Github user pvillard31 commented on the issue:
https://github.com/apache/nifi/pull/2538
````
[WARNING]
src/main/java/org/apache/nifi/processors/standard/FlattenJson.java:[21,8]
(imports) UnusedImports: Unused import -
com.github.wnameless.json.flattener.CharSequenceTranslatorFactory.
[WARNING]
src/main/java/org/apache/nifi/processors/standard/FlattenJson.java:[25,8]
(imports) UnusedImports: Unused import -
org.apache.commons.text.translate.CharSequenceTranslator.
[INFO]
------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO]
------------------------------------------------------------------------
[INFO] Total time: 04:39 min
[INFO] Finished at: 2018-03-13T17:43:28+01:00
[INFO]
------------------------------------------------------------------------
[ERROR] Failed to execute goal
org.apache.maven.plugins:maven-checkstyle-plugin:2.17:check (check-style) on
project nifi-standard-processors: You have 2 Checkstyle violations. -> [Help 1]
````
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