I was recently frustrated by very slow tests and timeouts in my Java code, that would often show a similar stack trace (if they actually timed out):

    io.vertx.core.VertxException: Thread blocked
     at java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
     at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:928)
     at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1323)
     at java.net.InetAddress.getLocalHost(InetAddress.java:1500)

I’ve highlighted the critical part. The Vertx bit is just what implements the timeout in my case. If you’re using a Mac and you see this, you’re probably having the same problem. You might also see Inet4AddressImpl in the stack trace instead of Inet6AddressImpl.

After a lot of web trawling and some help from a colleague (thanks Tim) I got to the bottom of it. I’m writing it up here, for my own benefit when I run into this again in the future, and because a lot of the existing resources weren’t clear and direct enough to solve my problem easily.

The fix

The slowness is caused by a domain name lookup that’s taking a few seconds each time, because for some reason your computer is asking the network about its own address, and timing out. I don’t fully understand the mechanics frankly, but the fix was simple in my case.

First, figure out what your computer thinks its hostname is, by running hostname in the terminal. Then use the value returned from that to add lines like this to your /etc/hosts file:

    # This works around slow lookup that we sometimes see in
    # java.net.Inet6AddressImpl.lookupAllHostAddr Sams-MacBook-Pro.local
    ::1 Sams-MacBook-Pro.local

This provides a direct answer for both IPv4 and IPv6, avoiding the slowness. This had the nice effect of bringing my Gradle build time down from 2 minutes to just 44 seconds, including all the tests.