? – Inquisitive Jul 03 '12 at 10:26
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See assylias' answer as to the standard. Do you want to know why its is defined this way? – Peter Lawrey Jul 03 '12 at 10:30
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@Subhra: Just open the link at-assylias provided - first paragraph :-) – home Jul 03 '12 at 10:33
@Peter +1 for bringing in the enum construct which is the best way of publishing a singleton. – Inquisitive Jul 03 '12 at 10:45
@Subhra Sometimes the simplest solution is also the best. ;) – Peter Lawrey Jul 03 '12 at 10:47

score 1 · Answer 3 · edited May 23 '17 at 11:58

I don't think it's a particularly good way of initialising this object (leaving aside the word 'safe', which I could interpret in may ways).

It's creating a singleton tied to the containing class, and the initialisation isn't controlled. Or rather, it initialises as soon as that class is loaded. Do you have complete control over that ?

I would rather:

not use a singleton. See here for more details on the difficulties of using singletons. I note that your question is tagged with thread-related keywords, and singletons create atheir own particular issues with threading.
let the client class control the initialisation of this object. You may wish to defer initialisation of this object (imagine if it suddenly becomes a heavyweight object, performs network calls upon construction etc.). Currently the above provides very little control over this.

Although I agree with your main point, I think "safe publication" in the context of multithreading if fairly explicit. — assylias, Jul 03 '12 at 10:33

What makes initializing an object reference from a static initializer to publish the object safely?

3 Answers3