Spring Boot Kotlin Playground

A collection of standalone Spring Boot examples written in Kotlin, demonstrating common backend development patterns, reactive programming, data access, messaging, security, external integrations, containerization, and Kubernetes deployment.

Project Overview

This repository is a playground for exploring Spring Boot development with Kotlin.

It is not a single application or a multi-module project. Each directory contains an independent Spring Boot project focused on a specific technology, configuration, or implementation pattern.

The examples cover topics such as:

• Kotlin Coroutines
• Spring WebFlux and reactive programming
• R2DBC and PostgreSQL
• Redis
• Apache Kafka and Spring Cloud Stream
• Elasticsearch
• Spring Cloud Gateway
• WebClient
• Reactive Security and JWT
• TLS and custom truststores
• SFTP integration
• Docker and multi-stage builds
• Kubernetes resources
• Logging and MDC
• Request and response processing

The projects are intended for learning, experimentation, troubleshooting, and use as references when implementing similar features in other Spring Boot applications.

Repository Structure

SpringBoot-Kotlin-Playground
├── SpringBoot-Cloud-*
├── SpringBoot-Coroutines-*
├── SpringBoot-Docker-*
├── SpringBoot-Elasticsearch-*
├── SpringBoot-K8s-*
├── SpringBoot-R2DBC-*
├── SpringBoot-Reactive-*
├── SpringBoot-SFTP-*
├── SpringBoot-WebClient-*
├── SpringBoot-WebFilter-*
└── ...

Each project generally contains its own:

project-name
├── src
├── build.gradle.kts
├── settings.gradle.kts
├── gradlew
├── gradlew.bat
├── README.md
└── docker-compose.yaml

Some projects may also include Dockerfiles, Kubernetes manifests, certificates, database scripts, or other supporting resources.

Examples

Spring Cloud Gateway

Project	Description
SpringBoot-Cloud-Gateway	Basic Spring Cloud Gateway configuration and request routing.
SpringBoot-Cloud-Gateway-AddHeaders	Adding or modifying HTTP headers through Spring Cloud Gateway.

Kafka and Spring Cloud Stream

Project	Description
SpringBoot-Cloud-Kafka-Delay	Implementing delayed message processing with Kafka.
SpringBoot-Cloud-Kafka-Header	Producing and consuming Kafka messages with custom headers.
SpringBoot-Cloud-Kafka-Multiple-Broker	Configuring an application to communicate with multiple Kafka brokers.
SpringBoot-Cloud-Kafka-Stream	Producing and consuming messages with Spring Cloud Stream and Kafka.

Kotlin Coroutines and Redis

Project	Description
SpringBoot-Coroutines-Functional-Endpoint	Building functional WebFlux endpoints with Kotlin Coroutines.
SpringBoot-Coroutines-Redis-Data	Accessing Redis using Kotlin Coroutines and reactive Redis support.

Docker

Project	Description
SpringBoot-Dockerfile	Containerizing a Spring Boot application with a Dockerfile.
SpringBoot-Docker-MultiStage	Building a smaller application image with a multi-stage Docker build.

Elasticsearch

Project	Description
SpringBoot-Elasticsearch-CoroutineRepository	Accessing Elasticsearch through coroutine-based repositories.
SpringBoot-Elasticsearch-ReactiveRepository	Working with Spring Data reactive Elasticsearch repositories.
SpringBoot-Elasticsearch-ReactiveTemplate	Querying Elasticsearch through ReactiveElasticsearchTemplate.

JasperReports

Project	Description
SpringBoot-Jasper-HelloWorld	A basic JasperReports integration example.

Kubernetes

Project	Description
SpringBoot-K8s-Deployment	Deploying a Spring Boot application with a Kubernetes Deployment.
SpringBoot-K8s-Env-Variable	Supplying application configuration through environment variables.
SpringBoot-K8s-Configmap-Env-Variable	Exposing ConfigMap values as container environment variables.
SpringBoot-K8s-Configmap-Mount-File	Mounting a Kubernetes ConfigMap as files inside a container.
SpringBoot-K8s-Secret-Variable	Exposing Kubernetes Secret values as environment variables.
SpringBoot-K8s-Secret-Mount-File	Mounting Kubernetes Secrets as files inside a container.
SpringBoot-K8s-Ingress	Exposing a Spring Boot application through Kubernetes Ingress.
SpringBoot-K8s-Volume-PVC	Mounting persistent storage using a PersistentVolumeClaim.

