Re: Q: Atomicity of class loading

From:

Eric Sosman <esosman@acm-dot-org.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 15 Dec 2006 18:00:28 -0500

Message-ID:

<a_-dnaIbEPKPtB7YnZ2dnUVZ_h63nZ2d@comcast.com>

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
esosman@acm-dot-org.invalid