Re: JavaMemoryModel: String literals and String.intern()

From: Thomas Hawtin (
Date: Fri Apr 16 2004 - 23:30:28 EDT

Eliot Moss wrote:
>>>>>>"Doron" == Doron Rajwan <> writes:
> Doron> Thread 1, Thread 2:
> Doron> synchronized ("some string") {
> Doron> ++a;
> Doron> }
> [...]
> Doron> 2. However, it is possible that the first thread
> Doron> finished, the GC collected the interned string, and
> Doron> then, the second thread started. In this case, the
> Doron> sync was on a different instance of the String.
> Doron, I don't get what you're saying. Even a copying GC has to preserve
> identity semantics in the face of locking objects. And so far as I know, an
> interned string will never be reclaimed anyway.

Interned Strings behave as normal garbage (if somewhat slower to be
collected than more typical objects). Early Sun JREs (1.0 and 1.1)
failed to collect interned Strings.

In the case above, the class' Class will have a reference to the String.
The Class cannot disappear while the method is executing. Therefore,
there is no data race. However you can easily create a race using

     synchronized ("some ".concat("string").intern()) {

which would be your own fault.

So synchronising an interned String remains a working way to achieve a
process global critical section. Synchronising on a StringBuffer, say,
assigned to a static final field works for a class' class loader global
scoped critical section.

Tom Hawtin

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