minimal async servlet example with undertow

main.kt

import io.undertow.Handlers
import io.undertow.Undertow
import io.undertow.servlet.Servlets
import io.undertow.servlet.Servlets.servlet
import java.util.*
import java.util.concurrent.CompletableFuture
import java.util.concurrent.ConcurrentHashMap
import java.util.concurrent.TimeUnit
import java.util.stream.Collectors
import javax.servlet.http.HttpServlet
import javax.servlet.http.HttpServletRequest
import javax.servlet.http.HttpServletResponse

class TimeoutServlet: HttpServlet() {
    override fun doGet(req: HttpServletRequest, resp: HttpServletResponse) {
        val reqId = req.getHeader("x-request-id") ?: UUID.randomUUID().toString()
        log("GET: reqId = '$reqId' on ${currentThread()}")
        val ctx = req.startAsync()
        val cf = CompletableFuture<String>()
            .orTimeout(10, TimeUnit.SECONDS);
        cf.handleAsync { t, u ->
            log("handle on ${currentThread()}")
            (ctx.response as? HttpServletResponse)?.apply {
                addHeader("x-request-id", reqId)
                writer.print("{completion: ${t ?: u}}")
                writer.close()
            }
            ctx.complete()
            completions.remove(reqId)
        }
        completions[reqId] = cf
    }

    private fun currentThread() = Thread.currentThread().name

    override fun doPost(req: HttpServletRequest, resp: HttpServletResponse) {
        val reqId = req.getHeader("x-request-id")
        when {
            reqId.isNullOrEmpty() -> resp.status = 400
            completions[reqId] == null -> resp.status = 404
            else -> {
                val completion = req.reader.lines().collect(Collectors.joining(System.lineSeparator()))
                completions[reqId]!!.complete(completion)
                log("POST '$completion' -> $reqId: ${completions[reqId].toString()}")
                resp.status = 200
            }
        }
    }

    override fun doOptions(req: HttpServletRequest, resp: HttpServletResponse) {
        resp.status = 200
        resp.writer.apply {
            write(completions.keys.joinToString(", ", prefix = "[", postfix = "]"))
            close()
        }
    }

    companion object {
        val completions = ConcurrentHashMap<String, CompletableFuture<String>>()
    }
}

fun main(args: Array<String>) {
    val servletContainer = Servlets.defaultContainer()
    val di = Servlets.deployment()
        .setClassLoader(TimeoutServlet::class.java.classLoader)
        .setContextPath("/")
        .setDeploymentName("main.war")
        .addServlets(
            servlet("servletName", TimeoutServlet::class.java)
                .addMapping("/*")
                .setLoadOnStartup(1)
                .setAsyncSupported(true)
        )

    val manager = servletContainer.addDeployment(di)
    manager.deploy()

    Undertow.builder()
        .addHttpListener(8080, "localhost")
        .setHandler(Handlers.path().addPrefixPath(
            "/", manager.start()
        )).build().start()
}

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>club.apibank</groupId>
    <artifactId>async-test</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>club.apibank async-test</name>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <kotlin.version>1.3.31</kotlin.version>
        <kotlin.code.style>official</kotlin.code.style>
        <junit.version>4.12</junit.version>
        <undertow.version>2.0.20.Final</undertow.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-stdlib</artifactId>
            <version>${kotlin.version}</version>
        </dependency>
        <dependency>
            <groupId>org.jetbrains.kotlin</groupId>
            <artifactId>kotlin-test-junit</artifactId>
            <version>${kotlin.version}</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>${junit.version}</version>
            <scope>test</scope>
        </dependency>

        <dependency>
            <groupId>io.undertow</groupId>
            <artifactId>undertow-core</artifactId>
            <version>${undertow.version}</version>
        </dependency>
        <dependency>
            <groupId>io.undertow</groupId>
            <artifactId>undertow-servlet</artifactId>
            <version>${undertow.version}</version>
        </dependency>
        <!--<dependency>-->
            <!--<groupId>io.undertow</groupId>-->
            <!--<artifactId>undertow-websockets-jsr</artifactId>-->
            <!--<version>${undertow.version}</version>-->
        <!--</dependency>-->
    </dependencies>

    <build>
        <sourceDirectory>src/main/kotlin</sourceDirectory>
        <testSourceDirectory>src/test/kotlin</testSourceDirectory>

        <plugins>
            <plugin>
                <groupId>org.jetbrains.kotlin</groupId>
                <artifactId>kotlin-maven-plugin</artifactId>
                <version>${kotlin.version}</version>
                <executions>
                    <execution>
                        <id>compile</id>
                        <phase>compile</phase>
                        <goals>
                            <goal>compile</goal>
                        </goals>
                    </execution>
                    <execution>
                        <id>test-compile</id>
                        <phase>test-compile</phase>
                        <goals>
                            <goal>test-compile</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

</project>