Steps to Reproduce

1. run the attached zul under websocket enabled

    <listener>
        <listener-class>org.zkoss.zkmax.au.websocket.WebSocketWebAppInit</listener-class>
    </listener>

2. click the button to write the session
(simulate session replication under clustering environment)

Current Result

Caused by: java.io.NotSerializableException: org.zkoss.zkmax.au.websocket.WebSocketEndPoint

• custom writeObject data (class "java.util.concurrent.ConcurrentHashMap")
• object (class "java.util.concurrent.ConcurrentHashMap", {org.zkoss.zkmax.au.websocket.WebSocketServerPush=org.zkoss.zkmax.au.websocket.WebSocketEndPoint@2ce4aa9e}

  )

• field (class "org.zkoss.zk.ui.impl.DesktopImpl$2", name: "_cache", type: "class java.util.concurrent.ConcurrentHashMap")
• object (class "org.zkoss.zk.ui.impl.DesktopImpl$2", org.zkoss.zk.ui.impl.DesktopImpl$2@3f64e691)
• custom writeObject data (class "org.zkoss.zk.ui.impl.DesktopImpl")
• object (class "org.zkoss.zk.ui.impl.DesktopImpl", [Desktop z_9a3GjeLa-urPJpg0Ym1JyQ:/ticket/14715.zul])
• field (class "org.zkoss.util.CacheMap$Value", name: "value", type: "class java.lang.Object")
• object (class "org.zkoss.util.CacheMap$Value", ([Desktop z_9a3GjeLa-urPJpg0Ym1JyQ:/ticket/14715.zul]@1641363540245))
• custom writeObject data (class "java.util.HashMap")
• object (class "java.util.LinkedHashMap", {z_9a3GjeLa-urPJpg0Ym1JyQ=([Desktop z_9a3GjeLa-urPJpg0Ym1JyQ:/ticket/14715.zul]@1641363540245)}

  )

• field (class "org.zkoss.util.CacheMap", name: "_map", type: "interface java.util.Map")
• custom writeObject data (class "org.zkoss.util.CacheMap")
• object (class "org.zkoss.zk.ui.impl.SimpleDesktopCache$Cache", {z_9a3GjeLa-urPJpg0Ym1JyQ=[Desktop z_9a3GjeLa-urPJpg0Ym1JyQ:/ticket/14715.zul]}

  )

• field (class "org.zkoss.zk.ui.impl.SimpleDesktopCache", name: "_desktops", type: "class org.zkoss.zk.ui.impl.SimpleDesktopCache$Cache")
• object (class "org.zkoss.zk.ui.impl.SimpleDesktopCache", org.zkoss.zk.ui.impl.SimpleDesktopCache@22700d9d)
• custom writeObject data (class "org.zkoss.zk.ui.http.SerializableSession")
• root object (class "org.zkoss.zk.ui.http.SerializableSession", org.zkoss.zk.ui.http.SerializableSession@bdc00d8)
  at java.base/java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1183)
  at java.base/java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:349)

Expected Result

no exception