[10] RFR: 8184603: Create ObjectStreamField signature lazily when possible

Peter Levart peter.levart at gmail.com
Wed Jul 19 09:49:18 UTC 2017

Hi Claes,

On 07/17/2017 02:16 PM, Claes Redestad wrote:
> 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.

You're right. There's already more than enough "implementation details" 
that pertain to JVM exposed through reflection API which was supposed to 
represent Java - the language - view of the world. JVM type signatures 
just happen to be used in serialization too, which is another 
implementation detail which might change in the future (with value types 
etc), so it's better to keep it private.

>> 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.

It could be, yes. We are trying to solve two issues here. One is the 
original 8184603 which is concerned with start-up overhead and your 
proposal is the right solution for it as it only delays the work to 
when/if it is needed. The other issue is overheads of repeatable 
signature interning. These are not frequent enough for cases that just 
create a bunch of ObjectStreamField instances assigned to static final 
fields, but I suspect are more frequent when signatures are being 
de-serialized from stream. At that time, we don't yet have a Class 
object to go with the signature and to use as a caching anchor, but we 
still want to keep the invariant of OSF signature(s) being interned 
Strings. If they really need to be interned right away in that case is a 
question which needs more studying of deserialization code.

>> 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.

Right. I suspect the intern() call bottleneck is most problematic when 
deserializing. All other cases could be optimized by caching the 
signature on the appropriate Class object(s) via ClassValue for example.

> 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.

Ok, we agree that no new public API for JVM signatures is desired and 
the problem of intern() calls bottleneck when deserializing should be 
researched more deeply. I agree that your solution is currently the best 
for the original issue.

> Thanks!
> /Claes

Regards, Peter

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