Re: Singletons?

Eric Sosman <esosman@acm-dot-org.invalid>
Tue, 13 Jun 2006 18:17:59 -0400
Eric Sosman wrote:

Mark Space wrote:

And if a new object contains no class variables, how much memory is

    Entirely up to the JVM. You can experiment with the JVMs
available to you, doing something like

    MyClass[] array = new MyClass[SOME_BIG_NUMBER];
    RunTime rt = RunTime.getRunTime();
    for (int i = 0; i < array.length; ++i) {
        if (i % 1000 == 0)
            System.out.println("After " + i
                + " objects, " + rt.freeMemory()
                + " bytes remain.");
        array[i] = new MyClass();

     Just as a point of possible interest, I tried this on my
system (JVM 1.5.0_04-b05, Windows XP SP2). A least-squares fit
says the free memory declines by 8.04 bytes per instantiation;
I'm willing to dismiss the point-oh-four as noise from things
like println(), and guess that this JVM consumes eight bytes
per "vacuous" object. YMMV.

    Also, I believe I've read that some JVMs optimize in ways
that can change the size of an object during its lifetime. The
thought is that the vast majority of objects are not synchronized,
so some JVMs (IIRC) defer allocating memory for a lock until it's
seen that the code actually synchronizes on the object. You might
be interested to modify the above test to make a second pass over
the array, just synchronizing on each object in turn to see if
the memory consumption changes.

     Tried this, too, and free memory dropped by 0.24 bytes per
synchronization. My guess is that it's more noise, and/or that
if the JVM uses any such optimization it's considerably more
subtle than "Oh, this object is being locked for the first time;
let's attach an appendage to it."

Eric Sosman

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