托管bean属性在不同的请求中变为空

Question

使用new ArrayList<E>()初始化List时遇到问题。这是我的班级： -

@ManagedBean(name = "clientBean")
@SessionScoped
public class ClientBean {

    public String msg = "", input, pendingMsg = "", user = "";
    public GossipClient client;
    public boolean inputDisabled = true, sendBtnDisabled = true, startBtnDisabled = false;
    String key = null;
    public List<ClientBeanDto> dtos;
    ClientBeanDto dto;
    String style;
    public List<OnlineList> onlineList;
    OnlineList userInfo;
    OnlineList newUser = null;

    public void start() throws UnknownHostException, IOException {
        client = new GossipClient(this);
        if (user != null && user != "" && client.connect()) {
            try {
                BufferedReader reader = new BufferedReader(new FileReader("D://key.txt"));
                key = reader.readLine();
                reader.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
            client.start();
            inputDisabled = sendBtnDisabled = false;
            startBtnDisabled = true;
            msg = pendingMsg = "";
            pendingMsg = user + ", you are connected!";
            client.sendNick(key + user);
            onlineList = new ArrayList<OnlineList>();
            dtos = new ArrayList<ClientBeanDto>();
        }
        .
        .
        .
    }

问题在于public List<OnlineList> onlineList。在start()方法中执行行onlineList = new ArrayList<OnlineList>();时，在调试模式下，我发现它的值为null，而下一行中的类似对象dtos被初始化并成功分配了一个id。 我似乎无法找到这种行为的任何原因，我没有得到任何错误/异常。任何智慧都将受到赞赏。

更新：每当点击xhtml（bean支持的）按钮时，list变为null。我找不到原因。 bean是SessionScoped，另一个列表（dtos）对象被保留。