java - getServletContext().getAttribute() 重置值？

Question

以下代码是我的示例 servlet 应用程序

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String userEmail = request.getPathInfo().split("/")[1];
    saveEmails(response, userEmail);

}
protected void saveEmails(HttpServletResponse response, String email) {
    ArrayList<String> userEmails = (ArrayList<String>)getServletContext().getAttribute("userEmails");
    if (userEmails == null) {
        System.out.println("creating a new linked list");
        ArrayList<String> newUserEmails = new ArrayList<String>();
        newUserEmails.add(email);
        getServletContext().setAttribute("userEmails", newUserEmails);
    } else {
        System.out.println("appending new email into linked list");
        getServletContext().setAttribute("userEmails", userEmails.add(email));
    }
    System.out.println(userEmails);
}

每当我发出第一个 (localhost/value1) 和第二个 (localhost/value2) 请求时 (getServletContext().getAttribute("userEmails")) 打印出以下内容

[value1]
[value1,value2]

但是，每当我发出第三个(localhost/value3)请求时，它总是将 LinkedList 转换为 boolean 值并打印出以下错误

HTTP Status 500 - java.lang.Boolean cannot be cast to java.util.LinkedList

我不确定发生了什么，我需要在 web.xml 中配置某些内容吗？

Answer 1

List.add() 返回一个 boolean 值，因此当您第二次调用 userEmails.add(email) 时，该属性将被替换为 Boolean .

第一次将属性放入上下文后，您无需继续设置该属性(除非您想替换整个列表)。只需更换即可

getServletContext().setAttribute("userEmails", userEmails.add(email));

与

userEmails.add(email);