Re: JavaMemoryModel: JMM problems with latch or copy-on-write?

From: David Smiley (
Date: Thu Mar 29 2001 - 09:03:40 EST

> You mean something like the following?
> Foo foo = new Foo();
> Singleton.FOO = foo;
> This situation is like your first example (simple boolean assignment), in
> that the compiler might outsmart you and hide the new foo from other
> threads.

This is distressing. I think I'm starting to understand this better now.
What can be done? Will the proposed semantics of volatile take the problem
away in the copy-on-write case assuming that foo is declared volatile?

As for the latch style problem what about this latch scenario:

class Span{
    volatile boolean init= false;//eventually becomes true permanently
    int start= 0, end= 0; //constraint: b >= a

    //assumed to be called only once ever
    void doInit(int a, int b) {
        if (b < a)
            throw IllegalArgumentException();
        start= a; end=b;
        init= true;

    //-1 if not ready, the span's length otherwise
    int getLength() {
        if (!init)
            return -1;
            return start-end;

Will some thread B calling getLength() ever see results other than what the
typical programmer would expect? i.e., I'm hoping that there won't be any
funky re-orderings surrounding the init= true.

-- David Smiley

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