Micronaut undertow startup failure - demo app

This is a very simple app to demonstrate an issue whereby a Micronaut app using the micronaut-http-server-undertow web server will fail to start in the following situation:

• micronaut.server.host set to 0.0.0.0 (i.e. bind to all addresses, not just localhost)
• host has a dual-stack IPv4/v6 network

Steps to reproduce

./gradlew shadowJar
java -jar build/libs/micronaut-undertow-startup-failure-0.1-all.jar

App startup fails with the following exception:

17:36:12.257 [main] ERROR io.micronaut.runtime.Micronaut - Error starting Micronaut server: For input string: "0:0:0:0:0:0:0:8080"
io.micronaut.http.server.exceptions.InternalServerException: For input string: "0:0:0:0:0:0:0:8080"
	at io.micronaut.servlet.undertow.UndertowServer.getURL(UndertowServer.java:108)
	at io.micronaut.runtime.Micronaut.lambda$start$2(Micronaut.java:95)
	at java.util.Optional.ifPresent(Optional.java:159)
	at io.micronaut.runtime.Micronaut.start(Micronaut.java:75)
	at io.micronaut.runtime.Micronaut.run(Micronaut.java:311)
	at io.micronaut.runtime.Micronaut.run(Micronaut.java:297)
	at com.example.Application.main(Application.java:8)
Caused by: java.net.MalformedURLException: For input string: "0:0:0:0:0:0:0:8080"
	at java.net.URL.<init>(URL.java:627)
	at java.net.URL.<init>(URL.java:490)
	at java.net.URL.<init>(URL.java:439)
	at java.net.URI.toURL(URI.java:1089)
	at io.micronaut.servlet.undertow.UndertowServer.getURL(UndertowServer.java:106)
	... 6 common frames omitted
Caused by: java.lang.NumberFormatException: For input string: "0:0:0:0:0:0:0:8080"
	at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
	at java.lang.Integer.parseInt(Integer.java:580)
	at java.lang.Integer.parseInt(Integer.java:615)
	at java.net.URLStreamHandler.parseURL(URLStreamHandler.java:222)
	at java.net.URL.<init>(URL.java:622)
	... 10 common frames omitted

Workarounds

Any of the following will avoid the error:

• Do not set micronaut.server.host in application.yml (though then the app binds only to localhost by default)
• Disable IPv6 with -Djava.net.preferIPv4Stack=true
• Configure the logger for io.micronaut.runtime.Micronaut to be WARN or higher
  • this fixes it because the failing call to UndertowServer.getURL is guarded by an if(LOG.isInfoEnabled())

Analysis

The root cause appears to be that in this case UndertowServer.getHost() is returning the string form of an IPv6 all-zeros address, which getURI blindly concatenates in URI.create(getScheme() + "://" + getHost() + ":" + getPort()) without wrapping it in square brackets. Safer would be to use the multi-argument URI constructor, which handles all necessary escaping:

new URI(getScheme(), null, getHost(), getPort(), null, null, null)