Welcome to The Main Thread — your strategic companion for navigating the evolving world of enterprise Java, software architecture, and AI-infused systems.
- Build fast, cloud-native apps with Quarkus
- Secure microservices with Vault, TOTP, and secrets management
- Integrate LangChain4j and AI services into Java applications
- Design resilient, scalable systems using Kubernetes and Dev Services
- Understand modern patterns like RAG (Retrieval-Augmented Generation), validation layers, and service guardrails
All content is hands-on, enterprise-relevant, and written for people building real systems—not prototypes.
Each folder in this repository matches an article from the Substack and includes:
- ✅ Complete project setups
- 💡 Example code with comments
- ⚙️ Quarkus configurations and Dev Services integration
- 📦 Maven projects you can run locally with Podman, Ollama, PostgreSQL, etc.
| Project | Java | Quarkus | Tests | Links | Build Status |
|---|---|---|---|---|---|
| age-everywhere | - | - | - | Article |  |
| ai-design-critic | - | - | - | Article |  |
| ai-detector | - | - | - | Article |  |
| ai-document-assistant | 21 | 3.29.4 | - | Article |  |
| ai-dungeon-master | - | - | - | Article |  |
| ai-email-simulator | - | - | - | Article |  |
| ai-error-handler-pii | - | - | - | Article |  |
| ai-memory | - | - | - | Article |  |
| ai-progress-tracker | 21 | 3.28.2 | - | Article |  |
| api-adapter-demo | 21 | 3.30.1 | ✅ 4/4 | Article |  |
| archunit-bce | - | - | - | Article |  |
| banner-studio | 21 | 3.29.4 | - | Article |  |
| blog-search | 21 | 3.30.1 | - | Article |  |
| carbon-quarkus | 21 | 3.30.1 | - | Article |  |
| christmas-card | 21 | 3.30.1 | - | Article |  |
| claims-intake | 21 | 3.30.5 | - | Article |  |
| countdown-orchestrator | 21 | 3.30.2 | - | Article |  |
| data-oriented-quarkus | 21 | 3.30.3 | ✅ 1/1 | Article |  |
| enterprise-rag-secure | 21 | 3.30.3 | - | Article |  |
| enterprise-rag | - | - | - | Article |  |
| event-sourcing-quarkus | 21 | 3.30.2 | ✅ 2/2 | Article |  |
| feature-key-service | 21 | 3.30.5 | - | Article |  |
| fleet-insure | 21 | 3.29.4 | - | Article |  |
| food-delivery | 21 | 3.30.2 | - | Article |  |
| htmx-tasks | 21 | 3.30.4 | - | Article |  |
| jigsaw-puzzle | 21 | 3.30.4 | - | Article |  |
| lean-dashboard | 21 | 3.30.5 | - | Article |  |
| metadata-stripper | 21 | 3.29.4 | ✅ 4/4 | Article |  |
| mtls-client-cert-guard | 21 | 3.30.5 | - | Article |  |
| multi-tenant-api | 21 | 3.30.1 | - | Article |  |
| nfl-game-predictor | 21 | 3.30.1 | ✅ 2/2 | Article |  |
| ooo-generator | 21 | 3.30.3 | - | Article |  |
| order-workflows | 21 | 3.30.4 | - | Article |  |
| pixel-art-converter | 21 | 3.30.3 | ✅ 1/1 | Article |  |
| polymorphic-json | 21 | 3.29.0 | ✅ 2/2 | Article |  |
| product-catalog | 21 | 3.30.5 | ✅ 1/1 | Article |  |
| qr-designer | 21 | 3.30.2 | - | Article |  |
| quarkus-actuator | 21 | - | - | Article |  |
| quarkus-jqassistant-demo | 21 | 3.30.3 | ✅ 12/12 | Article |  |
| quarkus-protobuf-demo | 21 | 3.30.1 | ✅ 3/3 | Article |  |
| recursive-comments | 21 | 3.29.4 | ✅ 6/6 | Article |  |
| request-filter-demo | 21 | 3.30.3 | ✅ 1/1 | Article |  |
| resource-reader | 17 | 3.22.1 | - | Article |  |
| santa-cli | 21 | 3.30.1 | - | Article |  |
| secret-santa | 21 | 3.30.2 | ✅ 5/5 | Article |  |
| shapeshifter-api | 21 | 3.30.1 | ✅ 4/4 | Article |  |
| stateless-api | 21 | 3.29.4 | - | Article |  |
| tech-store | 21 | 3.30.3 | ✅ 1/1 | Article |  |
| tflite-agent-macos | 25 | 3.30.3 | - | Article |  |
| timezone-storage-demo | 21 | 3.29.4 | - | Article |  |
| toml-ai-config | 21 | 3.30.3 | - | Article |  |
| url-shortener | 21 | 3.23.3 | ✅ 3/3 | Article |  |
| vaadin-task-manager | 21 | 3.30.1 | - | Article |  |
| validator-demo | 21 | 3.29.4 | ✅ 35/35 | Article |  |
| virtual-threads-demo | 21 | 3.30.2 | - | Article |  |
| voice-transcriber | 25 | 3.30.3 | - | Article |  |
| wishlist-app | 21 | 3.30.1 | - | Article |  |
| zombie-registry | 21 | 3.29.4 | - | Article |  |
Below are the example projects included in this repository, organized by topic. Each project folder is linked for easy access, and where available, a link to the corresponding Substack article is provided:
- ai-document-assistant/: Document Q&A system with LangChain4j, Ollama, and pgvector for RAG capabilities.
