Error 'java.lang.IllegalArgumentException: Plugin already initialized!' found

I'm trying to use a simple socket server to receive some commands(bukkit api). With the thread, the plugin can receive commands and send it to the main server, so that i can control the server. But when i tried to use a use a thread to solve the problem, an error happened:

CODE:

package tiance.auroracore;

import org.bukkit.Bukkit;
import org.bukkit.plugin.java.JavaPlugin;
import org.bukkit.scheduler.BukkitRunnable;
import tiance.auroracore.metrics.Metrics;

import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;

public final class AuroraCore extends JavaPlugin {

    private int port;// 默认服务器端口
    private String AcceptCommand;

    public AuroraCore() {
         this.port=9028;
         this.AcceptCommand=new String();
    }

    // 创建指定端口的服务器
    public AuroraCore(int port) {//构造方法
        this.port = port;//将方法参数赋值给类参数
        this.AcceptCommand=new String();
    }

    // 提供服务
    public void service() {//创建service方法
        ServerSocket server;
        try {
            server = new ServerSocket(port);//创建  ServerSocket类

        }
        catch (IOException e) {
            System.out.println("Error! Cannot start the server! Is the port already used?");
            return;
        }
        while (true) {
            try {// 建立服务器连接
                Socket socket = server.accept();// 等待客户连接
                try {
                    DataInputStream in = new DataInputStream(socket
                            .getInputStream());// 读取客户端传过来信息的DataInputStream
                    DataOutputStream out = new DataOutputStream(socket
                            .getOutputStream());// 向客户端发送信息的DataOutputStream
                    while (true) {
                        String accept = in.readUTF();// 读取来自客户端的信息
                        AcceptCommand = accept;
                        Bukkit.dispatchCommand(Bukkit.getConsoleSender(), AcceptCommand);
                    }
                } finally {// 建立连接失败的话不会执行socket.close();
                    socket.close();//关闭连接
                    server.close();//关闭
                }
            } catch (IOException e) {//捕获异常
                e.printStackTrace();
            }
        }
    }

    @Override
    public void onEnable() {

        new BukkitRunnable(){
            public void run(){
                new AuroraCore().service();//调用service方法
            }
        }.runTaskAsynchronously(this);

        int pluginId = 13929;
        Metrics metrics = new Metrics(this, pluginId);
        saveDefaultConfig();

    }

    @Override
    public void onDisable() {

    }
}

ERROR

[12:45:28 WARN]: [AuroraCore] Plugin AuroraCore v1.0.0 generated an exception while executing task 2
java.lang.IllegalArgumentException: Plugin already initialized!
    at org.bukkit.plugin.java.PluginClassLoader.initialize(PluginClassLoader.java:218) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:52) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at tiance.auroracore.AuroraCore.<init>(AuroraCore.java:19) ~[?:?]
    at tiance.auroracore.AuroraCore$1.run(AuroraCore.java:69) ~[?:?]
    at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftTask.run(CraftTask.java:76) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.craftbukkit.v1_16_R3.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:54) [craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) [?:1.8.0_281]
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) [?:1.8.0_281]
    at java.lang.Thread.run(Unknown Source) [?:1.8.0_281]
Caused by: java.lang.IllegalStateException: Initial initialization
    at org.bukkit.plugin.java.PluginClassLoader.initialize(PluginClassLoader.java:221) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.plugin.java.JavaPlugin.<init>(JavaPlugin.java:52) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at tiance.auroracore.AuroraCore.<init>(AuroraCore.java:19) ~[?:?]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[?:1.8.0_281]
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_281]
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) ~[?:1.8.0_281]
    at java.lang.reflect.Constructor.newInstance(Unknown Source) ~[?:1.8.0_281]
    at java.lang.Class.newInstance(Unknown Source) ~[?:1.8.0_281]
    at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:79) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:143) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:393) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:301) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at org.bukkit.craftbukkit.v1_16_R3.CraftServer.loadPlugins(CraftServer.java:379) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at net.minecraft.server.v1_16_R3.DedicatedServer.init(DedicatedServer.java:218) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:905) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$0(MinecraftServer.java:263) ~[craftbukkit-1.16.5.jar:3096a-Bukkit-af1a232]
    ... 1 more

I've edited the code again and again,but didn't found any solution. I've also seen some questions on the website, but none of them could solve my problem.I didn't found any repeat main class .I really don't know what to do! If someone can help, i will be very glad and thankful to him or her.