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