How to Read JSON File in Kotlin: Simple Guide

To read a JSON file in Kotlin, you can use libraries like kotlinx.serialization or Gson. First, read the file content as a string, then parse it into Kotlin objects using the chosen library's deserialization functions.

Syntax

Reading a JSON file in Kotlin involves two main steps: reading the file content as a string and then parsing that string into Kotlin objects.

Here is the general syntax using kotlinx.serialization:

• File.readText(): Reads the entire file content as a string.
• Json.decodeFromString<Type>(jsonString): Converts the JSON string into a Kotlin object of the specified type.

kotlin

import kotlinx.serialization.*
import kotlinx.serialization.json.*
import java.io.File

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

fun readJsonFile(filePath: String): Person {
    val jsonString = File(filePath).readText()
    return Json.decodeFromString<Person>(jsonString)
}

Example

This example shows how to read a JSON file named person.json containing a person's name and age, then print the parsed object.

kotlin

import kotlinx.serialization.*
import kotlinx.serialization.json.*
import java.io.File

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

fun main() {
    // Assume person.json content: {"name":"Alice","age":30}
    val jsonString = File("person.json").readText()
    val person = Json.decodeFromString<Person>(jsonString)
    println("Name: ${person.name}, Age: ${person.age}")
}

Output

Name: Alice, Age: 30

Common Pitfalls

• Not marking data classes with @Serializable: The Kotlin serialization library requires this annotation to work.
• File path errors: Make sure the JSON file path is correct and accessible.
• Incorrect JSON format: The JSON must match the Kotlin data class structure exactly.
• Using blocking IO on main thread: For Android or UI apps, avoid reading files on the main thread to prevent freezing.

kotlin

/* Wrong: Missing @Serializable annotation */
data class Person(val name: String, val age: Int)

/* Right: Add @Serializable annotation */
@Serializable
data class Person(val name: String, val age: Int)

Quick Reference

Summary tips for reading JSON files in Kotlin:

• Use File.readText() to get JSON as a string.
• Use kotlinx.serialization or Gson to parse JSON.
• Annotate data classes with @Serializable for kotlinx.serialization.
• Ensure JSON structure matches Kotlin classes.
• Handle exceptions for file reading and parsing errors.

Key Takeaways

Use File.readText() to read JSON file content as a string in Kotlin.

Parse JSON strings into Kotlin objects using kotlinx.serialization or Gson libraries.

Always annotate data classes with @Serializable when using kotlinx.serialization.

Ensure the JSON file path and format match your Kotlin data classes.

Handle errors and avoid blocking the main thread when reading files in UI apps.