HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException: java.lang.NullPointerException
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause java.lang.NullPointerException
nm.User.checkUser(User.java:77)
nm.user.action.UserLoginAction.execute(UserLoginAction.java:39)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.18 logs.
--------------------------------------------------------------------------------Apache Tomcat/6.0.18
org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause java.lang.NullPointerException
nm.User.checkUser(User.java:77)
nm.user.action.UserLoginAction.execute(UserLoginAction.java:39)
org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
org.apache.struts.action.ActionServlet.doPost(ActionServlet.java:432)
javax.servlet.http.HttpServlet.service(HttpServlet.java:637)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.18 logs.
--------------------------------------------------------------------------------Apache Tomcat/6.0.18
java.lang.NullPointerException
nm.User.checkUser(User.java:77)
nm.user.action.UserLoginAction.execute(UserLoginAction.java:39)
at java.lang.Integer.parseInt(Integer.java:415)
at java.lang.Integer.valueOf(Integer.java:553)
at com.xindun.controller.PagInationServlet.doGet(PagInationServlet.java:55)
at com.xindun.controller.PagInationServlet.doPost(PagInationServlet.java:79)
一般的,先看异常的信息,猜测异常原因,然后在从开始一行行向下看,一般先找到自己的代码然后再分析异常产生的原因,如果还没头绪,可以google或百度之。
java.lang.NullPointerException产生的原因是 访问一个变量的方法或属*,但这个变量现在是指向null的。
如
String x = null;
x.length();
那么x.length()行就会报空指针。所以遇到java.lang.NullPointerException首先是确定在哪一行发生了异常,然后看看这一行调用了哪个变量的属性或方法,然后分析哪个变量可能是null。
另外还有一种特殊情况
Integer i = null;
int x = i + 1;
在int x = i + 1;的地方也会报空指针。因为这里需要把i自动拆箱成一个int型,但i现在是null。
很常见的,一定是某个对象为NULL,所以导致这个异常了,
仔细找找就知道了。