sessionScope变量为空

HttpSession session = request.getSession(true); // 如果会话不存在，则创建一个新的会话

<session-config>
    <session-timeout>30</session-timeout> <!-- 单位为分钟 -->
</session-config>

session.setMaxInactiveInterval(30 * 60); // 设置会话超时时间为30分钟

// 将数据放入 sessionScope
session.setAttribute("key", value);

// 从 sessionScope 中获取数据
Object value = session.getAttribute("key");

synchronized (session) {
    session.setAttribute("key", value);
}

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

@WebServlet("/sessionExample")
public class SessionExampleServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession(true);

        // 将数据放入 sessionScope
        session.setAttribute("username", "JohnDoe");

        // 从 sessionScope 中获取数据
        String username = (String) session.getAttribute("username");

        response.getWriter().println("Username in session: " + username);
    }
}