servlet是否存在线程安全问题

        今天老师问了一句，servlet存在线程安全问题吗，

public class threadSafe extends HttpServlet{     private static final long serialVersionUID = 1L;     private volatile int num = 0;     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {         addOne();         response.getWriter().write("now access num : " + getNum());     }     /**       * 读取开销低       */      private int getNum() {         return num;     }     /**       * 其写入为非线程安全的，赋值操作开销高       */      private synchronized void addOne() {         num ++;     } }

synchronized关键字

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