Minecraft Plugin ClassNotFound Error from External JAR

I am trying to build a plugin for a Minecraft Spigot server that ultimately I would like to be able to communicate over serial with things connected to my PC (server is running locally on the PC as well).

I have been able to build and run the plugin and manipulate player/blocks in the game so I know the build process for my base plugin is working. My trouble started when I began trying to include an extra dependency: jSerialComm

I added the dependency entry in my pom.xml file:

<dependency>
    <groupId>com.fazecast</groupId>
    <artifactId>jSerialComm</artifactId>
    <version>[2.0.0,3.0.0)</version>
    <scope>provided</scope>
</dependency>

I added some basic code to import classes from jSerialComm and do something basic with them inside of a command in my plugin:

import com.fazecast.jSerialComm.SerialPort;

public class CommandCheck implements CommandExecutor {

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        if (sender instanceof Player){
            System.out.println(SerialPort.getCommPorts());
        }
        return false;
    }
}

This builds fine, I’m using InteliJ and it does recognize the SerialPort class (no red underlines).

But when this command runs in the game I get a Class Not Found error:

org.bukkit.command.CommandException: Unhandled exception executing command 'check' in plugin MyFirstPlugin v1.0-SNAPSHOT
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:47) ~[spigot-api-1.18.1-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:149) ~[spigot-api-1.18.1-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.craftbukkit.v1_18_R1.CraftServer.dispatchCommand(CraftServer.java:821) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.network.PlayerConnection.a(PlayerConnection.java:1939) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.network.PlayerConnection.a(PlayerConnection.java:1778) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.network.PlayerConnection.a(PlayerConnection.java:1759) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.network.protocol.game.PacketPlayInChat.a(PacketPlayInChat.java:46) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.network.protocol.game.PacketPlayInChat.a(PacketPlayInChat.java:1) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.network.protocol.PlayerConnectionUtils.lambda$0(PlayerConnectionUtils.java:30) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.TickTask.run(SourceFile:18) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.util.thread.IAsyncTaskHandler.c(SourceFile:151) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.util.thread.IAsyncTaskHandlerReentrant.c(SourceFile:23) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.b(MinecraftServer.java:1158) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.c(MinecraftServer.java:1) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.util.thread.IAsyncTaskHandler.y(SourceFile:125) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.bf(MinecraftServer.java:1137) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.y(MinecraftServer.java:1130) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.util.thread.IAsyncTaskHandler.c(SourceFile:134) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.x(MinecraftServer.java:1114) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:1038) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at net.minecraft.server.MinecraftServer.lambda$0(MinecraftServer.java:304) ~[spigot-1.18.1-R0.1-SNAPSHOT.jar:3380-Spigot-8965a50-0ba6b90]
        at java.lang.Thread.run(Thread.java:833) [?:?]
Caused by: java.lang.NoClassDefFoundError: com/fazecast/jSerialComm/SerialPort
        at com.foamyguy.myfirstplugin.CommandCheck.onCommand(CommandCheck.java:21) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[spigot-api-1.18.1-R0.1-SNAPSHOT.jar:?]
        ... 21 more
Caused by: java.lang.ClassNotFoundException: com.fazecast.jSerialComm.SerialPort
        at org.bukkit.plugin.java.PluginClassLoader.loadClass0(PluginClassLoader.java:147) ~[spigot-api-1.18.1-R0.1-SNAPSHOT.jar:?]
        at org.bukkit.plugin.java.PluginClassLoader.loadClass(PluginClassLoader.java:99) ~[spigot-api-1.18.1-R0.1-SNAPSHOT.jar:?]
        at java.lang.ClassLoader.loadClass(ClassLoader.java:520) ~[?:?]
        at com.foamyguy.myfirstplugin.CommandCheck.onCommand(CommandCheck.java:21) ~[?:?]
        at org.bukkit.command.PluginCommand.execute(PluginCommand.java:45) ~[spigot-api-1.18.1-R0.1-SNAPSHOT.jar:?]
        ... 21 more

I have unzipped and looked inside of the built jar file and the jSerialComm jar file is successfully being included within my plugin jar:

What do I need to do in order to successfully use an external JAR file (jSerialComm specifically) from inside of a Minecraft plugin? Or alternatively, is there some built-in way that I can connect and communicate over serial ports without needing an external JAR and therefore not needing anything "special" in order to work?

>Solution :

Even if the JAR is present in your plugin, the classes of the JAR are not loaded in the classpath and Spigot cannot access the classes.

You can use a plugin, such as the maven-shade-plugin, which copies all classes from your API-JAR to your Plugin-JAR.

First, set the scope from provided to compile.

<dependency>
    <groupId>com.fazecast</groupId>
    <artifactId>jSerialComm</artifactId>
    <version>[2.0.0,3.0.0)</version>
    <scope>compile</scope> <!-- -->
</dependency>

Then add the maven-shade-plugin under build > plugins inside your pom.xml

<build>
    <!-- ... -->
    <plugins>
      <!-- ... -->
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-shade-plugin</artifactId>
        <version>3.1.0</version>
        <executions>
          <execution>
            <phase>package</phase>
            <goals>
              <goal>shade</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
      <!-- ... -->
    </plugins>
    <!-- ... -->
  </build>

If you now build the jar (using e. g. man clean package), there should be a "fat-" jar file in the target/ folder that contains the classes of the API as well as your classes.

Contents of your plugin before:

├ com
  ├ foamguy
    └ myfirstplugin
      └ ...
├ plugin.yml
└ jSerialComm-xxx.jar

Contents of your plugin after:

├ com
  ├ foamguy
    └ myfirstplugin
      └ ...
  └ fazecast
    └ jSerialComm
      └ ...
└ plugin.yml