Tomcat 7:WebSocket升级不起作用

我正在尝试在复盆子PI上运行WebSockets应用程序。我已经下载并解压缩了Tomcat 7.0.67。然后我启动了TomcatManager并部署了我的“wsock.jar”,它只包含一个文件:

// ChatServer.java
package wsock;

import javax.websocket.OnClose;
import javax.websocket.OnMessage;
import javax.websocket.OnOpen;
import javax.websocket.Session;
import javax.websocket.server.ServerEndpoint;

@ServerEndpoint("/chat")
public class ChatServer {
    @OnOpen
    public void onOpen(Session session) {
        System.err.println("Opened session: " + session.getId());
    }

    @OnClose
    public void onClose(Session session) {
        System.err.println("Closed session: " + session.getId());
    }

    @OnMessage
    public String onMessage(String message) {
        System.err.println("Message received: " + message);
        return "{ \"message\": \"Hello World\" }";
    }
}

在我的本地Tomcat 7(当前在Windows 10上)上部署时,它可以工作:

ws = new WebSocket('ws://localhost:8080/wsock/chat');
WebSocket { url: "ws://localhost:8080/wsock/chat", readyState: 0, bufferedAmount: 0, onopen: null, onerror: null, onclose: null, extensions: "", protocol: "", onmessage: null, binaryType: "blob" }

在我的树莓PI上部署时:

ws = new WebSocket('ws://raspberrypi:8080/wsock/chat');
WebSocket { url: "ws://raspberrypi:8080/wsock/chat", readyState: 0, bufferedAmount: 0, onopen: null, onerror: null, onclose: null, extensions: "", protocol: "", onmessage: null, binaryType: "blob" }
Firefox can't establish a connection to the server at ws://raspberrypi:8080/wsock/chat.

发送的请求是:

Host: raspberrypi:8080
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:43.0) Gecko/20100101 Firefox/43.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de,en-US;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
DNT: 1
Sec-WebSocket-Version: 13
Origin: http://raspberrypi:8080
Sec-WebSocket-Extensions: permessage-deflate
Sec-WebSocket-Key: 0a80/+qiZ3mJ03bDgSV5kg==
Connection: keep-alive, Upgrade
Pragma: no-cache
Cache-Control: no-cache
Upgrade: websocket

但答案不包含升级:

Content-Language: en
Content-Length: 971
Content-Type: text/html;charset=utf-8
Date: Fri, 01 Jan 2016 11:42:35 GMT
Server: Apache-Coyote/1.1

有趣的是,当我连接到示例WebSocket Chat(随Tomcat服务器一起提供)时,它可以工作:

ws = new WebSocket('ws://raspberrypi:8080/examples/websocket/chat');
WebSocket { url: "ws://raspberrypi:8080/examples/webs…", readyState: 0, bufferedAmount: 0, onopen: null, onerror: null, onclose: null, extensions: "", protocol: "", onmessage: null, binaryType: "blob" }

那么,这里怎么了?我错过了什么?我能做些什么来解决这个问题?