Working with JSON

What is JSON

JSON is a common format to store and transmit data. The properties of an object are represented as a string.

For example, the object new Exercise("Running", 30, "m") would be represented in JSON as

{
  "activity": "Running",
  "time": {
    "value": 30,
    "unit": "m"
  }
}

Using Jackson

A popular library to work with JSON in Java is called Jackson. We install it by adding thee dependency in pom.xml.

<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.15.0</version>
</dependency>

Jackson gives us access to a powerful ObjectMapper class. To use it, we need to import it:

import com.fasterxml.jackson.databind.ObjectMapper;

Converting to JSON

If we have an object, we can turn it into JSON using the Jackson library.

WARNING

Any method beginning with get will be serialised into the JSON object. You can decorate unwanted getters with @JsonIgnore.

java

Exercise exercise = new Exercise("Running", 30, "m");

// Convert to JSON
ObjectMapper objectMapper = new ObjectMapper();
String json = objectMapper.writeValueAsString(exercise)
System.out.println(json)

output

{
  "activity": "Running",
  "time": {
    "value": 30,
    "unit": "m"
  }
}

Parsing JSON

If we have a string of JSON and we want to convert it into an object, we pass the JSON and the target object to Jackson's .readValue() method.

WARNING

Jackson will call the class constructor with no arguments, so you must provide a default constructor in your class: public MyClass() {}.

String json = "{ \"activity\": \"Running\", \"time\": { \"value\": 30, \"unit\": \"m\" } }";

// Parse JSON string into Exercise object
ObjectMapper objectMapper = new ObjectMapper();
Exercise exercise = objectMapper.readValue(json, Exercise.class);

JSON keys

If you find that the keys in the JSON don't match up with the keys in the object you're mapping to, you can use a special decorator.

class Exercise {
  @JsonProperty("Name")
  private String activity;
}

Now, parsing a JSON which has { "Name": "swimming" } will map swimming into the activity property of the Exercise class.

JSON Lists

JSON supports list like structures.

[
  {
    "activity": "Running",
    "time": {
      "value": 30,
      "unit": "m"
    }
  },
  {
    "activity": "Swimming",
    "time": {
      "value": 1,
      "unit": "h"
    }
  }
]

To parse this into a list of exercises, we need to import com.fasterxml.jackson.core.type.TypeReference;. This is used to help Jackson figure out the type of List<Exercise> and parse the JSON string correctly.

Suppose we have the contents of the above JSON in a string called exercisesJSON.

ObjectMapper objectMapper = new ObjectMapper();
List<Exercise> exercises = objectMapper.readValue(exercisesJSON, new TypeReference<List<Exercise>>() {});

INFO

new TypeReference<List<Exercise>>() {} creates an anonymous subclass of TypeReference, which helps Jackson keep track of the type inside the List.