ExecuteScript - JSON-to-JSON Revisited (with Jython)

I've received some good comments about a couple of previous blog posts on using the ExecuteScript processor in NiFi (0.5.0+) to perform JSON-to-JSON transformations. One post used Groovy and the other used Javascript.

Since then I've received some requests for a Jython example for ExecuteScript, so I figured I'd do the same use case again (JSON to JSON) so folks can see the differences in the languages when performing the same operations :)

The approach has been covered in detail in the other posts, so I will talk a bit about the Jython-specific stuff and then get right to the code.

One major caveat here is that I don't know Python :)  I learned enough to get the script working, but please let me know how to better do any of this. I stopped touching the script once it worked, so it's very possible there are unnecessary imports, classes, etc.

The first thing to do is to bring in the Jython libraries you will need, as well as importing the Java and NiFi classes to be used:

import json
import java.io
from org.apache.commons.io import IOUtils
from java.nio.charset import StandardCharsets
from org.apache.nifi.processor.io import StreamCallback

I didn't need to import java.lang.String as Jython does type coercion. I probably couldn't call getBytes() on that string unless Jython knew to coerce the object to a Java String, but that's ok because we can call bytearray("myString".encode('utf-8')) to achieve the same results.

The next task was to create a StreamCallback object for use in session.write(). I created a Jython class for this and overrode the interface method:

class PyStreamCallback(StreamCallback):
  def __init__(self):
        pass
  def process(self, inputStream, outputStream):
        # ...

After that, I read in and parsed the JSON text with IOUtils then json.loads(), then performed all the operations on the various parts of the object/dictionary. Finally I generated a new JSON string with json.dumps(), encoded to UTF-8, got the byte array, and wrote it to the output stream.

The resulting script is as follows:

import json
import java.io
from org.apache.commons.io import IOUtils
from java.nio.charset import StandardCharsets
from org.apache.nifi.processor.io import StreamCallback

class PyStreamCallback(StreamCallback):
  def __init__(self):
        pass
  def process(self, inputStream, outputStream):
    text = IOUtils.toString(inputStream, StandardCharsets.UTF_8)
    obj = json.loads(text)
    newObj = {
          "Range": 5,
          "Rating": obj['rating']['primary']['value'],
          "SecondaryRatings": {}
        }
    for key, value in obj['rating'].iteritems():
      if key != "primary":
        newObj['SecondaryRatings'][key] = {"Id": key, "Range": 5, "Value": value['value']}
              
    outputStream.write(bytearray(json.dumps(newObj, indent=4).encode('utf-8'))) 

flowFile = session.get()
if (flowFile != None):
  flowFile = session.write(flowFile,PyStreamCallback())
  flowFile = session.putAttribute(flowFile, "filename", flowFile.getAttribute('filename').split('.')[0]+'_translated.json')
  session.transfer(flowFile, REL_SUCCESS)

The template is available as a Gist (here). I welcome all suggestions on how to make this better, and please share any scripts you come up with!

Cheers!