{"id":443,"date":"2025-03-25T18:48:04","date_gmt":"2025-03-25T18:48:04","guid":{"rendered":"https:\/\/imcodinggenius.com\/?p=443"},"modified":"2025-03-25T18:48:04","modified_gmt":"2025-03-25T18:48:04","slug":"auto-reload-in-ktor","status":"publish","type":"post","link":"https:\/\/imcodinggenius.com\/?p=443","title":{"rendered":"Auto-reload in Ktor"},"content":{"rendered":"

Together, we will learn how to enable the Ktor development mode<\/strong> in several ways and how auto-reload<\/strong> can help with the Kotlin HTML DSL app<\/strong> example. <\/p>\n

Quick Ktor Setup<\/h2>\n

As the first step, let\u2019s quickly prepare a basic Ktor application responding with HTML.<\/p>\n

To do so, let\u2019s navigate to the https:\/\/start.ktor.io<\/a> page and configure the artifact first:<\/p>\n

The choice is up to you here. From my end, I will be using the latest Ktor version with the YAML config and without the version catalog. <\/p>\n

Then, let\u2019s add the Kotlin HTML DSL plugin necessary for our simple example:<\/p>\n

Lastly, let\u2019s download the zip package, extract it, and import it to our IDE. <\/p>\n

If you are working in IntelliJ IDEA, then please make sure that you use JDK 23, too. <\/p>\n

You can do that in two places: <\/p>\n

Settings -> Build, Execution, Deployment -> Build Tools -> Gradle. Here, please set the Gradle JVM to 23<\/p>\n

Project Structure -> Project Settings -> Project. On this page, please set the SDK to 23 and Language level to \u201cSDK default\u201d<\/p>\n

After all, please sync the gradle project and wait for the process to finish.<\/p>\n

Expose Endpoint Returning HTML<\/h2>\n

With all of that done, let\u2019s delete the unwanted files that were generated automatically, like Routing.kt, or Templating.kt and expose the endpoint:<\/p>\n

fun Application.module() {<\/p>\n

routing {
\n get(«\/») {
\n call.respondHtml {
\n body {
\n + «Hello world!»
\n }
\n }
\n }
\n }<\/p>\n

}<\/p>\n

As we can see, whenever we reach the root path (by default, the localhost:8080), we should see the \u201cHello world!\u201d text.<\/p>\n

The Problem<\/h2>\n

Now, let\u2019s change the text, for example, to contain only a \u201cHello!\u201d message. <\/p>\n

What happens if we refresh the page? Nothing!<\/strong> The page does not reflect our code change until we restart the server manually! <\/p>\n

And, especially when dealing with HTML, it may become a pretty tedious task.<\/p>\n

Solution- Ktor auto-reload<\/h2>\n

It won\u2019t be a big surprise if I say that the Ktor auto-reload<\/strong> feature may be a great solution here? <\/p>\n

But what is that?<\/p>\n

Well, long story short, it its a feature that we can turn on by enabling the development mode<\/strong> in Ktor. And when enabled, it reloads application classes on code changes<\/strong>. <\/p>\n

To be more specific, according to the documentation, Ktor listens for the changes to the following files:<\/p>\n

\/build\/classes\/kotlin\/main\/META-INF
\n\/build\/classes\/kotlin\/main\/com\/your-package-namn
\n\/build\/classes\/kotlin\/main\/com
\n\/build\/classes\/kotlin\/main
\n\/build\/resources\/main<\/p>\n

And as we can see, by the code changes<\/strong>, we mean the generated outputs and artifacts.<\/strong> But the good news is that with gradle, we can easily automate the generation, too.<\/p>\n

Lastly, I just wanted to mention that application logs are also pretty clear about what we need to do in order to enable auto-reload: <\/p>\n

2025-03-25 06:40:49.929 [main] INFO Application — Autoreload is disabled because the development mode is off.
\n2025-03-25 06:40:50.072 [main] INFO Application — Application started in 0.374 seconds.
\n2025-03-25 06:40:50.363 [DefaultDispatcher-worker-2] INFO Application — Responding at http:\/\/127.0.0.1:8080<\/p>\n

Enable development mode<\/h2>\n

With all of that said, let\u2019s get back to the practice part. <\/p>\n

As the first step, we must enable the development mode. And we can do that in a few ways, so let me walk you through them.<\/p>\n

Hardcoded Approach<\/h2>\n

When we navigate to the build.gradle.kts file, we should see the following: <\/p>\n

application {
\n mainClass = «io.ktor.server.netty.EngineMain»<\/p>\n

val isDevelopment: Boolean = project.ext.has(«development»)
\n applicationDefaultJvmArgs = listOf(«-Dio.ktor.development=$isDevelopment»)
\n}<\/p>\n

So the easy and the dummy approach would be to simply hardcode the value- -Dio.ktor.development=true: <\/p>\n

application {
\n mainClass = «io.ktor.server.netty.EngineMain»
\n applicationDefaultJvmArgs = listOf(«-Dio.ktor.development=true»)
\n}<\/p>\n

