> For the complete documentation index, see [llms.txt](https://amartyushov.gitbook.io/tech/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://amartyushov.gitbook.io/tech/frameworks/spring/servlet-containers-clients.md).

# Servlet containers, clients

## Servlet containers

<table data-full-width="true"><thead><tr><th width="295">Servlet Container</th><th width="138">Blocking HTTP Requests</th><th>Non-Blocking HTTP Requests</th><th width="122">WebSockets Support</th><th>Works Well with Kotlin Coroutines</th></tr></thead><tbody><tr><td><strong>Jetty</strong></td><td>✅ Yes</td><td>✅ Yes (via async I/O)</td><td>✅ Yes</td><td>✅ Yes (with Coroutine-based APIs)</td></tr><tr><td><strong>Tomcat</strong></td><td>✅ Yes</td><td>✅ Yes (via async servlets)</td><td>✅ Yes</td><td>⚠️ Limited (needs adapters like <code>ktor-server-tomcat</code>)</td></tr><tr><td><strong>Undertow</strong></td><td>✅ Yes</td><td>✅ Yes (reactive streams)</td><td>✅ Yes</td><td>✅ Yes (used in <code>ktor-server-undertow</code>)</td></tr><tr><td><strong>Netty (is not a servlet container</strong>—it's an <strong>event-driven network application framework</strong> that can be used to build non-blocking servers and clients<strong>)</strong></td><td>⚠️ Not its primary model</td><td>✅ Yes (fully event-driven)</td><td>✅ Yes</td><td>✅ Yes (integrates well with Ktor and coroutines)</td></tr></tbody></table>

####

## Clients

<table data-full-width="true"><thead><tr><th width="171">HTTP Client</th><th width="148">Blocking Requests</th><th width="141">Non-Blocking Requests</th><th width="170">Coroutine Friendly</th><th>Additional Notes</th></tr></thead><tbody><tr><td><strong>RestTemplate</strong></td><td>✅ Yes</td><td>❌ No</td><td>⚠️ Limited</td><td>Synchronous API, designed for Spring MVC.</td></tr><tr><td><strong>WebClient (</strong>Reactor Netty (default in Spring Boot)<strong>)</strong></td><td>⚠️ Limited</td><td>✅ Yes</td><td>✅ Yes</td><td>Non-blocking, reactive web client from Spring WebFlux.</td></tr><tr><td><strong>Ktor</strong></td><td>✅ Yes</td><td>✅ Yes</td><td>✅ Yes</td><td>Designed for Kotlin; supports both blocking and non-blocking calls.</td></tr><tr><td><strong>OkHttp</strong></td><td>✅ Yes</td><td>✅ Yes</td><td>⚠️ Limited</td><td>Primarily synchronous but supports asynchronous requests; good for Kotlin coroutines.</td></tr><tr><td><strong>Apache HttpClient</strong></td><td>✅ Yes</td><td>⚠️ Limited</td><td>⚠️ Limited</td><td>Synchronous by default; can use <code>Future</code> for async.</td></tr><tr><td><strong>Retrofit</strong></td><td>✅ Yes</td><td>✅ Yes</td><td>✅ Yes</td><td>Works with OkHttp, designed for REST APIs; supports coroutines via extensions.</td></tr><tr><td><strong>Vert.x Web Client</strong></td><td>❌ No</td><td>✅ Yes</td><td>✅ Yes</td><td>Fully non-blocking and designed for asynchronous programming.</td></tr></tbody></table>

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`


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