运行后 直接输出WebSocket connection is closed.

这可能是由于 WebSocket 连接没有在 `connect` 方法中成功建立造成的。你可以在 `on_error` 方法中添加打印语句，以查看是否有任何错误消息。另外，确保 WebSocket 服务器正在运行并监听正确的地址和端口。下面是修改后的示例：

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!")

如果 WebSocket 服务器正在运行，但你仍然无法连接，请检查防火墙设置或代理设置是否阻止了 WebSocket 连接。