Sponge Plugin Issues

build.gradle

plugins {
  id 'com.github.johnrengelman.shadow' version '2.0.2'
  id 'java'
}

repositories {
  mavenCentral()
  maven {
    name = 'sponge'
    url = 'https://repo.spongepowered.org/maven'
  }
}

group = 'ca.brennie'
version = '0.1.0-SNAPSHOT'
description = 'An HTTP JSON API'

dependencies {
  compileOnly 'org.spongepowered:spongeapi:7.0.0'
  implementation 'com.sparkjava:spark-core:2.7.1'
  implementation 'javax.servlet:javax.servlet-api:3.1.0'
}

jar {
  onlyIf { false }
}

shadowJar {
  dependencies {
    exclude dependency("org.slf4j.*:")
    // uncommenting this fixes the issue?
    // relocate 'javax.servlet', 'ca.brennie.vendor.javax.servlet'

    exclude '/about.html'
    exclude '/jetty-dir.css'
  }
}

build.dependsOn shadowJar

HttpApiPlugin.java

package ca.brennie.mchttpapi;

import org.spongepowered.api.event.Listener;
import org.spongepowered.api.event.game.state.GameStartedServerEvent;
import org.spongepowered.api.event.game.state.GameStoppingServerEvent;
import org.spongepowered.api.plugin.Plugin;

import static spark.Spark.*;

@Plugin(
    id = "mchttpapi",
    version = "0.1.0",
    name = "MC HTTP API",
    description = "An HTTP API for interacting with a Minecraft server."
)
public class HttpApiPlugin {
    @Listener
    public void onServerStart(final GameStartedServerEvent event) {
        // Uncommenting the following also fixes the issue.
        // ClassLoader loader = HttpApiPlugin.class.getClassLoader();
        // try {
        //    loader.loadClass("javax/servlet/ServletContext");
        // } catch (final ClassNotFoundException e) {}

        get("/", (req, rsp) -> "Hello, world");
    }

    @Listener
    public void onServerStopping(final GameStoppingServerEvent event) {
        stop();
    }
}