  JSON is an essential file and data format in do-mains that span scientific
computing, web APIs or configuration management. Its popularity has motivated
significant software development effort to build multiple libraries to process
JSON data. Previous studies focus on performance comparison among these
libraries and lack a software engineering perspective.We present the first
systematic analysis and comparison of the input / output behavior of 20 JSON
libraries, in a single software ecosystem: Java/Maven. We assess behavior
diversity by running each library against a curated set of 473 JSON files,
including both well-formed and ill-formed files. The main design differences,
which influence the behavior of the libraries, relate to the choice of data
structure to represent JSON objects and to the encoding of numbers. We observe
a remarkable behavioral diversity with ill-formed files, or corner cases such
as large numbers or duplicate data. Our unique behavioral assessment of JSON
libraries paves the way for a robust processing of ill-formed files, through a
multi-version architecture.
