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Spring Boot, Spring Cloud and Reactor for C# developers — Mission

Why I’m learning this

Section titled “Why I’m learning this”

The Java course taught me the language. The Java code I actually meet, though, is rarely plain Java: it is a Spring Boot service, often a reactive one, behind a Spring Cloud gateway. Its annotations do the work that Program.cs does explicitly in ASP.NET Core, and its Mono and Flux look like Task and IAsyncEnumerable without behaving like them.

I want to read and write those services with the same confidence as an ASP.NET Core application: know where each bean comes from, predict on which thread a pipeline runs, and tell a real design choice from a habit.

Who this course is for

Section titled “Who this course is for”

You are an experienced C# developer. You know ASP.NET Core, its dependency injection, IOptions, IHostedService, health checks, HttpClient, IAsyncEnumerable, and probably Polly and YARP. You have followed Java for C# developers, or you are comfortable with Java 25, Maven and JUnit. This course sends you back to that course’s lessons instead of repeating them: lesson 9 for virtual threads, lesson 10 for Maven, lesson 11 for JUnit, Mockito and BlockHound.

The running example

Section titled “The running example”

The lessons build small services around music theory: a scales service that returns the notes and chords of a mode, a progression stream, a chord chart client. The domain is a plain Java module, music, with no Spring and no Reactor in it. By the end of the course, the pieces fit together like this:

flowchart LR
    client[Browser or HTTP client] --> gateway[Spring Cloud Gateway]
    gateway --> scales["Scales service (WebFlux)"]
    gateway --> progressions["Progressions service (server-sent events)"]
    progressions -->|WebClient| scales
    scales --> db[(R2DBC database)]
    config[Config server] -.-> scales
    config -.-> progressions
    progressions -->|Spring Cloud Stream| broker[[Message broker]]

Lessons 1 to 4 build the first two boxes on the right; the later lessons add the others. Everything runs on localhost, started by the tests, without Docker.

By the end of this course, I will be able to

Section titled “By the end of this course, I will be able to”
  • create a Spring Boot application and explain where each of its beans, properties and endpoints comes from;
  • map ASP.NET Core’s dependency injection, configuration, options and health checks onto Spring’s;
  • write, test and debug Reactor pipelines, and say on which thread each step runs;
  • choose between Spring MVC on virtual threads and WebFlux for a given service;
  • build reactive HTTP APIs and clients with WebFlux and WebClient;
  • put a gateway, centralised configuration, service discovery and resilience in front of those services with Spring Cloud;
  • observe and test a set of Spring services the way I would a set of ASP.NET Core services.

Outline

Section titled “Outline”
# Lesson You already know
1 Spring Boot seen from ASP.NET Core Program.cs, IServiceCollection, appsettings.json, IOptions, health checks
2 Reactor: Mono and Flux Task, IAsyncEnumerable, LINQ, Rx.NET
3 Reactor under the hood the thread pool, ConfigureAwait, channels, Polly, AsyncLocal
4 WebFlux minimal APIs, HttpClient, server-sent events
5 Reactive data access with R2DBC and Spring Data Entity Framework Core, IAsyncEnumerable from a DbContext
6 Spring Cloud Gateway YARP
7 Centralised configuration with Spring Cloud Config configuration providers, Azure App Configuration
8 Service discovery and client-side load balancing .NET Aspire service discovery, IHttpClientFactory
9 Resilience with Resilience4j and Spring Cloud Circuit Breaker Polly, Microsoft.Extensions.Http.Resilience
10 Messaging with Spring Cloud Stream MassTransit, Azure Service Bus clients
11 Observability with Micrometer OpenTelemetry for .NET, dotnet-counters
12 Testing a set of Spring services WebApplicationFactory, integration tests

Journal: what I tried, what surprised me, what I still need to verify.

Resources

Section titled “Resources”