- ai-dungeon-master/: Interactive text-based adventure game with AI dungeon master using LangChain4j and Ollama for dynamic storytelling.
- ai-email-simulator/: Automating email generation and task extraction with LangChain4j and Ollama.
Substack article - ai-error-handler-pii/: AI-powered exception handling, PII redaction, and user-friendly error messages.
Substack article - ai-memory/: AI memory management and conversation context handling with LangChain4j.
- ai-whitepaper-generator/: AI-powered white paper generator using LangChain4j, Ollama, and iText for PDF generation.
Substack article - captains-log-generator/: Star Trek-style captain's log generator using AI with stardate calculation and mood-based entries.
- credit-card-validator/: AI-powered credit card validation using Luhn algorithm, brand detection, and LangChain4j chat interface.
- dad-joke-generator/: AI-powered dad joke generator with LangChain4j and Ollama for guaranteed groans.
- data-agent/: AI agent for CSV data analysis using tool-calling and local LLMs.
Substack article - dns-ai/: AI-powered DNS lookup and analysis tool using LangChain4j with DNS record querying capabilities.
- embedding-tutorial/: Text similarity and embeddings with LangChain4j and Ollama.
Substack article - expense-splitter/: AI-powered expense splitting with receipt image processing using LangChain4j and Ollama.
- flight-tracker-ollama/: AI-powered flight tracking with real-time aviation data and natural language queries.
- gospel-in-code/: Bible analysis application with semantic comparison, sentiment analysis, and vector embeddings.
- intelligent-ticketing/: AI-powered customer support system with ticket classification, knowledge base search, and automated processing using embeddings.
- jvm-inspector-ai/: AI-powered JVM monitoring tool with thread analysis, deadlock detection, and intelligent diagnostics using LangChain4j.
- llm-filter-demo/: Content moderation system using Bloom filters for fast pre-filtering and LLM for final content safety analysis.
- llm-observability/: LLM monitoring and observability tools for tracking AI model performance and usage.
- quarkus-chain-of-thought/: AI reasoning application implementing structured chain-of-thought problem solving.
- quarkus-chatbot/: Real-time AI chatbot with LangChain4j and WebSocket.
Substack article - quarkus-image-describer/: Describes images with a LangChain4j service. Substack article
- quarkus-langchain4j-observability/: Observability and monitoring for LangChain4j applications in Quarkus.
- quarkus-meme-generator/: AI-powered meme generator using LangChain4j and image processing.
- quarkus-supervisor-ai/: AI supervisor system with multiple specialized agents using Kafka messaging for distributed AI workflow orchestration.
- semantic-llm-router/: Intelligent LLM request routing based on semantic analysis and model capabilities.
- sentiment-analysis/: REST API for sentiment analysis with LangChain4j and Ollama.
Substack article - structured-ollama-tutorial/: Structured data extraction with Ollama and LangChain4j.
Substack article - summarization-agent/: AI-powered text summarization agent with document processing capabilities.
- text-analytics/: Character interaction graph analysis for Huckleberry Finn using Quarkus, OpenNLP, and Ollama for sentiment analysis.
Substack article - grumbles-live/: Humorous AI news anchor built with LangChain4j. Substack article
- happy-place-app/: Sentiment-based content generator using local LLMs. Substack article
- openapi-ollama-doc-generator/: Generate docs from OpenAPI using Ollama. Substack article
- ai-design-critic/: AI-powered design critique and feedback system. Substack article
- collaborative-editor/: Real-time collaborative text editor using CRDT (Conflict-free Replicated Data Types) with WebSocket synchronization and AI writing assistance.
- csv-uploader/: Dynamic CSV ingestion and schema creation in PostgreSQL.
Substack article - file-upload-progress/: Chunked file uploads with real-time progress tracking using Server-Sent Events (SSE).
