This is a plugin for Javalin to track some basic HTTP request/response metrics. It is written using a mix of Java and Kotlin.

Currently the following metrics are being recorded:

• Request time
• Response size

A unique request ID is also generated for each request, which is set in the Request-Id header of the HTTP response.

The plugin also registers several routes related to metrics:

• /metrics displays a simple Thymeleaf page for viewing aggregated metrics (min, max and avg values for time and size). There is also a form for viewing stats for a single request ID.
• /metrics/:request-id will return the duration and sizeInBytes for an individual request in json format. This is used by the form on the /metrics page.

The plugin can be built as a jar using ./gradlew assemble and added to your project.

Simply register the plugin during creation of your Javalin application:

public class App {
    public static void main(String[] args) {
        Javalin app = Javalin.create()
          .register(new MetricsPlugin())
          .start(7000);
        app.get("/", ctx -> ctx.result("Hello World"));
    }
}

Check out ExampleApp.kt for an example application.

• Path matching in the used version of Javalin (2.8.0) doesn't seem to work well with more complex regular expressions, so I was unable to exclude calls to the /metrics endpoint itself from metrics collection.
• There is no bounding on the map holding the recorded statistics so memory consumption could be an issue for long running applications. It would be advisable to evaluate using a ring buffer for storage in order to automatically age out the oldest entries.
• The metrics page is extremely basic and could use some polish.
• The plugin isn't currently being published to a Maven repo so bringing this into your own project isn't the smoothest experience.