How to read and write JSON data with Jackson Core?

About

This tutorial shows you how to read and write a JSON data structure with the core package of the Jackson Java serialization library.

The core package is the package that manages all basic serialization and deserialization operations at a low level.

Steps

Dependencies

Add the library as a dependency.

Example with Gradle

implementation 'com.fasterxml.jackson.core:jackson-core:2.10.1'

JsonFactory

Creation of a reusable (and thread-safe, once configured) JsonFactory instance

JsonFactory factory = new JsonFactory();
// configure, if necessary:
factory.enable(JsonParser.Feature.ALLOW_COMMENTS);

JsonFactory factory = objectMapper.getFactory();

Reading / Parsing

All reading is done using JsonParser (or its sub-classes, in case of data formats other than JSON), an instance of which is constructed by JsonFactory.

The Input file is:

{
  "id":4985773666,
  "text":"You need to add it as your \"Favourite\"",
}

The code to parse this Json file with JsonParser:

JsonFactory jsonFactory = new JsonFactory();
Path path = Paths.get("src", "test", "resources", "db-fs", "json");
Path jsonFile = path.resolve("file.json");
JsonParser jsonParser = jsonFactory.createParser(jsonFile.toFile());

// Sanity check: verify that we got "Json Object":
if (jsonParser.nextToken() != JsonToken.START_OBJECT) {
  throw new IOException("Expected data to start with an Object");
}

// Iterate over object fields:
while (jsonParser.nextToken() != JsonToken.END_OBJECT) {
  String fieldName = jsonParser.getCurrentName();
  // Let's move to value
  jsonParser.nextToken();
  switch (fieldName) {
	case "id":
	  System.out.println("Id: " + jsonParser.getLongValue());
	  break;
	case "text":
	  System.out.println("Text: " + jsonParser.getText());
	  break;
	default:
	  throw new IOException("Unrecognized field '" + fieldName + "'");
  }
}

jsonParser.close(); // important to close both parser and underlying File reader

• Output:

Id: 4985773666
Text: You need to add it as your "Favourite"

Writing

JsonFactory jsonFactory = new JsonFactory();
Path jsonFile = Files.createTempFile("write", ".json");
JsonGenerator jsonGenerator = jsonFactory.createGenerator(jsonFile.toFile(), JsonEncoding.UTF8);
jsonGenerator.useDefaultPrettyPrinter(); // enable indentation

jsonGenerator.writeStartObject();
jsonGenerator.writeNumberField("id", 1223);
jsonGenerator.writeStringField("text", "a text");
jsonGenerator.writeEndObject();
jsonGenerator.close();

System.out.println(new String(Files.readAllBytes(jsonFile)));

Output:

{
  "id" : 1223,
  "text" : "a text"
}

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