Hi Peter!
On 2017-07-15 14:08, Peter Levart wrote:
It seems that interning signature(s) is important for correctness (for
example, in ObjectOutputStream.writeTypeString(str) the 'str' is used
to lookup a handle so that handles are put into stream instead of the
type signature(s) for multiple references to the same type). Looking
up objects in handles table is based on identity comparison.
Yes, interned signatures is important for correctness (and performance?)
of the current serialization implementation.
But there might be a way to obtain a singleton signature String per
type and still profit. By adding a field to java.lang.Class and
caching the JVM signature there. This would also be a useful public
method, don't you think?
I have a nagging feeling that we should be careful about leaking
implementation details about the underlying VM through public APIs,
since making changes to various specifications is hard enough as it is.
Out of 191 ObjectStreamField constructions I found in JDK sources,
there are only 39 distinct field types involved, so the number if
intern() calls is reduced by a factor of ~5. There's no need to cache
signature in ObjectStreamField(s) this way any more, but there must
still be a single final field for ObjectStreamField(s) constructed
with explicit signature(s).
Here's how this looks like in code:
http://cr.openjdk.java.net/~plevart/misc/Class.getJvmTypeSignature/webrev.01/
Could this be done as a ClassValue instead of another field on Class? My
guess is only a small number of classes in any given app will be directly
involved in serialization, so growing Class seems to be a pessimization.
What do you think?
I wonder what workloads actually see a bottleneck in these String.intern
calls, and *which* String.intern calls we are bottlenecking on in these
workloads. There's still a couple of constructors here that won't see a
speedup.
I think we need more data to ensure this is actually worthwhile to pursue,
or whether there are other optimizations on a higher level that could
be done.
Thanks!
/Claes
