getServletContext（）。getAttribute（）是否会重置值？

Question

The following code is my sample servlet application

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);
}

Whenever I make the first (localhost/value1) and second (localhost/value2) requests it (getServletContext().getAttribute("userEmails")) prints out the following

[value1]
[value1,value2]

However whenever I make the third (localhost/value3) request it always converts LinkedList into boolean and prints out the following error

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

I'm not sure what's going on, do I need to configure something in my web.xml?

Answer 1

List.add() returns a boolean, so on the second time when you call userEmails.add(email) the attribute is replaced by a Boolean .

You don't need to keep setting the attribute after you've put it in the context the first time (unless you want to replace the whole list). Just replace

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

with

userEmails.add(email);