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How to ignore specific fields while serializing Kotlin objects to JSON using GSON in Kotlin Android

How to ignore specific fields while serializing Kotlin objects to JSON using GSON in Kotlin Android.

Here's a step-by-step tutorial on how to ignore specific fields while serializing Kotlin objects to JSON using GSON in Kotlin Android.

Step 1: Add GSON Dependency​

First, you need to add the GSON dependency to your project. Open your project's build.gradle file and add the following line to the dependencies block:

implementation 'com.google.code.gson:gson:2.8.7'

Sync your project to download the GSON library.

Step 2: Create a Kotlin Data Class​

Next, create a Kotlin data class that represents the object you want to serialize to JSON. For example, let's say we want to serialize a Person object with name, age, and email fields. 

data class Person(val name: String, val age: Int, val email: String)

Step 3: Customize Serialization with @Expose Annotation​

To ignore specific fields during serialization, you need to use the @Expose annotation from GSON. By default, GSON serializes all fields, but with this annotation, you can specify which fields to include or ignore.

import com.google.gson.annotations.Expose

data class Person(
    @Expose val name: String,
    @Expose val age: Int,
    @Expose(serialize = false) val email: String
)

In this example, we are using the @Expose annotation on all fields. We also set serialize = false for the email field, indicating that it should be ignored during serialization.

Step 4: Create a GSON Instance​

Now, create a GSON instance that will handle the serialization process. You can do this by calling GsonBuilder().excludeFieldsWithoutExposeAnnotation().create().

import com.google.gson.GsonBuilder

val gson = GsonBuilder().excludeFieldsWithoutExposeAnnotation().create()

The excludeFieldsWithoutExposeAnnotation() method tells GSON to ignore fields that are not annotated with @Expose.

Step 5: Serialize Kotlin Object to JSON​

To serialize the Kotlin object to JSON, simply call the toJson() method on the GSON instance, passing in the object you want to serialize.

val person = Person("John Doe", 25, "[email protected]")
val json = gson.toJson(person)

The toJson() method will return a JSON string representation of the person object, ignoring the email field.

Step 6: Verify the JSON Output​

You can verify the JSON output to ensure that the email field is ignored. You can print the JSON string to the console or display it in a UI element.

Log.d("JSON", json)

The output should be something like:

{
   "name":"John Doe",
   "age":25
}

This confirms that the email field was ignored during serialization.

That's it! You have successfully ignored specific fields while serializing Kotlin objects to JSON using GSON in Kotlin Android.