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Roadmap: Kotlin — Backend Web, Spring Boot, and Android Development

A phased learning path from zero to expert across the full Kotlin ecosystem.

Table of Contents

Phase 1: Kotlin Language Foundations

Modules: 01–03
Duration: ~25 hours
Goal: Become fluent in Kotlin as a language — independent of any framework or platform.

By the end of Phase 1 you will write idiomatic Kotlin: null-safe code, data classes, sealed class hierarchies, coroutines, extension functions, and DSLs. You will understand why Kotlin exists, how it compares to Java, and how the JVM executes it.

Module Focus
01 Introduction Why Kotlin, null safety, basic syntax, first programs
02 Kotlin Fundamentals OOP, collections, sealed classes, generics
03 Kotlin Advanced Coroutines, extension functions, DSL building, delegation

Milestone: Pass all Phase 1 tests with ≥70%. Write a small command-line tool in pure Kotlin before moving to Phase 2.

Phase 2: Spring Boot Backend

Modules: 04–08
Duration: ~35 hours
Goal: Build production-ready Spring Boot backend services in Kotlin.

Phase 2 takes the Kotlin fluency from Phase 1 and applies it to the Spring Boot ecosystem: REST APIs, database access, authentication, advanced patterns, and Gradle build configuration.

Module Focus
04 Spring Boot Fundamentals IoC/DI, REST controllers, auto-configuration
05 Spring Data and Persistence JPA, repositories, transactions, PostgreSQL
06 Spring Security JWT, OAuth2, method-level security
07 Spring Advanced Actuator, caching, Kafka, WebFlux
08 Gradle and Build Tooling Kotlin DSL, multi-module projects

Milestone: Deploy a functioning REST API to a local Docker container. It must authenticate with JWT, query a real PostgreSQL database, and have passing integration tests.

Phase 3: Android Development

Modules: 09–10
Duration: ~25 hours
Goal: Build modern Android apps using Jetpack Compose, Hilt, Room, and Retrofit.

Phase 3 applies Kotlin knowledge to Android. The Kotlin language skills from Phase 1 transfer directly; coroutines (Module 03) and the dependency injection concepts from Module 04 reappear in an Android context.

Module Focus
09 Android Fundamentals Jetpack Compose, Navigation, Activities, lifecycle
10 Android Advanced ViewModel, StateFlow, Room, Retrofit, Hilt, Material3

Milestone: Build and install a working Android app on a real device or emulator that fetches data from the REST API built in Phase 2.

Phase 4: Expert and Cross-Platform

Modules: 11–12
Duration: ~20 hours
Goal: Master testing, and synthesize all skills in a full-stack capstone project.

Phase 4 closes the loop: comprehensive testing (Module 11) followed by the Capstone (Module 12), where everything comes together: a Spring Boot API, an Android client, and a shared Kotlin Multiplatform library.

Module Focus
11 Testing in Kotlin JUnit5, Mockk, Spring slices, TestContainers, Espresso
12 Capstone Project Full-stack project: Spring Boot API + Android app + KMP shared library

Milestone: A working, tested, documented, full-stack Kotlin project on GitHub.

How This Topic Fits Broader Learning

This topic assumes basic programming knowledge and optional Java exposure. It connects to:

  • [[postgresql]] — used in Module 05 for real database persistence; deep understanding of PostgreSQL improves your Spring Data work
  • [[graphql-rest]] — REST design principles used across Modules 04–07; GraphQL is an optional extension of your API design skills
  • [[qa-testing]] — general testing philosophy that Module 11 applies concretely to Kotlin
  • [[go]] — after mastering Kotlin, Go is a natural complement for learning a simpler compiled language used in infrastructure

After completing this topic, natural progressions are:

  • DevOps / Platform Engineering — deploy your Spring Boot apps with Docker, Kubernetes, and CI/CD
  • Microservices Architecture — extend Module 07's Kafka integration into a full event-driven architecture
  • Kotlin Multiplatform (advanced) — extend Module 12's KMP foundation into iOS and desktop targets