Chris,
Great to hear! For the following steps, I'm assuming it will be a
one-argument function called "jsonPath" that takes a String containing a
JSON Path expression to be evaluated on the "subject" of the function. So I
am picturing it used like:
${ my.json.attribute:jsonPath("$.path.to.my.data") }
To add such a function to EL, you'll likely want to do something like the
following:
1) Add a token for the function name to the Lexer:
https://github.com/apache/nifi/blob/master/nifi-commons/nifi-expression-language/src/main/antlr3/org/apache/nifi/attribute/expression/language/antlr/AttributeExpressionLexer.g#L120
2) Add the token to the oneArgString rule:
https://github.com/apache/nifi/blob/master/nifi-commons/nifi-expression-language/src/main/antlr3/org/apache/nifi/attribute/expression/language/antlr/AttributeExpressionParser.g#L77
3) Add a JsonPathEvaluator class (or whatever its called) to the functions
package:
https://github.com/apache/nifi/tree/master/nifi-commons/nifi-expression-language/src/main/java/org/apache/nifi/attribute/expression/language/evaluation/functions
4) Add a case to Query.buildFunctionEvaluator() to create and use a new
JsonPathEvaluator:
https://github.com/apache/nifi/blob/master/nifi-commons/nifi-expression-language/src/main/java/org/apache/nifi/attribute/expression/language/Query.java#L1061
5) Add unit test(s) to TestQuery to test your function:
https://github.com/apache/nifi/blob/master/nifi-commons/nifi-expression-language/src/test/java/org/apache/nifi/attribute/expression/language/TestQuery.java
Also whatever dependencies you bring in (Jayway, e.g.) will need to be
added to the EL module's POM:
https://github.com/apache/nifi/blob/master/nifi-commons/nifi-expression-language/pom.xml.
If a parent POM has declared a version already (such as the NiFi parent POM
declaring jayway 2.0.0 at present), you may want to keep that unless you
need to override for some reason.
Looking forward to your contribution, please let me/us know if you run into
any trouble or have any questions.
Cheers,
Matt
On Tue, Mar 22, 2016 at 10:59 AM, McDermott, Chris Kevin (MSDU -
STaTS/StorefrontRemote) <chris.mcderm...@hpe.com> wrote:
> Hi folks,
>
> I’m a newbie NiFi user but long time Java developer. I’ve entered a Jira
> issue to extend the expression language to add a function to evaluate a
> Json path against the subject. Looking at the code I feel this is very
> doable, and I am up for task so I’m planning on tackling it. I’ve read the
> developers guide, etc. Please, any advice and direction is most welcome.
>
> Thanks,
>
> Chris McDermott.
>
