RFR: JDK-8234634: JShell could leverage the better NullPointerException analysis

[Jan Lahoda]

Actually, Robert expressed concerns about this offline. The current 
proposal is to not use the -XX parameter, but prepare jshell for the 
state where that will be default (by using -g:vars and cleaning of the 
exception messages). The users will still be able to enable the better 
behavior using -R-XX...:

http://cr.openjdk.java.net/~jlahoda/8234634/webrev.01/

Jan

On 27. 11. 19 14:18, Maurizio Cimadamore wrote:
> I like it very much - great work.
> 
> I get what you say about depending on XX options, but this is, I think, 
> a nice usability boost, and I'm not too picky about what we have to do 
> in order to get it :-)
> 
> Maurizio
> 
> On 22/11/2019 10:52, Jan Lahoda wrote:
>> Hi,
>>
>> Maurizio suggested on core-libs-dev that JShell could leverage the 
>> better NullPointerException analysis. This patch tried to do it. The 
>> outcome for the user is e.g.:
>> ---
>> jshell> String s = null;
>> s ==> null
>>
>> jshell> s.length()
>> |  Exception java.lang.NullPointerException: Cannot invoke 
>> "String.length()" because "s" is null
>> |        at (#2:1)
>>
>> jshell> String test(Object o) { return o.toString(); }
>> |  created method test(Object)
>>
>> jshell> test(null)
>> |  Exception java.lang.NullPointerException: Cannot invoke 
>> "Object.toString()" because "o" is null
>> |        at test (#3:1)
>> |        at (#4:1)
>> ---
>>
>> There are three parts of this patch:
>> 1. enabling -g:vars, so that the NPE analysis has more data to work on
>> 2. enabling -XX:+ShowCodeDetailsInExceptionMessages, so that the 
>> enhanced analysis runs. It feels a little bit unfortunate to depend on 
>> a non-standard VM option, but this is only done in the tool (not in 
>> the engine), so it may be OK.
>> 3. processing of the NPE message to strip references to the synthetic 
>> JShell classes.
>>
>> Webrev: http://cr.openjdk.java.net/~jlahoda/8234634/webrev.00/
>> JBS: https://bugs.openjdk.java.net/browse/JDK-8234634
>>
>> How does this look?
>>
>> Thanks,
>>     Jan