Local execution mode and local variables

[Robert Field]

Hi Christian,

Thanks for your question.

Let me start by saying local execution mode is not the intended mode of use.

Interesting case.

Local execution mode cannot do redefinition, wherein a class is updated 
in-place, so updates are done by replacing the class, when replaced 
instances of the old class are lost, hence the null you are seeing.

Class A is tagged for update because of a false-positive matching the 
name of the two variables.  Usually false positives are harmless because 
the class is redefined with itself, but, per the above, local execution 
loses the instance.

I am submitting a bug because there are several options for handling 
this better:

       https://bugs.openjdk.java.net/browse/JDK-8198573

The false positive could be detected in the compiler tree or bytecode.

More importantly, in a case requiring an actual replace in normal 
execution mode the instance is lost which is surprising.  Instance 
recreation could be examined.  At a minimum, the replacement event 
should be exposed to JShell tool users.

Thanks,
Robert


On 02/22/18 06:54, Christian Stein wrote:
> Hi,
>
> The following script leads to unexpected loss
> of an object reference when jshell is launched
> with option "--execution local":
>
>    class A { int a() { int error = 0; return error; } }
>    A a = new A()
>    System.out.println("before error | a = " + a)
>    int error = 4711
>    System.out.println("after error  | a = " + a)
>    System.out.println("           error = " + error)
>    /exit
>
> Normal mode:
>
> jshell execution-local-fails.jsh
>
> before error | a = REPL.$JShell$11$A at 7d9d1a19
> after error  | a = REPL.$JShell$11$A at 7d9d1a19
>             error = 4711
>
>
> Local mode:
>
> jshell --execution local execution-local-fails.jsh
>
> before error | a = REPL.$JShell$11$A at 25df00a0
> after error  | a = null
>             error = 4711
>
>
>
> Is this behaviour expected?
>
> Tested on 9.0.1, 9.0.4 and current 10+44.
>
> Work around in "local mode": use a different
> name for the local variable.
>
> Cheers,
> Christian