What are CRF (Conditional Random Field)
Stanford CoreNLP is a very good implementation of CRF (In Natural Language Processing domain). 

https://nlp.stanford.edu/software/CRF-NER.html  . CRF specific implementation is : https://github.com/stanfordnlp/CoreNLP/blob/master/src/edu/stanford/nlp/ie/crf/CRFClassifier.java

Few other resources : 

https://www.analyticsvidhya.com/blog/2018/08/nlp-guide-conditional-random-fields-text-classification/

https://github.com/lancifollia/crf

https://towardsdatascience.com/conditional-random-field-tutorial-in-pytorch-ca0d04499463