Android java.net.SocketException: Permission denied

I'm an android beginner. I want to create a ServerSocket, but it always calls "java.net.SocketException: Permission denied" back on line "serverSocket = new ServerSocket(12345);" I want to know why, thank you.

ServerThread.java

public class ServerThread extends Thread {
    ServerSocket serverSocket ;
    @Override
    public void run() {
        try {
            serverSocket = new ServerSocket(12345);
            while(true) {
                System.out.println("IN");
                Socket socket = serverSocket.accept(); // null
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

MainActivity.java

public class MainActivity extends AppCompatActivity {

   @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new ServerThread().start();
    }
}