Save Data to a JSON File

Saving Data to a JSON File

We’ll use the toJson(Object src, Appendable writer) method from the Gson class to convert a Java data type into JSON and store it in a file. The Gson() constructor creates a Gson object with default configuration:

Gson gson = new Gson();

Now, we can call toJson() to convert and store Java objects.

Primitives

Saving primitives to a JSON file is pretty straight-forward using GSON:

gson.toJson(123.45, new FileWriter(filePath));

Custom Objects

public class User {
    private int id;
    private String name;
    private transient String nationality;
}

If a field is marked transient, it’s ignored by default and not included in the JSON serialization or deserialization. As a result, the nationality field isn’t present in the JSON output.

Also by default, Gson omits null fields during serialization. So if we consider this example:

gson.toJson(new User(1, null, "Unknown"), new FileWriter(filePath));

the file output will be:

{"id":1}

Collections

We can store a collection of objects in a similar manner:

User[] users = new User[] { new User(1, "Mike"), new User(2, "Tom") };
gson.toJson(users, new FileWriter(filePath));

Using GsonBuilder

In order to tweak the default Gson configuration settings, we can utilize the GsonBuilder class.

This class follows the builder pattern, and it’s typically used by first invoking various configuration methods to set desired options, and finally calling the create() method:

Gson gson = new GsonBuilder()
  .setPrettyPrinting()
  .create();

Here, we’re setting the pretty print option which is by default set to false. Similarly, to include null values in serialization, we can call serializeNulls().

Results

  • SaveJsonFileTest