At 10:51 AM 4/9/2003, Joseph Bowbeer wrote:
>[ After reading Jerry's response, too. ]
>
>Bill Pugh writes:
>
> > Now within the code for send, a VM might try to
> > optimize the call to out.write into a tail call to out.write.
> > The VM could also inline the call to send.
>
>Can we argue that these optimizations should be disallowed in the presence
>of a finalizer?
>
>Naively, I ask: Why do we have to assume that the compiler will be blind to
>finalizers?
>
Because that is the implication of what the JLS says about it.
"Optimizing transformations of a program can be designed that reduce the
number of objects that are reachable to be less than those which would
naively be considered reachable".
>----- Original Message -----
>From: "Bill Pugh" <pugh@cs.umd.edu>
>To: <javamemorymodel@cs.umd.edu>
>Sent: Wednesday, April 09, 2003 7:43 AM
>Subject: JavaMemoryModel: A problematical case for finalizers
>
>
>
>In talking about finalizers with Doug Lea, I mentioned a scenario
>which I thought everyone knew about but Doug hadn't considered. He
>suggested I share it
>with everyone.
>
>Consider
>
>public class Foo {
> OutputStream out;
>
> public Foo(...) {
> out = ...;
> }
>
> public void send(...) {
> byte [] buf = ...;
> out.write(buf);
> }
> protected void finalize() {
> out.close();
> }
> }
>
>Now consider the code:
>
> { Foo f = new Foo(...);
> f.send(...);
> }
>
>Now within the code for send, a VM might try to optimize the call to
>out.write into a tail call to out.write.
>
>The VM could also inline the call to send.
>
>In this situation, there wouldn't be any live reference to the Foo
>object from the stack during the call to write. However, the write
>method could take a very long time to execute if it is pushing a
>large amount of data down a narrow pipe. So the Foo object could be
>finalized, and the output stream be closed, while the output stream
>is still being written to.
>
>Are we happy to declare such code broken?
>
>Note that declaring the send and finalize methods as synchronized
>with fix the problem. So would a call to System.keepAlive(this) at
>the end of the send method, if people are interested in that.
>
>Bill
>
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