Re: Q: Atomicity of class loading

"Daniel Pitts" <>
15 Dec 2006 15:06:44 -0800
Eric Sosman wrote:

Daniel Pitts wrote:

G. Ralph Kuntz, MD wrote:

Is class loading and class variable initialization atomic?

If I have a class defined as follows:

public class X {
  private static final X INSTANCE = new X();

  private X() {}

  public static X getInstance() {
    return INSTANCE;

can I be sure that when the class loads as a result of the call
X.getInstance(),only one instance will be created?

The short answer is no.

Two different class loaders might load the class X at different times.
And in that case you would end up with more than one instance.

     But they will be of different classes that happen to share
a single name. Try it: load X twice with two class loaders, call
both getInstance methods, call getClass on each of the instances,
and compare the Class objects.

A better approach would be to use dependency injection (a form of
inversion of control). Create the instance at initialization time in
the main method, for example), and pass it where it is needed.

     In the presence of multiple class loaders, I don't see why
this would change anything. Also, I don't see why one would
saddle every class that wants to use X with the job of writing
X's initialization code.

Eric Sosman

class X loaded by classloader A would indeed be "different" then classX
loaded by classloader B, however, the question was about the number of
possible instances in the VM, and I was pointing out how it is possible
to not be unique.

I think you missed the point.
Only one class is responsible for initializing the X instance. the X
class itself. And only one class (or layer, depending on how you view
it) is responsible for creating the single X instance.. The
responsible layer is the framework. Or, at the very least, the
framework could create a thread-safe singleton factory object.

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