Right Way To Shadow Jar When Using Jetty With Http4k

(x-post from my blog here)

If you’ve been spring boot for a while, you’re probably familiar with Spring’s bootJar functionality that lets you create a new executable “fat jar” with all its dependencies pre defined. However, while I was building a kotlin app using http4k, which doesn’t come with any such built in tooling I had to resort to using the shadowJar plugin to build a fat jar.

However post deploy, the app stopped starting up and I noticed the following error in the logs:

Exception in thread “main” java.lang.ExceptionInInitializerError
    at org.eclipse.jetty.http.MimeTypes$Type.<init>(MimeTypes.java:98)
    at org.eclipse.jetty.http.MimeTypes$Type.<clinit>(MimeTypes.java:56)
    at org.eclipse.jetty.http.MimeTypes.<clinit>(MimeTypes.java:175)
    at org.eclipse.jetty.server.handler.ContextHandler.doStart(ContextHandler.java:806)
    at org.eclipse.jetty.servlet.ServletContextHandler.doStart(ServletContextHandler.java:275)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:72)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:169)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:110)
    at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:100)
    at org.eclipse.jetty.websocket.server.WebSocketHandler.doStart(WebSocketHandler.java:84)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:72)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.start(ContainerLifeCycle.java:169)
    at org.eclipse.jetty.server.Server.start(Server.java:407)
    at org.eclipse.jetty.util.component.ContainerLifeCycle.doStart(ContainerLifeCycle.java:110)
    at org.eclipse.jetty.server.handler.AbstractHandler.doStart(AbstractHandler.java:100)
    at org.eclipse.jetty.server.Server.doStart(Server.java:371)
    at org.eclipse.jetty.util.component.AbstractLifeCycle.start(AbstractLifeCycle.java:72)
    at org.http4k.server.Jetty$toServer$3.start(jetty.kt:33)
    at io.taggit.AppKt.main(App.kt:175)
    at io.taggit.AppKt.main(App.kt)
Caused by: java.lang.ArrayIndexOutOfBoundsException: 1
    at org.eclipse.jetty.http.PreEncodedHttpField.<clinit>(PreEncodedHttpField.java:68)
    … 20 more

Perplexed, since when I used the same plugin with another one of my projects I had never noticed this error.

After much googling, turns out Jetty bundles HttpField encoders by referencing META-INF/services/org.eclipse.jetty.http.HttpFieldPreEncoder so when shadow jar created the fat jar it didn’t have the data that Jetty was looking for which in turn meant Jetty threw an initialisation exception.

The fix?

Adding the mergeServiceFiles() attribute to the shadowJar task, as per the shadow documentation here.

shadowJar {
    baseName = ‘taggit-api’
    zip64 = true
    mergeServiceFiles()
}

And voila, the service was back up and running again. This is becauase multiple libraries potentially use the same service descriptor files (usually META-INF) and in case of creating fat jars its generally desired to merge the service descriptors to make sure all libraries have their service descriptors loaded at runtime.

P.S. - It might be worth mentioning here as well that the reason I didn’t encounter the same error for one of my other services is due to that project using Netty as the underlying webserver.

Cover Photo by Markus Spiske on Unsplash