- panache-primevue-tutorial/: Beautiful PrimeVue table backed by Quarkus and Panache with filterable, sortable Vue 3 frontend.
Substack article - quarkus-docling-converter/: Document conversion service using Docling library to convert various formats to text.
- reactive-db-app/: Reactive database operations and streaming with Quarkus and reactive programming patterns.
- reactive-streaming-example/: Reactive streams implementation with backpressure handling and event processing.
- resource-reader/: File resource handling, stream processing, and error handling.
Substack article - temporary-file-sharing/: Secure temporary file sharing service with download limits and automatic cleanup using MinIO object storage.
- qr-code-demo/: QR code generation, image processing, and REST endpoints.
Substack article - hibernate-search-orm-elasticsearch-quickstart/: Hibernate Search quickstart with Elasticsearch. Substack article
- quarkus-hibernate-filters/: Query filtering using Hibernate ORM. Substack article
- color-palette-extractor/: Color palette extraction from images.
- pdf-processing-pipeline/: PDF processing and document analysis pipeline.
- substack-discovery/: Substack content discovery and analysis tool.
- api-key-manager/: Secret Agent themed API key management system with secure key generation, rotation, and usage tracking.
- custom-error-pages-classic/: Custom error pages and JSON error responses with Qute and ExceptionMappers.
Substack article - jwt-case/: JWT-based authentication and authorization.
Substack article - secure-diary/: Secure diary application with Bouncy Castle encryption, protecting thoughts one encrypted entry at a time.
Substack article - security-jpa-quickstart/: JPA-based user authentication and PostgreSQL integration.
Substack article - security-jpa-quickstart2/: Alternative JPA-based authentication example.
- self-learning-waf/: Self-learning Web Application Firewall using DeepLearning4j for reactive Java security without static rules.
Substack article - totp-vault/: TOTP and Vault-based secrets management.
Substack article - zero-trust-uploads/: Zero-trust file uploads with ClamAV antivirus scanning using secure in-memory pipeline before storage.
Substack article - quarkflix-guards/: Hands on CDI Interceptors tutorial Substack article
- quarkus-response-tutorial/: HTTP Response handling Substack article
- quarkus-otp/: TOTP (Time-based One-Time Password) implementation with Quarkus. Substack article
- secure-service/: Secure service implementation with authentication and authorization.
- fraud-detector/: Fraud detection system with machine learning capabilities.
- breadcrumb-navigation/: Dynamic breadcrumb navigation component in Quarkus with zero-configuration auto-generation from URI paths.
Substack article - composite-key-app/: Demonstrates composite keys with Hibernate Panache and student enrollment modeling.
- flyway-adventure/: Database migrations with Flyway.
Substack article - hll-tutorial/: Scalable unique counting with HyperLogLog algorithm for counting millions of things with kilobytes of memory.
Substack article - http-header-tutorial/: HTTP header handling tutorial with request/response manipulation and GZIP compression.
- quarkus-dto-tutorial/: DTO mapping and transformation.
Substack article - quarkus-graphql/: Building GraphQL APIs in Quarkus.
Substack article - quarkus-interceptor-tutorial/: Custom request interceptors, logging, and auditing.
Substack article - quarkus-timezone-tutorial/: Timezone handling best practices with Quarkus and Hibernate Panache for proper date management.
Substack article - validation-example/: Form validation with Jakarta Validation and Qute.
Substack article - i18n-demo/: Database-backed internationalization.
Substack article - chirper/: Simple microblogging demo. Substack article
- credit-line-app/: Workflow example managing credit line approvals. Substack article
- greeting-operator/: Kubernetes operator for greeting resources. Substack article
- mission-control/: Mission planning service with async tasks. Substack article
- multi-tenant-todo-app/: Multi-tenant TODO API. Substack article
- quarkus-panache-transaction-tutorial/: Transaction patterns with Hibernate Panache. Substack article
- dynamic-agent-spawner/: Dynamic agent spawning example. Substack article
- dynamic-renderer/: Dynamic rendering service. Substack article
- url-shortener/: URL shortening service. Substack article
- wizard-app/: Wizard-style application UI. Substack article
- a2a-demo/: Agent-to-Agent communication demonstration.
- age-everywhere/: Age calculation and date handling service. Substack article
- approval-workflow/: Business process management and approval workflows.
- archunit-bce/: Architecture testing with ArchUnit for business capability enforcement. Substack article
- audit-app/: Application auditing and monitoring system. Substack article
- banner-studio/: ASCII banner generation and styling tool. Substack article
- car-filter-app/: Car filtering and search application.
- cdi-events-demo/: CDI events and observer pattern demonstration.