Now, when we run the app with .\/gradlew run, we will see that the message disappeared, proving the development mode is enabled:<\/p>\n

2025-03-25 07:10:15.883 [main] INFO Application — Application started in 0.41 seconds.
\n2025-03-25 07:10:16.142 [main] INFO Application — Responding at http:\/\/127.0.0.1:8080
\n<==========—> 83% EXECUTING [1m 13s]<\/p>\n

But first, this approach does not work if you run the server in IntelliJ by using the green icon because, by default, it does not invoke any gradle command.<\/p>\n

Secondly, it is hardcoded, meaning we don\u2019t have too much control over it in various environments. <\/p>\n

Program Argument<\/h2>\n

Another option is to bring back what we already had in the build.gradle.kts: <\/p>\n

val isDevelopment: Boolean = project.ext.has(«development»)
\napplicationDefaultJvmArgs = listOf(«-Dio.ktor.development=$isDevelopment»)<\/p>\n

And from now on, the only thing we need is the -Pdevelopment flag for the gradlew run command:<\/p>\n

.\/gradlew run -Pdevelopment<\/p>\n

It\u2019s clearly better and gives us more flexibility. <\/p>\n

VM options<\/h2>\n

Another option that we have on the table is to directly use the VM option we saw above- -Dio.ktor.development=true. <\/p>\n

For that purpose, we can even completely get rid of the previous snippet from application in build.gradle.kts: <\/p>\n

application {
\n mainClass = «io.ktor.server.netty.EngineMain»
\n}<\/p>\n

And now, whenever invoking the gradlew run command, we will simply put the whole arg: <\/p>\n

.\/gradlew «-Dio.ktor.development=true» run<\/p>\n

Moreover<\/strong>, when working with IntelliJ, we can edit our configuration to pass the same option: <\/p>\n

So we can see that this option will work with both gradle, as well as the IntelliJ IDEA runs.<\/strong> <\/p>\n

Config File<\/h2>\n

Lastly, we can also set that in the application.yaml:<\/p>\n

ktor:
\n development: true
\n application:
\n modules:
\n — com.codersee.ApplicationKt.module
\n deployment:
\n port: 8080<\/p>\n

It works exactly the same, and with environment variables can give us a lot of flexibility, too. <\/p>\n

Automate Build<\/h2>\n

Alright, so at this point, our app is up and running<\/strong> with the development mode ON.<\/strong> <\/p>\n

However, when we make any code change, nothing still happens!<\/strong> <\/p>\n

Well, as I mentioned previously, the Ktor auto-reload feature looks for the output files. Meaning we must build the project to see the changes. <\/p>\n

And we can do that by either running the .\/gradlew build or by navigating to the Build tab in IntelliJ: <\/p>\n

And although this works and we can now see changes without the restart, it is still not too convenient. <\/p>\n

Well, the good news we can slightly adjust the gradlew build command and automate that:<\/strong><\/p>\n

.\/gradlew -t build -x test<\/p>\n

With the -t flag (or —continuous), we enable the continuous build mode.<\/strong> Gradle will watch for changes in the source files, and whenever a relevant change is detected, it will automatically rerun the build task. <\/p>\n

Additionally, we exclude the test task using the -x flag to speed up the process a little <\/p>\n

If we would like to add that as a run configuration to IntelliJ, then it is really easy, too: <\/p>\n

And basically, that\u2019s all! At this point, we can add the necessary HTML changes without restarting the Ktor server.<\/strong> <\/p>\n

Narrowing Down Watched Files<\/h2>\n

As I mentioned above, Ktor listens for the file updates in the following directories: <\/p>\n

\/build\/classes\/kotlin\/main\/META-INF
\n\/build\/classes\/kotlin\/main\/com\/your-package-namn
\n\/build\/classes\/kotlin\/main\/com
\n\/build\/classes\/kotlin\/main
\n\/build\/resources\/main<\/p>\n

If we would like to narrow down the list of watched directories, then we can do that, for example, in the YAML config file:<\/p>\n

ktor:
\n deployment:
\n watch:
\n — resources<\/p>\n

The only thing we must specify is the part of the watched patch that we want to keep. <\/p>\n

Summary<\/h2>\n

And that\u2019s all for this quick tutorial on how to enable the auto-reload feature in Ktor.<\/strong><\/p>\n

If you found this content useful, then check out my Ktor Server Pro course<\/a>\u2013 the most comprehensive Ktor course on the market. <\/p>\n

Thank you for being here, and see you around! <\/p>\n

The post Auto-reload in Ktor<\/a> appeared first on Codersee — Kotlin on the backend<\/a>.<\/p>","protected":false},"excerpt":{"rendered":"

Together, we will learn how to enable the Ktor development mode in several ways and how auto-reload can help with the Kotlin HTML DSL app example. Quick Ktor Setup As the first step, let\u2019s quickly prepare a basic Ktor application responding with HTML. To do so, let\u2019s navigate to the … <\/p>\n

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