RFR [10]: 8186517: sun.nio.cs.StandardCharsets$Aliases and ClassMap can be lazily loaded

[Peter Levart]

Hi Sherman,

On 08/24/2017 05:56 PM, Xueming Shen wrote:
>> While looking at the code of StandardCharsets, I noticed a real 
>> danger of using a thread-unsafe object while it is being modified. 
>> The method charsets() returns an Iterator<Charset> which lazily 
>> invokes an unsynchronized lookup() method for each charset as it 
>> returns them. lookup() uses a Cache (a PreHashedMap) which is not a 
>> thread-safe object. private lookup() is called from a synchronized 
>> block of public charsetForName() method, but it is invoked 
>> unsynchronized from iterator.
>
> There were tasty initialization/synchronization/deadlock issues here 
> in the past (decade ago,
> happened only on customers' site with specific use scenarios). Our 
> system initialization
> "sequence" has been changed and the way the standard charset provider 
> is being loaded has
> been changed since, it might be reasonable to reconsider how this part 
> of code should work,
> but I would still be very cautious here as the initial goal is for 
> startup performance.

It would be nice if startup performance improvement extended to programs 
using just constants from java.nio.charset.StandardCharsets too.

Here's a variant of that:

http://cr.openjdk.java.net/~plevart/jdk10-dev/8186517_StandardCharsets_opt/webrev.02/

Regards, Peter

>
> -sherman
>
>>
>> One way to fix it would be to enclose the lookup() invocation in 
>> Iterator.next with a synchronized block, but then I thought that 
>> maybe a PreHashedMap is not the right tool for the job. What if the 
>> cache was simply a ConcurrentHashMap which need not be pre-populated 
>> with all possible keys (saving on footprint) and could therefore be 
>> constructed eagerly. The whole lookup() could then be unsynchronized. 
>> Here's what I came up with:
>>
>> http://cr.openjdk.java.net/~plevart/jdk10-dev/8186517_StandardCharsets_opt/webrev.01/ 
>>
>>
>> I also ensured that the same instance of Charset object gets returned 
>> for all aliases of a charset (including standard 3). 

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