Servlet containers, clients

Servlet containers

Servlet Container	Blocking HTTP Requests	Non-Blocking HTTP Requests	WebSockets Support	Works Well with Kotlin Coroutines
Jetty	✅ Yes	✅ Yes (via async I/O)	✅ Yes	✅ Yes (with Coroutine-based APIs)
Tomcat	✅ Yes	✅ Yes (via async servlets)	✅ Yes	⚠️ Limited (needs adapters like ktor-server-tomcat)
Undertow	✅ Yes	✅ Yes (reactive streams)	✅ Yes	✅ Yes (used in ktor-server-undertow)
Netty (is not a servlet container—it's an event-driven network application framework that can be used to build non-blocking servers and clients)	⚠️ Not its primary model	✅ Yes (fully event-driven)	✅ Yes	✅ Yes (integrates well with Ktor and coroutines)

Clients

HTTP Client	Blocking Requests	Non-Blocking Requests	Coroutine Friendly	Additional Notes
RestTemplate	✅ Yes	❌ No	⚠️ Limited	Synchronous API, designed for Spring MVC.
WebClient (Reactor Netty (default in Spring Boot))	⚠️ Limited	✅ Yes	✅ Yes	Non-blocking, reactive web client from Spring WebFlux.
Ktor	✅ Yes	✅ Yes	✅ Yes	Designed for Kotlin; supports both blocking and non-blocking calls.
OkHttp	✅ Yes	✅ Yes	⚠️ Limited	Primarily synchronous but supports asynchronous requests; good for Kotlin coroutines.
Apache HttpClient	✅ Yes	⚠️ Limited	⚠️ Limited	Synchronous by default; can use Future for async.
Retrofit	✅ Yes	✅ Yes	✅ Yes	Works with OkHttp, designed for REST APIs; supports coroutines via extensions.
Vert.x Web Client	❌ No	✅ Yes	✅ Yes	Fully non-blocking and designed for asynchronous programming.

Spring’s WebClient is a non-blocking HTTP client, but it can use different underlying engines for handling HTTP requests.

What determines WebClient's underlying engine?

It depends on which Spring Boot starter you include:

1. If using spring-boot-starter-webflux (Reactive)

   • WebClient defaults to Netty (via Reactor Netty).

   • Netty handles HTTP connections asynchronously.

2. If using spring-boot-starter-web (Blocking, Tomcat)

   • WebClient uses Apache HttpClient or Java’s HttpURLConnection.

   • It can still be non-blocking at the application level, but the underlying client might not be fully asynchronous.

     • If you’re making a few HTTP requests per second → You can use a blocking client inside coroutines with Dispatchers.IO