报错了 websocket._exceptions.WebSocketConnectionClosedException: socket is already closed.

这个错误通常是由于在 WebSocket 连接已关闭的情况下，尝试发送或接收消息。为了防止这个错误，你可以在发送消息之前检查 WebSocket 连接的状态，如果连接已关闭，则不发送消息。下面是一个修改后的示例：

import websocket
import threading

class WebSocketClient:

    def __init__(self, url):
        self.url = url
        self.ws = None
        self.connect()

    def connect(self):
        self.ws = websocket.WebSocketApp(self.url,
                                         on_open=self.on_open,
                                         on_message=self.on_message,
                                         on_error=self.on_error,
                                         on_close=self.on_close)
        self.wst = threading.Thread(target=self.ws.run_forever)
        self.wst.daemon = True
        self.wst.start()

    def on_open(self, ws):
        print("WebSocket connection established")

    def on_message(self, ws, message):
        print("Received message:", message)

    def on_error(self, ws, error):
        print("WebSocket error:", error)

    def on_close(self, ws):
        print("WebSocket connection closed")

    def send(self, message):
        if self.ws and self.ws.sock and self.ws.sock.connected:
            self.ws.send(message)
        else:
            print("WebSocket connection is closed.")

    def close(self):
        self.ws.close()

if __name__ == "__main__":
    ws = WebSocketClient("ws://localhost:8080")
    ws.send("Hello, WebSocket Server!")

在 `send` 方法中，我们添加了一些代码来检查 WebSocket 连接的状态。如果连接已关闭，则不发送消息，并打印一条错误信息。这样，在尝试发送消息之前，我们始终会检查 WebSocket 连接的状态，从而避免出现 `WebSocketConnectionClosedException` 异常。