Why Are My JUnit Tests Running So Slow?

This is mostly a note to myself, but often when I setup a new Linux install, I find that JUnit tests run significantly slower than usual. The CPU is nearly entirely idle and there’s almost no IO-wait making it hard to work out what’s going on.

The problem is that JUnit (or something in our tests or test runner) is doing a DNS lookup on the machine’s hostname. That lookup should be really fast, but if you’re connected to a VPN it may search the remote DNS server which takes time and makes the tests take much longer than they should.

The solution is to make sure your hostname is in /etc/hosts pointing to 127.0.0.1 (and that /etc/hosts is searched first but it usually is). Then the lookup is lightning fast and the tests start maxing out the CPU like they should.