Java Web 中分别使用session，application和文件存储方式实现网站计数器功能，写出代码

使用session实现网站计数器：

在Servlet中：

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    HttpSession session = request.getSession();
    Integer count = (Integer) session.getAttribute("count");
    if (count == null) {
        count = 1;
    } else {
        count++;
    }
    session.setAttribute("count", count);
    response.getWriter().print("You are the " + count + "th visitor.");
}

使用application实现网站计数器：

在Servlet中：

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    ServletContext application = getServletContext();
    Integer count = (Integer) application.getAttribute("count");
    if (count == null) {
        count = 1;
    } else {
        count++;
    }
    application.setAttribute("count", count);
    response.getWriter().print("You are the " + count + "th visitor.");
}

使用文件存储方式实现网站计数器：

public class CounterUtil {
    private static final String FILE_NAME = "counter.txt";

    public static int getCount() {
        int count = 0;
        File file = new File(FILE_NAME);
        if (file.exists()) {
            try (BufferedReader reader = new BufferedReader(new FileReader(file))) {
                String line = reader.readLine();
                if (line != null) {
                    count = Integer.parseInt(line.trim());
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        count++;
        try (BufferedWriter writer = new BufferedWriter(new FileWriter(file))) {
            writer.write(String.valueOf(count));
        } catch (IOException e) {
            e.printStackTrace();
        }
        return count;
    }
}

在Servlet中：

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    int count = CounterUtil.getCount();
    response.getWriter().print("You are the " + count + "th visitor.");
}