Reading JSON objects from a large file

Question

I am looking for a JSON Parser that can allow me to iterate through JSON objects from a large JSON file (with size few hundreds of MBs). I tried JsonTextReader from Json.NET

Answer 1

Let's assume you have a json array similar to this:

[{"text":"0"},{"text":"1"}......]

I'll declare a class for the object type

public class TempClass
{
    public string text;
}

Now, the deserializetion part

JsonSerializer ser = new JsonSerializer();
ser.Converters.Add(new DummyConverter<TempClass>(t =>
    {
       //A callback method
        Console.WriteLine(t.text);
    }));

ser.Deserialize(new JsonTextReader(new StreamReader(File.OpenRead(fName))), 
                typeof(List<TempClass>));

And a dummy JsonConverter class to intercept the deserialization

public class DummyConverter<T> : JsonConverter
{
    Action<T> _action = null;
    public DummyConverter(Action<T> action)
    {
        _action = action;
    }
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(TempClass);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        serializer.Converters.Remove(this);
        T item = serializer.Deserialize<T>(reader);
        _action( item);
        return null;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

Answer 2

I would use this library JSON.net. The command for Nuget is as follows -> Install-Package Newtonsoft.Json

Answer 3

This is one of the use cases I contemplated for my own parser/deserializer.

I've recently made a simple example (by feeding the parser with JSON text that is read thru a StreamReader) of deserializing this JSON shape:

{ 
"fathers" : [ 
{ 
  "id" : 0,
  "married" : true,
  "name" : "John Lee",
  "sons" : [ 
    { 
      "age" : 15,
      "name" : "Ronald"
      }
    ],
  "daughters" : [ 
    { 
      "age" : 7,
      "name" : "Amy"
      },
    { 
      "age" : 29,
      "name" : "Carol"
      },
    { 
      "age" : 14,
      "name" : "Barbara"
      }
    ]
  },
{ 
  "id" : 1,
  "married" : false,
  "name" : "Kenneth Gonzalez",
  "sons" : [
    ],
  "daughters" : [
    ]
  },
{ 
  "id" : 2,
  "married" : false,
  "name" : "Larry Lee",
  "sons" : [ 
    { 
      "age" : 4,
      "name" : "Anthony"
      },
    { 
      "age" : 2,
      "name" : "Donald"
      }
    ],
  "daughters" : [ 
    { 
      "age" : 7,
      "name" : "Elizabeth"
      },
    { 
      "age" : 15,
      "name" : "Betty"
      }
    ]
  },

  //(... etc)
  ]
}

... into these POCOs:

https://github.com/ysharplanguage/FastJsonParser#POCOs

(i.e., specifically: "FathersData", "Father", "Son", "Daughter")

That sample also presents:

(1) a sample filter on the relative item index in the Father[] array (e.g., to fetch only the first 10), and

(2) how to populate dynamically a property of the father's daughters, as the deserialization of their respective father returns - (that is, thanks to a delegate that the caller passes on to the parser's Parse method, for callback purposes).

For the rest of the bits, see:

ParserTests.cs : static void FilteredFatherStreamTestDaughterMaidenNamesFixup()

(lines #829 to #904)

The performance I observe on my humble laptop (*) for parsing some ~ 12MB to ~ 180MB JSON files and deserializing an arbitrary subset of their content into POCOs

(or into loosely-typed dictionaries (just (string, object) key/value pairs) also supported)

is anywhere in the ballpark from ~ 20MB/sec to 40MB/sec (**).

(e.g., ~ 300 milliseconds in the case of the 12MB JSON file, into POCOs)

More detailed info available here:

https://github.com/ysharplanguage/FastJsonParser#Performance

'HTH,

(*) (running Win7 64bit @ 2.5Ghz)

(**) (the throughput is quite dependent on the input JSON shape/complexity, e.g., sub-objects nesting depth, and other factors)