求救 Servlet 调用出问题了 急！

Question

type Exception reportmessagedescription The server encountered an internal error () that prevented it from fulfilling this request.exceptionjava.lang.NullPointerException
star.moon.HandleGuess.doPost(HandleGuess.java:17)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)note The full stack trace of the root cause is available in the Apache Tomcat/6.0.14 logs.

Answer 1

NullPointerException 出现空值 NULL 指针！

Answer 2

这是我学习的时候，写的代码。
package star.moon;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class HandleGuess extends HttpServlet
{
public void init(ServletConfig config) throws ServletException
{
super.init(config);
}
    public void doPost(HttpServletRequest request,HttpServletResponse response) throws ServletException , IOException
{
HttpSession session =request.getSession(true);
String str=request.getParameter("clientGuessLetter");
Character guessLetter=str.trim().charAt(0);
Character saveLetter=(Character)session.getAttribute("savaLetter");
char realLetter=saveLetter.charValue();
if(Character.isUpperCase(guessLetter))
{
guessLetter=Character.toLowerCase(guessLetter);
}
if(guessLetter<realLetter)
{
session.setAttribute("message","您猜小了");
response.sendRedirect("inputLetter.jsp");
}
if(guessLetter>realLetter)
{
session.setAttribute("message","您猜大了");
response.sendRedirect("inputLetter.jsp");
}
if(guessLetter==realLetter)
{
session.setAttribute("message","您猜对了");
response.sendRedirect("inputLetter.jsp");
}
}
public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException
{
doPost(request,response);
}
}

Answer 3

先判断下你的session中到底有没有你想取得那个值吧
Character saveLetter=(Character)session.getAttribute("savaLetter"); 
如果session中没有savaLetter，那么你取出来的值应该是null;
所以下面一行代码saveLetter.charValue(); 就出空指针异常了。
呵呵

Answer 4

测试一下request.getParameter("clientGuessLetter"); 是不是返回的是null。