Configuration and Logging

Project	Description
SpringBoot-Property-Binding	Binding application properties to Kotlin configuration classes.
SpringBoot-Logging-Level	Configuring application and package-specific logging levels.
SpringBoot-MDC-Logging	Adding contextual information to logs with MDC.
SpringBoot-MDC-Logstash	Producing structured logs for Logstash with MDC values.
SpringBoot-Modify-Request	Reading or modifying incoming HTTP request data.

R2DBC

Project	Description
SpringBoot-R2DBC-CoroutineRepository	Accessing a relational database through coroutine-based R2DBC repositories.
SpringBoot-R2DBC-R2dbcTemplate	Performing reactive database operations with R2dbcEntityTemplate.

Reactive Web, Security, and TLS

Project	Description
SpringBoot-Reactive-ExceptionHandler-V1	Handling exceptions in a Spring WebFlux application.
SpringBoot-Reactive-ExceptionHandler-V2	An alternative global exception-handling implementation for WebFlux.
SpringBoot-Reactive-Logging-Request-Response	Logging reactive HTTP request and response information.
SpringBoot-ReactiveSecurity-JWT	Securing reactive endpoints with Spring Security and JWT.
SpringBoot-Reactive-SSL	Configuring HTTPS for a reactive Spring Boot application.
SpringBoot-Reactive-SSL-TrustStore-Client	Configuring a custom client truststore for reactive HTTP requests.
SpringBoot-Reactive-SSL-TrustStore-Global	Applying custom truststore configuration globally.

SFTP Integration

Project	Description
SpringBoot-SFTP-CachedSession	Reusing and caching SFTP sessions.
SpringBoot-SFTP-SharedSession	Sharing an SFTP session across integration operations.
SpringBoot-SFTP-InboundChannel	Receiving or downloading files through an SFTP inbound channel adapter.
SpringBoot-SFTP-InboundChannel-DSL	Configuring an SFTP inbound flow with the Spring Integration DSL.

WebClient

Project	Description
SpringBoot-WebClient	Calling external HTTP APIs with Spring WebClient.
SpringBoot-WebClient-Customize	Customizing WebClient and its underlying HTTP client.
SpringBoot-WebClient-Filter	Intercepting WebClient requests and responses with exchange filters.
SpringBoot-WebClient-ConnectionPool	Configuring Reactor Netty connection pooling for WebClient.

WebFilter

Project	Description
SpringBoot-WebFilter	Intercepting reactive HTTP requests with Spring WebFilter.
SpringBoot-WebFilter-Order	Controlling the execution order of multiple WebFilters.

Utilities

Project	Description
SpringBoot-ZipFile-Password	Creating or extracting password-protected ZIP files.

Getting Started

Clone the repository:

git clone https://github.com/tirmizee/SpringBoot-Kotlin-Playground.git
cd SpringBoot-Kotlin-Playground

Select the example you want to explore:

cd SpringBoot-WebClient

Run the application with the included Gradle Wrapper:

./gradlew bootRun

On Windows:

gradlew.bat bootRun

To build the project without starting it:

./gradlew clean build

Running Supporting Services

Some examples require supporting services such as PostgreSQL, Redis, Kafka, Elasticsearch, SFTP, or a mock HTTP server.

When a project includes a docker-compose.yaml or docker-compose.yml file, start its dependencies with:

docker compose up -d

Then run the Spring Boot application:

./gradlew bootRun

Stop the supporting services with:

docker compose down

Always review the selected project's README.md, application.yaml, and Docker Compose configuration before running it.

Requirements

The exact requirements may vary between projects because the examples were created independently and may use different Spring Boot, Kotlin, Java, and dependency versions.

Commonly required tools include:

• JDK
• Docker and Docker Compose
• Git
• IntelliJ IDEA or another Kotlin-compatible IDE
• A local Kubernetes environment for Kubernetes examples

Check the following files inside each project for the required versions and dependencies:

build.gradle.kts
settings.gradle.kts
application.yaml
docker-compose.yaml
README.md

Important Notes

• Each directory is an independent application.
• Run Gradle commands from inside the selected project directory.
• Configuration values and credentials are intended for local demonstrations only.
• Some projects demonstrate older Spring APIs or dependency versions.
• Review and update dependencies before using an example in a production application.
• Production systems should use secure secret management instead of storing credentials directly in configuration files.
• The examples focus on individual concepts and may intentionally omit production concerns unrelated to the demonstrated topic.

Purpose

This repository can be used as:

• A Kotlin and Spring Boot learning playground
• A reference for implementing specific Spring features
• A collection of small proof-of-concept applications
• A starting point for troubleshooting integrations
• A source of examples for reactive and coroutine-based development

Author

Created by tirmizee.