JSON Parser

This is a simple but complete implementation of a JSON parser written in Java. It satisfies the original JSON grammar defined at json.org

Tests

I test my parser against the example JSON files at json.org for sanity. Additionally, I test against the JSON checker test suite. I adapted hundreds of tests from the nst repository. I used this blog post from the same author, as a guide. In total, my parser is validated by 358 tests.

Sample Usage

To use this parser, you should first create a JSONParser object. You can initialize it using either

a JSON file:

JSONParser parser = new JSONParser(jsonFile);

or a string of JSON:

JSONParser parser = new JSONParser("{\"key\": \"value\"}");

You can then parse the JSON file by calling one of the following methods (depending on what the top level value of the JSON file is):

• parser.parseString()

• parser.parseBoolean()

• parser.parseInteger()

• parser.parseDouble()

• parser.parseJSONObject()

• parser.parseJSONArray()

Here is an example:

{
    "data": {
        ...
    },
    "status": 200,
    "metadata": [...]
}

File jsonFile = new File("sample.json");
try {
    JSONParser parser = new JSONParser(jsonFile);
    JSONObject jsonObject = parser.parseJSONObject();
    JSONObject data = jsonObject.getJSONObject("data");
    Integer status = data.getInteger("status");
    JSONArray metadata = data.getJSONArray("metadata");
} catch (IOException e) {
    System.err.println("There was an error opening the file");
} catch (ParserException e) {
    System.err.println("Error parsing the json file: " + e.getLocalizedMessage());
}

The parser will throw a ParserException if there are any problems parsing the JSON.

There is a larger example in Main.java using sample.json.

JSONObject

With a JSONObject, you can get its values by using:

• jsonObject.getString(key)

• jsonObject.getBoolean(key)

• jsonObject.getInteger(key)

• jsonObject.getDouble(key)

• jsonObject.getJSONObject(key)

• jsonObject.getJSONArray(key)

JSONArray

With a JSONArray, you can get its items by using:

• jsonArray.getString(index)

• jsonArray.getBoolean(index)

• jsonArray.getInteger(index)

• jsonArray.getDouble(index)

• jsonArray.getJSONObject(index)

• jsonArray.getJSONArray(index)

Performance Benchmarks

Using a sample json file of about 1100 lines generated from json-generator. Here are the times it took to parse:

org.json: 18ms

my json parser: 73ms

You can run this experiment yourself in the Main.java file, by uncommenting the runBenchmarks method.

Notes

The maximum allowed depth is 1024. This is arbitrary and can be redefined in the JSONParser.java class.