UDP support was added to the 1.1.0 release of project Reactor, but the web has been pretty quiet about it since. There are some good existing technologies out there if you need to write a UDP server, but those aren’t part of the sweet Reactor project… or integrate seamlessly with Spring Boot!

I have created a very simple, starter server on Github and will run through the pieces:

Reactor Spring Support
If you open the build.gradle file in the project you will notice the “org.projectreactor.spring:reactor-spring-context” dependency. This project implicitly creates the Environment and puts it into your application context when you use the @EnableReactor annotation on the configuration class. It makes wiring everything together very easy by providing annotations for specifying consumers, selectors, and etc. when you’re ready to start doing something with the incoming packets. Of course, it also does the lifecycle stuff you would expect with a Spring project, so check out their documentation for more.

The Datagram Server
Much like the Reactor TCP server, you shouldn’t instantiate an implementation of DatagramServer directly. Instead use the DatagramServerSpec to build the server and configure it. We aren’t doing anything interesting with the incoming packets for this example, just logging them when they arrive.

The CountDownLatch
As noted in this example from Spring, the TCP and UDP servers are non-blocking so this is necessary so the main thread doesn’t exit.

Give It a Run
Use the Gradle wrapper to build, and then run the jar (also be sure you are using JDK 8):

Spring boot will start up and you should see logging from the framework. Look for a statement that looks something like:

r.net.netty.udp.NettyDatagramServer      : BIND /0:0:0:0:0:0:0:0:{SOME_PORT_NUMBER}

This will tell you what port number the server is listening, keep this in mind for the next step.

Test It Out
Netcat is very convenient for sending simple UDP packets for situations like this. In a separate shell type:

Where $UDP_SERVER_PORT is the port we collected from the log output in the previous step.