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From "Dmitri Blinov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (JEXL-194) synchronize on iterableValue in foreach statement
Date Fri, 20 May 2016 15:07:13 GMT

    [ https://issues.apache.org/jira/browse/JEXL-194?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15293504#comment-15293504
] 

Dmitri Blinov commented on JEXL-194:
------------------------------------

Another train of thoughts...

As an implementation to the extension of statement executions in JEXL we could introduce the
use of annotations to each statement or block in the script, i.e. 

{code}
@synchronized(items) {for (x : items) ...}

@silent @lenient {null.tryMe()}
{code}

>From the JexlEngine point of view, each annotation could be implemented in the form of
Interceptor interface, for example

{code}
public interface Interceptor {
   public Object onStart(JexlStatement block, String annotation, Object.. args);
   public void onCatch(JexlStatement block, Exception ex); 
   public void onFinally(JexlStatement block); 
}
{code}

JexlEngine could be given a method to register annotation interceptor based on annotation
name, and the one for default interceptor. The engine should call each Interceptor.onStart()
method in the order the annotations are applied to the statement in the script, finishing
with the default interceptor if registered. The Interceptor.onCatch() method should be called
only if execution had thrown any exception. Interceptor.onFinally() method should be called
after the statement execution regardless of any exceptions, to provide guaranteed resource
unlocking/cleaning if necessary. The Interceptor.onCatch() and Interceptor.onFinally() methods
should be called in backward order respectively.

The JexlStatement is the proposed new interface to somehow identify the statement or block
of code which may also provide some info about it's stack frame.

The annotation syntax should allow for zero, one or more parameters. Those parameters should
be evaluated before interceptor execution and submitted to the Interceptor.onStart() method.

In the absence of any matching interceptors corresponding to the annotation name, JexlEngine
should simply ignore that, or write some diagnostic message in the log file.

Such implementation could provide developers with excellent tool to add various checks and
enhancements to scripting without pushing for new features that eventually would mess up the
basic code. The JEXL-185 issue could also be dropped since tracing could be added easily via
interceptors.



> synchronize on iterableValue in foreach statement
> -------------------------------------------------
>
>                 Key: JEXL-194
>                 URL: https://issues.apache.org/jira/browse/JEXL-194
>             Project: Commons JEXL
>          Issue Type: Improvement
>    Affects Versions: 3.0
>            Reporter: Dmitri Blinov
>            Assignee: Henri Biestro
>            Priority: Minor
>
> Since it is a requirement to synchronize on simple Collections and synchronized Collections
while iterating over them and since jexl has no instrument to control synchronization in script,
I think its reasonable to implement synchronization in jexl itself on iterableValue. In case
of concurrent collections it will possibly block other threads only if they are synchronizing
on those collections themselves, which will be complementary to required synchronization in
jexl.



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