Arek Burdach created NIFI-8161:
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             Summary: NiFi EL: migration from SimpleDateFormat to 
DateTimeFormatter
                 Key: NIFI-8161
                 URL: https://issues.apache.org/jira/browse/NIFI-8161
             Project: Apache NiFi
          Issue Type: Improvement
            Reporter: Arek Burdach


In NiFi Expression Language we are commonly using SimpleDateFormat. It is old, 
inefficient approach: due to mutability of SimpleDateFormat it need to be 
recreated for each operation. Also this format is lax in some places where user 
would like to be more strict. In my opinion better approach would be usage of 
java8 DateTimeFormatter instead.

I've done some benchmarks that you can check on your own in 
FormatEvaluatorBenchmark. Results on my 8-core, i7-1065G7:
- before change (SimpleDateFormat): 11.230 ± 5.407  us/op
- after switching to DateTimeFormatter API: 4.747 ± 0.426  us/op
- after introduction of preparation of formatter for literal formats: 2.025 ± 
0.055  us/op

This change is not 100% transparent so some changes might be necessary in users 
code. Most of differences are visible in modifications that I've made in 
TestQuery tests:
-  back tick (`) for escaping of extra characters is not supported anymore - 
only single quote is supported
- "repeated" syntax like "dd" for days strictly check if two digits were 
provided -  if someone need to use more lax syntax, need to use single "d" 
syntax



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