Introduce SPI to configure the SpEL Indexer

[jackmiking]

The SpEL indexer doesn't provide support for Jackson's ArrayNode, and it seems to be non-extensible; whereas, a Jackson ObjectNode can be operated on within a SpEL expression via PropertyAccessor support.

[sbrannen]

Can you please describe your use case and explain why you want to parse Jackson types in a SpEL expression?

[jackmiking]

well, we are building up a code-less project which should store request context in JSON since it's data structure is completely dynamic. And this request context will be used by some handler by using config data.

For example, if the request context looks like this: {"a": {"b": [{"c": "dd"},{"c": "ee"}]}}.
there might be a handler which want to make a choice for the next handler base on the value of "a.b[0].c".

it is easy to implement only the fetch logic here. However we might also want to execute some functions and even a calculation. It is hard to finish it from scratch, and here SpEL comes into play, and that is why we need an extended mode for the Indexer class.

[sbrannen]

How are you evaluating your SpEL expressions?

Do you have custom setup for the SpelExpressionParser, EvaluationContext, etc.?

[jackmiking]

yes, the context can config property accessor,however, that is no the case for arrayNode. an array item is accessed by the Indexer class which doesn't allow for extending