- coffee-api/: Coffee shop API with OpenAPI documentation. Substack article
- community-app/: Community management application. Substack article
- fleet-insure/: Fleet insurance management system.
- granular-rbac-tutorial/: Granular Role-Based Access Control implementation.
- greet-hypermedia/: Hypermedia API implementation with HATEOAS. Substack article
- meme-service/: Meme generation and management service. Substack article
- museum/: Museum management and exhibition system.
- negotiation-simulator/: Business negotiation simulation system.
- order-service/: Order processing and management service.
- poll-doodle/: Polling and scheduling application. Substack article
- property-testing/: Property-based testing with Jqwik.
- qualifiers-lab/: CDI qualifiers and dependency injection lab.
- quantum-identicons/: Quantum-inspired identicon generation.
- quarkus-agentic/: Agentic AI system implementation.
- quarkus-ai-debugger/: AI-powered debugging assistant.
- quarkus-api-testing/: API testing framework and examples. Substack article
- quarkus-coverage-tutorial/: Code coverage analysis and reporting.
- quarkus-guardrails/: AI guardrails and safety measures.
- quarkus-ldap-demo/: LDAP authentication integration. Substack article
- quarkus-mcp/: Model Context Protocol implementation. Substack article
- quarkus-mutation-demo/: Mutation testing demonstration.
- quarkus-read-replica/: Database read replica configuration.
- quarkus-roq-tutorial/: Roq framework tutorial and examples.
- quarkus-soap-multi/: Multi-protocol SOAP service implementation.
- quarkus-todo-app/: Todo application with CRUD operations.
- quarkus-versioning-tutorial/: API versioning strategies and implementation.
- quarky-cart/: Shopping cart implementation.
- renarde-tasks/: Task management with Renarde framework. Substack article
- resilient-ai/: AI resilience and fault tolerance patterns. Substack article
- rest-deprecation-practice/: REST API deprecation strategies.
- rest-testing-demo/: REST API testing with RestAssured and Pact. Substack article
- roq-author-website/: Author website built with Roq framework.
- unicode-greetings/: Unicode and internationalization examples.
- very-simple-app/: Minimal Quarkus application example. Substack article
- xa-transaction-tutorial/: XA transaction management tutorial.
- year-end-invoices/: Year-end invoice processing system.
- error-handling-app/: RFC 7807 standardized error handling in Quarkus for secure, standards-based API error responses.
Substack article - geonames-search/: High-performance autocomplete search with Quarkus, Panache, and PostgreSQL for geographic locations.
Substack article - health-checker/: Website health monitoring service using MicroProfile Health, YAML configuration, and HttpClient for uptime checking.
Substack article - household-chore-assignment/: Fair household chore assignment using the Gale-Shapley algorithm for stable matching.
Substack article - quarkus-sftp-compose/: Smart SFTP file upload system with PostgreSQL metadata storage using Quarkus Dev Services.
Substack article - realtime-monitor/: JVM heap memory monitoring and dashboard UI.
Substack article - quote-cli/: CLI tool for quotes or text processing.
Substack article - cloud-metrics/: Tenant-aware metrics and rate limiting demo.
- matrix-test/: Simple Quarkus demo inspired by The Matrix.
- quarkus-coverage-tutorial/: Jacoco code coverage setup.
- quarkus-otel-adventure/: OpenTelemetry observability and distributed tracing implementation with Quarkus microservices.
- quarkus-task-runner/: Background task execution and scheduling with Quarkus and job processing patterns.
- quarkus-unleash-tutorial/: Feature flags and toggles implementation using Unleash with Quarkus for controlled feature rollouts.
- api-perf-resilience/: API performance and resilience patterns. Substack article
- bsky-javafeed-generator/: Bluesky Java feed generator.
- bsky-javafeed-generator-reactive/: Reactive Bluesky Java feed generator.
- caching-tutorial/: Caching strategies and implementation.
- camel-rest-sql/: Apache Camel REST SQL integration.
- camel-rss-quarkus/: Apache Camel RSS integration with Quarkus.
- log-to-obs/: Logging to observability platforms.
- mutiny-test/: Mutiny reactive programming testing.
This repo will grow alongside the publication. Have an idea for a new tutorial or example you'd like to see covered? 👉 Open an Issue and suggest it!
🤝 Contributing
Contributions are welcome! If you'd like to:
- Fix a bug or improve code → fork the repo and open a Pull Request
- Share feedback or ideas → join the discussion in Issues
- Comment on articles or reach out to me directly!
Every contribution, big or small, helps improve the learning experience for everyone following The Main Thread.
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- 🔗 LinkedIn: Markus Eisele
- 📰 Substack: Enterprise Java and Quarkus
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