Plugin I made reports java.lang.NoClassDefFoundError

Question

I'm trying to call a class that starts a websocket in another file when the Minecraft plugin runs, but I'm getting an error. What's the problem? please help me.. (use maven)

package test.minecraft;

import org.bukkit.plugin.java.JavaPlugin;

public final class test_Minecraft extends JavaPlugin {

    @Override
    public void onEnable() {
        // Plugin startup logic
        this.saveDefaultConfig();
        chat backchat = new chat();
        backchat.st();
    }

    @Override
    public void onDisable() {
        // Plugin shutdown logic
    }

}

package test.minecraft;

import org.java_websocket.client.WebSocketClient;
import org.java_websocket.handshake.ServerHandshake;
import java.net.URI;
import java.net.URISyntaxException;

public class chat {
    private static WebSocketClient client;

    public static void st(String chatId) {
        connectWebSocket();

    private static void connectWebSocket(S) {
        try {
            URI uri = new URI("wss://example.com");
            client = new WebSocketClient(uri) {
                @Override
                public void onOpen(ServerHandshake serverHandshake) {
                    System.out.println("Connected to websocket");
                }

                @Override
                public void onMessage(String s) {
                    JSONObject jsonObject = new JSONObject(s);
                    System.out.println(s);

                }

                @Override
                public void onClose(int i, String s, boolean b) {
                    System.out.println("Connection closed");
                }

                @Override
                public void onError(Exception e) {
                    e.printStackTrace();
                }
            };
            client.connect();
        } catch (URISyntaxException e) {
            e.printStackTrace();
        }
    }
}

I'm trying to call a class that starts a websocket in another file when the Minecraft plugin runs.